Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

INTRODUCTION linearizes the LED’s output flux and eliminates the LED’s This application note presents isolation amplifier circuit time and temperature. The galvanic isolation between the designs useful in industrial, instrumentation, medical, and input and the output is provided by a second PIN photodiode communication systems. It covers the IL300’s coupling (pins 5, 6) located on the output side of the coupler. The specifications, and circuit topologies for photovoltaic and output current, IP2, from this photodiode accurately tracks the photoconductive amplifier design. Specific designs include photocurrent generated by the servo photodiode. unipolar and bipolar responding amplifiers. Both single Figure 1 shows the package footprint and electrical ended and differential amplifier configurations are discussed. schematic of the IL300. The following sections discuss the Also included is a brief tutorial on the operation of key operating characteristics of the IL300. The IL300 photodetectors and their characteristics. performance characteristics are specified with the Galvanic isolation is desirable and often essential in many photodiodes operating in the photoconductive mode. measurement systems. Applications requiring galvanic isolation include industrial sensors, medical transducers, and IL300 mains powered switchmode power supplies. Operator safety 1 8 and signal quality are insured with isolated interconnections. These isolated interconnections commonly use isolation 2 K2 7 amplifiers. K1 Industrial sensors include thermocouples, strain gauges, and pressure transducers. They provide monitoring signals to a 3 6 process control system. Their low level DC and AC signal must be accurately measured in the presence of high 4 5 common-mode noise. The IL300’s 130 dB common mode IP1 IP2 rejection (CMR), ± 50 ppm/°C stability, and ± 0.01 % linearity provide a quality link from the sensor to the controller input. 17752 Safety is an important factor in instrumentation for medical Fig. 1 - IL300 Schematic patient monitoring. EEG, ECG, and similar systems demand SERVO GAIN - K1 high insulation safety for the patient under evaluation. The IL300’s 7500 V withstand test voltage (WTV) insulation, DC The typical servo photocurrent, IP1, as a function of LED response, and high CMR are features which assure safety current, is shown in figure 2. This graph shows the typical for the patient and accuracy of the transducer signals. non-servo LED-photodiode linearity is ± 1 % over an LED The aforementioned applications require isolated signal drive current range of 1 to 30 mA. This curve also shows that processing. Current designs rely on A to D or V to F the non-servo photocurrent is affected by ambient converters to provide input/output insulation and noise temperature. The photocurrent typically decreases by isolation. Such designs use transformers or high-speed - 0.5 % per °C. The LED’s nonlinearity and temperature optocouplers which often result in complicated and costly characteristics are minimized when the IL300 is used as a solutions. The IL300 eliminates the complexity of these servo linear amplifier. isolated amplifier designs without sacrificing accuracy or stability. 300 The IL300’s 200 kHz bandwidth and gain stability make it an excellent candidate for subscriber and data phone 250 interfaces. Present OEM switch mode power supplies are 0 °C 25 °C approaching 1 MHz switching frequencies. Such supplies 200

rrent (µA) 50 °C need output monitoring feedback networks with wide u 75 °C bandwidth and flat phase response. The IL300 satisfies 150 these needs with simple support circuits. o Photoc

v 100 OPERATION OF THE IL300 50

The IL300 consists of a high-efficiency AlGaAs LED emitter IP1 - Ser coupled to two independent PIN photodiodes. The servo 0 photodiode (pins 3, 4) provides a feedback signal which 0 5 10 15 20 25 30 17753 controls the current to the LED emitter (pins 1, 2). This IF - LED Current - mA photodiode provides a photocurrent, IP1, that is directly proportional to the LED’s incident flux. This servo operation Fig. 2 - Servo Photocurrent vs. LED Current

www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1004 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler

The servo gain is defined as the ratio of the servo conditions can be determined by using the minimum value photocurrent, IP1, to the LED drive current, IF. It is called K1, for K1 (0.005) and the normalization factor from figure 4. The and is described in equation 1. example is to determine IP1 (min.) for the condition of K1 at (1) TA = 75 °C, and IF = 6 mA. K1= IP1 ⁄ IF NK1() IF = 6mAT, A = 75 °C = 0.72⋅ NK1() IF ,TA (3) The IL300 is specified with an IF = 10 mA, TA = 25 °C, and K1 MIN() IF ,TA = K1 MIN()() 0.005 ⋅ NK1() 0.72 (4) Vd = - 15 V. This condition generates a typical servo K1 MIN() I ,T = 0.0036 (5) photocurrent of IP1 = 70 µA. This results in a typical F A K1 = 0.007. The relationship of K1 and LED drive current is Using K1(IF, TA) = 0.0036 in equation 1 the minimum IP1 can shown in figure 3. be determined.

IP1MIN= K1 MIN()() IF ,TA ⋅ IF (6)

IP1MIN= 0.0036⋅ 6 mA (7) 0.010 IP1MIN() IF = 6 mA, TA = 75 °C = 21.6 µA (8) 0° The minimum value IP1 is useful for determining the F 0.008 /I 25° maximum required LED current needed to servo the input P1 50° stage of the isolation amplifier. 0.006 75° 100° OUTPUT FORWARD GAIN - K2 o Gain - I v 0.004 Figure 1 shows that the LED's optical flux is also received by a PIN photodiode located on the output side (pins 5, 6) of the 0.002

K1- Ser coupler package. This detector is surrounded by an optically transparent high-voltage insulation material. The coupler 0 construction spaces the LED 0.4 mm from the output PIN 0.1 1 10 100 photodiode. The package construction and the insulation 17754 IF - L ED Current (mA) material guarantee the coupler to have a withstand test voltage of 7500 V peak. Fig. 3 - Servo Gain vs. LED Current K2, the output (forward) gain is defined as the ratio of the The servo gain, K1, is guaranteed to be between 0.005 output photodiode current, IP2, to the LED current, IF. K2 is minimum to 0.011 maximum of an IF = 10 mA, TA = 25 °C, shown in equation 9. and VD = 15 V. K2= IP1 ⁄ IF (9) The forward gain, K2, has the same characteristics of the servo gain, K1. The normalized current and temperature 1.2 performance of each detector is identical. This results from Normalized to: 0 ° I = 10 mA, 25 ° using matched PIN photodiodes in the IL300’s construction. 1.0 F TA = 25 °C

o Gain 50 °

v TRANSFER GAIN - K3 0.8 75 ° 100 ° The current gain, or CTR, of the standard phototransistor 0.6 optocoupler is set by the LED efficiency, transistor gain, and optical coupling. Variation in ambient temperature alters the

ormalized Ser 0.4 N LED efficiency and phototransistor gain and results in CTR drift. Isolation amplifiers constructed with standard K1 - 0.2 N phototransistor optocouplers suffer from gain drift due to changing CTR. 0.0 0.1 1 10 100 Isolation amplifiers using the IL300 are not plagued with the

17755 drift problems associated with standard phototransistors. IF - LED Current (mA) The following analysis will show how the servo operation of Fig. 4 - Normalized Servo Gain vs. LED Current the IL300 eliminates the influence of LED efficiency on the Figure 4 presents the normalized servo gain, NK1(IF, TA), as amplifier gain. a function of LED current and temperature. It can be used to The input-output gain of the IL300 is termed transfer gain, determine the minimum or maximum servo photocurrent, IP1, K3. Transfer gain is defined as the output (forward) gain, K2, given LED current and ambient temperature. The actual divided by servo gain, K1, as shown in equation 10. servo gain can be determined from equation 2. K3= K2⁄ K1 (10) (2) The first step in the analysis is to review the simple optical K1() IF ,TA = K1() data sheet limit ⋅ NK1() IF ,TA The minimum servo photocurrent under specific use servo feedback amplifier shown in figure 5.

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1005 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

The circuit consists of an operational amplifier, U1, a resistance amplifier. The common inverting trans resistance feedback resistor R1, and the input section of the IL300. The amplifier is shown in figure 6. The output photodiode is servo photodiode is operating in the photoconductive mode. operated in the photoconductive mode. The photocurrent, The initial conditions are: IP2, is derived from the same LED that irradiates the servo photodetector. The output signal, Vout, is proportional to the Va ==Vb 0 . Initially, a positive voltage is applied to the nonirritating input output photocurrent, IP2, times the trans resistance, R2. (Va) of the op amp. At that time the output of the op amp will Vout = - IP2 ⋅ R2 (17) swing toward the positive Vcc rail, and forward bias the LED. IP2 = K2⋅ IF (18) As the LED current, I , starts to flow, an optical flux will be F Combining equations 17 and 18 and solving for I is shown generated. The optical flux will irradiate the servo photodiode F in equation 19. causing it to generate a photocurrent, IP1. This photocurrent will flow through R1 and develop a positive voltage at the IF = - Vout ⁄ ()K2⋅ R2 (19) inverting input (Vb) of the op amp. The amplifier output will start to swing toward the negative supply rail, - VCC. When the magnitude of the Vb is equal to that of Va, the LED drive IL300 8 current will cease to increase. This condition forces the 7 circuit into a stable closed loop condition. K2 7 3 + Vout VCC U2 6 3 7 6 + + VCC 1 IL300 V VCC 2 a 6 5 - U1 4 V 2 IP2 in K1 V I F R2 b - IP2 V 3 2 CC 4 17757 4 I Fig. 6 - Optical Servo Amplifier IP1 P1 R1 The input-output gain of the isolation amplifier is determined by combining equations 16 and 19.

IF = Vin ⁄ ()K1⋅ R1 (16)

IF = - Vout ⁄ ()K2⋅ R2 (19) 17756 Vin ⁄ ()K1⋅ R1 = - Vout ⁄ ()K2⋅ R2 (20) Fig. 5 - Optical Servo Amplifier Vout ⁄ Vin = – ()K2⋅ R2 ⁄ ()K1⋅ R1 (21) When Vin is modulated, Vb will track Vin. For this to happen the photocurrent through R1 must also track the change in Note that the LED current, IF, is factored out of equation 21. This is possible because the servo and output photodiode Va. Recall that the photocurrent results from the change in LED current times the servo gain, K1. The following currents are generated by the same LED source. This equations can be written to describe this activity. equation can be simplified further by replacing the K2/K1 ratio with IL300’s transfer gain, K3. V ==V V =0 (11) a b in V ⁄ V = - K3⋅ () R2⁄ R1 (22) I = I ⋅ K1 (12) out in P1 F The IL300 isolation amplifier gain stability and offset drift V = I ⋅ R1 (13) b P1 depends on the transfer gain characteristics. Figure 7 shows The relationship of LED drive to input voltage is shown by the consistency of the normalized K3 as a function of LED combining equations 11, 12, and 13. current and ambient temperature. The transfer gain drift as a function of temperature is ± 0.005 %/°C over a 0 °C to 75 °C Va = IP1 ⋅ R1 (14) range. Vin = IF ⋅⋅K1 R1 (15) I = V ⁄ ()K1⋅ R1 (16) Figure 8 shows the composite isolation amplifier including F in the input servo amplifier and the output trans resistance Equation 16 shows that the LED current is related to the input amplifier. This circuit offers the insulation of an optocoupler voltage Vin. A changing Va causes a modulation in the LED and the gain stability of a feedback amplifier. flux. The LED flux will change to a level that generates the necessary servo photocurrent to stabilize the optical feedback loop. The LED flux will be a linear representation of the input voltage, Va. The servo photodiode’s linearity controls the linearity of the isolation amplifier. The next step in the analysis is to evaluate the output trans www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1006 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler

Package assembly variations result in a range of K3. 1.010 Because of the importance of K3, Vishay offers the transfer Normalized to IF = 10 mA, TA = 25 °C gain sorted into ± 5 % bins. The bin designator is listed on 0 °C Non - servoed the IL300 package. The K3 bin limits are shown in table 1. 1.005 This table is useful when selecting the specific resistor 25 °C values needed to set the isolation amplifier transfer gain.

1.000 50 °C TABLE 1 - K3 TRANSFER GAIN BINS BIN TYP. MIN. MAX. 75 °C 0.995 A 0.59 0.56 0.623 K3 - Transfer Gain (K2/K1) B 0.66 0.623 0.693

0.990 C 0.73 0.693 0.769 0 5 10 15 20 25 D 0.81 0.769 0.855

17758 IF - LED Current (mA) E 0.93 0.855 0.95 F 1.0 0.95 1.056 Fig. 7 - Normalized Servo Transfer Gain G 1.11 1.056 1.175 An instrumentation engineer often seeks to design an H 1.24 1.175 1.304 isolation amplifier with unity gain of Vout/Vin = 1.0. The I 1.37 1.304 1.449 IL300’s transfer gain is targeted for: K3 = 1.0. J 1.53 1.449 1/61

3 7 + + VCC 1 IL300 8 Va 6 3 7 U1 2 K2 7 Vin + I K1 Vout Vb F V 6 – CC U2 3 6 2 V 4 CC VCC 2 4 5 – 4 IP1 IP2 IP1 R1 IP2 R2

17759

Fig. 8 - Composite Amplifier ISOLATION AMPLIFIER DESIGN PHOTOVOLTAIC ISOLATION AMPLIFIER TECHNIQUES The transfer characteristics of this amplifier are shown in The previous section discussed the operation of an isolation figure 9. amplifier using the optical servo technique. The following The input stage consists of a servo amplifier, U1, which section will describe the design philosophy used in controls the LED drive current. The servo photodiode is developing isolation amplifiers optimized for input voltage operated with zero voltage bias. This is accomplished by range, linearity, and noise rejection. connecting the photodiodes anode and cathode directly to The IL300 can be configured as either a photovoltaic or U1’s inverting and non-inverting inputs. The characteristics photoconductive isolation amplifier. The photovoltaic of the servo amplifier operation are presented in figure 9a topology offers the best linearity, lowest noise, and drift and figure 9b. The servo photocurrent is linearly proportional performance. Isolation amplifiers using these circuit to the input voltage, IP1 = Vin ⁄ R1 . Figure 9b shows the configurations meet or exceed 12 bit A to D performance. LED current is inversely proportional to the servo transfer Photoconductive photodiode operation provides the largest gain, IF = IP1 ⁄ K1 . The servo photocurrent, resulting from coupled frequency bandwidth. The photoconductive the LED emission, keeps the voltage at the inverting input of configuration has linearity and drift characteristics U1 equal to zero. The output photocurrent, IP2, results from comparable to a 8 to 9 bit A to D converter. the incident flux supplied by the LED. Figure 9c shows that the magnitude of the output current is determined by the output transfer gain, K2. The output voltage, as shown in

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1007 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler figure 9d, is proportional to the output photocurrent IP2. The The second is the servo transfer gain, K1 · R1. The amplifier output voltage equals the product of the output photocurrent gain is the first divided by the second. See equation 23. times the output amplifier’s trans resistance, R2.

IP1 I V When low offset drift and greater than 12 bit linearity is IF P2 out desired, photovoltaic amplifier designs should be considered. The schematic of a typical positive unipolar 1 1 K2 R2 photovoltaic isolation amplifier is shown in figure 10. R1 K1 The composite amplifier transfer gain (Vo/Vin) is the ratio of 0 + 0 I + 0 I + 0 I + two products. The first is the output transfer gain, K2 · R2. Vin P1 F P2 a b c d 17760 Fig. 9 - Positive Unipolar Photovoltaic Isolation Amplifier Transfer Characteristics

VCC 1 k

+ Ω 3 6 R2 OP-07 1 IL300 8 V R1 10 kΩ in 2 IF 10 kΩ - 2 K2 7 K1 + Voltage IP1 3 6 - 2 6 4 5 OP-07 IP1 IP2 V 3 out + 17761

Fig. 10 - Positive Unipolar Photovoltaic Aamplifier Vout K2⋅ R1 ------= ------(23) The input resistor, R1, is set by the input voltage range and Vin K1⋅ R1 the peak servo photocurrent, IP1. Equation 23 shows that the composite amplifier transfer gain Thus R1 is equal to: is independent of the LED forward current. The K2/K1 ratio reduces to IL300 transfer gain, K3. This relationship is R1 = Vin/IP1 included in equation 24. This equation shows that the R1 = 1.0/105 µA composite amplifier gain is equal to the product of the IL300 R1 = 9.524 kΩ gain, K3, times the ratio of the output to input resistors. R1 is rounded to 10 kΩ. Vout K3⋅ R2 ------= ------(24) Vin R1 Designing this amplifier is a three step process. First, given 1.0 the input signal span and U1’s output current handling 0.9 capability, the input resistor R1 can be determined by using ) 0.8 V the circuit found in figure 9 and the following typical 0.7 characteristics:

oltage ( 0.6 V

OP-07 out = ± 15 mA t

u 0.5 tp

L300 K1 = 0.007 u 0.4

K2 = 0.007 - O

t 0.3 u K3 = 1.0 o V 0.2 V 0 ≥ + 1.0 V in 0.1 The second step is to determine servo photocurrent, IP1, 0.0 resulting from the peak input signal swing. This current is the 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

17762 product of the LED drive current, IF, times the servo transfer Vin - Input Voltage (V) gain, K1. For this example the Ioutmax is equal to the largest LED current signal swing, i.e., IF = Ioutmax. Fig. 11 - Photovoltaic Amplifier Transfer Gain IP1 =K1 · Ioutmax IP1 = 0.007 · 15 mA IP1 = 105 µA www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1008 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler

1 0 - 1 T = 25 °C - 2 A - 3 - 4 - 5

de Response (dB) - 6 u - 7

Amplit - 8 - 9 - 10 101 102 103 104 105 17763 F - Frequency (Hz)

Fig. 12 - Photovoltaic Amplifier Frequency Response

1 kΩ IL300 + 1 8 3 6 IF OP-07 2 K2 7 R2 R1 K1 10 kΩ Vin 2 10 kΩ – 3 6 - Voltage IP1 4 5 – IP1 IP2 2 6 OP-07 V 3 out 17764 +

Fig. 13 - Negative Unipolar Photovoltaic Isolation Amplifier The third step in this design is determining the value of the The negative unipolar photovoltaic isolation amplifier transfer trans resistance, R2, of the output amplifier. R2 is set by the characteristics are shown in figure 14. This amplifier, as composite voltage gain desired, and the IL300’s transfer shown in figure 13, responds to signals in only one quadrant. gain, K3. Given K3 = 1.0 and a required Vout/Vin = G = 1.0, If a positive signal is applied to the input of this amplifier, it the value of R2 can be determined. will forward bias the photodiode, causing U1 to reverse bias R2 = (R1 · G) / K3 the LED. No damage will occur, and the amplifier will be cut off under this condition. This operation is verified by the R2 = (10 kΩ · 1.0) /1.0 transfer characteristics shown in figure 14. R2 = 10 kΩ When the amplifier in figure 9 is constructed with OP-07 I I I I operational amplifiers it will have the characteristics shown in P1 F P2 0 P2 + figure 11 and figure 12.The frequency response is shown in -1 1 figure 12. This amplifier has a small signal bandwidth of K2 -R2 45 kHz. R1 K1 - The amplifier in figure 9 responds to positive polarity input -0V 0 I + 0 I + in P1 F Vout signals. This circuit can be modified to respond to negative 17765 a b c d polarity signals. The modifications of the input amplifier include reversing the Fig. 14 - Negative Unipolar Photovoltaic Isolation Amplifier Transfer Characteristics polarity of the servo photodiode at U1’s input and connecting the LED so that it sinks current from U1’s output. The non inverting isolation amplifier response is maintained by reversing the IL300’s output photodiodes connection to the input of the trans resistance amplifier. The modified circuit is shown in figure 13.

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1009 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

+ 3 6 R2 U1 1 kΩ 1 IL300a 8 Ω R1 10 k 2 10 kΩ – 2 K2a 7 K1a Vin 3 6 – 2 6 4 5 U2 IP1a IP2a Vout 3 + 1 IL300b 8

2 K2b 7 K1b

3 6

4 5 I IP1b P2b

17766 Fig. 15 - Bipolar Input Photovoltaic Isolation Amplifier

IFa IP2a Vout IP1a + + + + 1 1 R1 K1a K2a R2

0 I 0 + 0 0 + P2a V I + I + in P1a I Fa IP1b I IP2B 0 + Fb IP2B + + +

-1 1 -R2 R1 K1b K2b – – V 0 0 + 0 + in IP1b I Fb Vout 17767 a b c d

Fig. 16 - Bipolar Input Photovoltaic Isolation Amplifier Transfer Characteristics A bipolar responding photovoltaic amplifier can be Whenever the need to couple bipolar signals arises a pre constructed by combining a positive and negative unipolar biased photovoltaic isolation amplifier is a good solution. By amplifier into one circuit. This is shown in figure 15. This pre biasing the input amplifier the LED and photodetector will amplifier uses two IL300s with each detector and LED operate from a selected quiescent operating point. The connected in anti parallel. The IL300a responds to positive relationship between the servo photocurrent and the input signals while the IL300b is active for the negative signals. voltage is shown in figure 17. The operation of the IL300s and the U1 and U2 is shown in I the transfer characteristics given in figure 16. Bipolar input P1 I photovoltaic isolation amplifier transfer characteristics P1Q The operational analysis of this amplifier is similar to the 1 positive and negative unipolar isolation amplifier. This simple R1 circuit provides a very low offset drift and exceedingly good linearity. The circuit’s useful bandwidth is limited by - + crossover distortion resulting from the photodiode stored 17768 Vin charge. With a bipolar signal referenced to ground and using Fig. 17 - Transfer Characteristics Pre Biased Photovoltaic a 5 % distortion limit, the typical bandwidth is under 1 kHz. Bipolar Amplifier Using matched K3s, the composite amplifier gain for positive and negative voltage will be equal. www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1010 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler

+ 3 6 2N3906 OP-177 R2 R1 1 IL300 8 2 0.1 µF Ω 10 kΩ - 10 k VCC 100 Ω 2 K2 7 Input K1 3 6 - 0.1 µF 100 µA 2 6 4 5 OP-177 IP1 IP2 100 µA 3 Output +

R2 GAIN =K3 R1 FS = ± 1 V 100 µA 100 µA current source 17770 Fig. 18 - Pre Biased Photovoltaic Isolation Amplifier

The quiescent operation point, IP1Q, is determined by the Figure 20 shows the amplifier found in figure 18 including two dynamic range of the input signal. This establishes maximum modified Howland current sources. The first source pre LED current requirements. The output current capability of biases the servo amplifier, and the second source is the OP-07 is extended by including a buffer transistor connected to U2’s inverting input which matches the input between the output of U1 and the LED. The buffer transistor pre bias. minimizes thermal drift by reducing the OP-07 internal power dissipation if it were to drive the LED directly. I I V This is shown in figure 18. The bias is introduced into the P1 F IP2 out inverting input of the servo amplifier, U1. The bias forces the + 1 LED to provide photocurrent, I , to servo the input back to a R1 1 P1 K1 K2 I zero volt equilibrium. The bias source can be as simple as a 0 + P2 series resistor connected to VCC. Best stability and minimum - 0 + 0 + 0 + R2 - IP1 IF offset drift is achieved when a good quality current source is Vin used. 17769 a b c d Fig. 19 - Pre Biased Photovoltaic Isolation Amplifier Transfer Characteristics

+ 3 6 2N3906 OP-07 R1 IL300 R2 2 1 8 Ω – 100 pF 10 kΩ 10 k VCC 100 Ω 2 K2 7 Input K1 3 6 – 100 pF 100 µA 2 6 Output 4 5 OP-07 12 kΩ IP1 IP2 3 100 µA +

+ 100 µA current 100 µA current 3 6 source 100 µA source OP-07 Ω 2 R2 12 k – GAIN = K3 LM313 R1 V - CC 1.2 V FS = ± 1 V + 3 6 2N4340 OP-07 2 – LM313 0.01 µF V - CC 1.2 V

2N4340

17771 0.01 µF Fig. 20 - Pre Biased Photovoltaic Isolation Amplifier

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1011 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

+ 3 6 2N3906 OP-07 R2 R1 1 IL300 8 2 100 pF 10 kΩ – 10 kΩ VCC 100 Ω 2 K2 7 Input + K1 3 6 – 100µA 2 100 pF 6 OP-07 Ω 4 5 12 k IP1 IP2 100 µA 3

+ 10 k

+ 100 µA current Ω 3 6 source OP-07 2 10 kΩ 10 kΩ – LM313 VCC- 1.2 V + 3 6 2N4340 OP-07 2 – 10 k 0.01µF Output Ω – 2 OP-07 10 kΩ 6 10 k 3 + 10 k 100 µA Ω Ω IL300 Ω 12 k 1 8 + 3 VCC 100 Ω 2 K2 7 OP-07 K1 6 Input - 2 R4 3 6 – 100 pF 10 kΩ – 2 100 pF 4 5 R3 OP-07 IP1 IP2 6 10 kΩ 3 + 2N3906

17772 Fig. 21 - Differential Pre Biased Photovoltaic Isolation Amplifier The previous circuit offers a DC/AC coupled bipolar isolation UNIPOLAR ISOLATION AMPLIFIER amplifier. The output will be zero volts for an input of zero The circuit shown in figure 22 is a unipolar photoconductive volts. This circuit exhibits exceptional stability and linearity. amplifier and responds to positive input signals. The This circuit has demonstrated compatibility with 12 bit A/D gain of this amplifier follows the familiar form of converter systems. The circuit’s common mode rejection is Vout ⁄ Vin ==GK3R2R1⋅()⁄ . R1 sets the input signal range determined by CMR of the IL300. When higher common in conjunction with the servo gain and the maximum output mode rejection is desired one can consider the differential current, I , which U1 can source. Given this, I . o 0max = Fmax amplifier shown in figure 21. R1 can be determined from equation 28. This amplifier is more complex than the circuit shown in R1= V ⁄ ()K1⋅ I (28) figure 20. The complexity adds a number of advantages. inmax 0max First the CMR of this isolation amplifier is the product of the The output section of the amplifier is a voltage follower. The IL300 and that of the summing differential amplifier found in output voltage is equal to the voltage created by the output the output section. Note also that the need for an offsetting photocurrent times the photodiode load resistor, R2. This bias source at the output is no longer needed. This is due to resistor is used to set the composite gain of the amplifier as differential configuration of the two IL300 couplers. This shown in equation 29. amplifier is also compatible with instrumentation amplifier R2= () R1⋅ G ⁄ K3 (29) designs. It offers a bandwidth of 50 kHz, and an extremely This amplifier is conditionally stable for given values of R1. good CMR of 140 dB at 10 kHz. As R1 is increased beyond 10 kΩ, it may become necessary to frequency compensate U1. This is done by placing a small PHOTOCONDUCTIVE ISOLATION capacitor from U1’s output to its inverting input. This circuit AMPLIFIER uses a 741 op amp and will easily provide 100 kHz or greater The photoconductive isolation amplifier operates the bandwidth. photodiodes with a reverse bias. The operation of the input network is covered in the discussion of K3 and as such will not be repeated here. The photoconductive isolation amplifier is recommended when maximum signal bandwidth is desired. Bipolar photoconductive isolation amplifier. www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1012 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler

3 7 V IL300 + + CC 1 8 V 7 V a 6 CC U1 V 2 K2 7 – in K1 I F 2 6 Vb – V U2 VCC 3 6 CC 2 V 4 V 3 out CC + 4 5 IP2 IP1 IP1 R1 4 IP2 R2

17773 Fig. 22 - Unipolar Photoconductive Isolation Amplifier

+ Vref2 V VCC in 3 - V IL300 + 7 CC 1 8 R1 7 VCC 6 100 Ω U1 2 K2 7 + 3 R2 741 K1 V V 6 2 CC CC U2 – 4 3 6 - V 741 CC V 20 pF 2 out 4 5 - IP1 IP2 4

- VCC R3

17774 R4

-Vref1 Fig. 23 - Bipolar Photoconductive Isolation Amplifier BIPOLAR ISOLATION AMPLIFIER and uses inexpensive signal diodes as reference sources. Many applications require the isolation amplifier to respond The input signal is attenuated by 50 % by a voltage divider to bipolar signals. The generic inverting isolation amplifier formed with R1 and R2. The solution for R3 is given in shown in figure 23 will satisfy this requirement. Bipolar signal equation 32. R3 0.5 V V I K1 (32) operation is realized by pre biasing the servo loop. The pre = ()inmax + ref1 ⁄ ()F ⋅ bias signal, Vref1, is applied to the inverting input through For this design R3 equals 30 kΩ. The output trans resistance R3. U1 forces sufficient LED current to generate a voltage is selected to satisfy the gain requirement of the composite across R3 which satisfies U1’s differential input isolation amplifier. With K3 = 1, and a goal of unity transfer requirements. The output amplifier, U2, is biased as a trans gain, the value of R4 is determined by equation 33. resistance amplifier. The bias or offset, V , is provided to ref2 R4= [] R3⋅⋅ G() R1+ R2 ⁄ ()K3⋅ R2 (33) compensate for bias introduced in the servo amplifier. Much like the unipolar amplifier, selecting R3 is the first step R4 = 60 kΩ in the design. The specific resistor value is set by the input From equation 31, Vref2 is shown to be twice Vref1. Vref2 is voltage range, reference voltage, and the maximum output easily generated by using two 1N914 diodes in series. current, Io, of the op amp. This resistor value also affects the This amplifier is simple and relatively stable. When better bandwidth and stability of the servo amplifier. output voltage temperature stability is desired, consider the The input network of R1 and R2 form a voltage divider. U2 is isolation amplifier configuration shown in figure 25. This configured as a inverting amplifier. This bipolar amplifier is very similar in circuit configuration except that the photoconductive isolation amplifier has a transfer gain given bias is provided by a high quality LM313 band gap reference in equation 30. source. Vout K3⋅⋅ R4 R2 This circuit forms a unity gain non-inverting photoconductive ------= ------(30) isolation amplifier. Along with the LM113 references and low Vin R3⋅ () R1+ R2 Equation 31 shows the relationship of the Vref1 to Vref2. offset OP-07 amplifiers the circuit replaces the 741 op amps. A 2N2222 buffer transistor is used to increase the OP-07’s V = ()V ⋅ R4 ⁄ R3 (31) ref2 ref1 LED drive capability. The gain stability is set by K3, and the Another bipolar photoconductive isolation amplifier is shown output offset is set by the stability of OP-07s and the in figure 24. It is designed to accept an input signal of ± 1 V reference sources.

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1013 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

Figure 26 shows a novel circuit that minimizes much of the generated on the output side to the input servo amplifier. This offset drift introduced by using two separate reference isolation amplifier uses dual op amps to minimize parts sources. This is accomplished by using an optically coupled count. Figure 26 shows the output reference being supplied tracking reference technique. The amplifier consists of two by a voltage divider connected to VCC. The offset drift can be optically coupled signal paths. One IL300 couples the input reduced by using a band gap reference source to replace the to the output. The second IL300 couples a reference voltage voltage divider.

+ VCC 13.7 kΩ +

1N914 22 µF V CC - V V CC in 3 7 IL300 + 1 8 30 kΩ 100 Ω U1 6 7 30 kΩ 2 K2 7 + VCC 741 K1 3 V + V + VCC 6 out 2 CC U2 - 4 3 6 20 pF 741 - VCC 2 4 5 - - VCC IP1 IP2 4 30 kΩ 14.3 kΩ R3 60 kΩ

- VCC 20 pF 1N914 22 µF +

17775 Fig. 24 - Bipolar Photoconductive Isolation Amplifier

7 VCC V 3 CC V + Ω in 18 kΩ 1 kΩ 6.8 k 6 OP07 2N2222 1.5 kΩ 2 20pF - LM313 4 10 µF - VCC 1 kΩ Gain 2 kΩ 1 kΩ V Ω CC IL300 2 k R3 1 8 18 kΩ 6.8 kΩ 10 kΩ 2 7 2 K2 7 V CC - V Ω K1 CC 6 - V 10 k CC Ω V 2 k CC 3 6 OP07 Offset 0.1 µF LM313 47 µF 3 + 4 5 IP1 IP2 4 - VCC

20 kΩ

17776

Fig. 25 - High Stability Bipolar Photoconductive Isolation Amplifier

www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1014 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler

+ VCC 3 7 + 0.1 V 470 Ω IL300 90 kΩ 6 1 8 10 kΩ OP77 Gain adjust + VCC 1 V 2 2 K2 7 V - 4 7.5 kΩ 5 kΩ in 900 kΩ K1 + V - VCC 20 pF CC 3 6 10 V - VCC Ω 9 M - V 4 5 4 10 kΩ CC IP1 IP2 100 V 2 - OP77 6 + ± 0 to 100 mV 3 7 Output + VCC

- V +V CC ref2 220 pF 4 IP2 IP1 + 5 4 + V 3 ref1 + V CC - VCC 10 kΩ Ω 6 3 OP77 4.7 k 4 - V 2 6 CC K1 - 7 K2 2 OP77 7 2 6 V 470 Ω CC + VCC 5 kΩ 8 1 + adjust IL300 7 3 73.2 kΩ + VCC Zero 1 kΩ 17777 Tracking Reference Fig. 26 - Bipolar Photoconductive Isolation Amplifier with Tracking Reference One of the principal reasons to use an isolation amplifier is to output that combines optically coupled differential signals. reject electrical noise. The circuits presented thus far are of The circuit shown in figure 28 illustrates the circuit. a single ended design. The common mode rejection, CMRR, Op amps U1 and U5 form a differential input network. U4 of these circuits is set by the CMRR of the coupler and the creates a 100 µA, IS, current sink which is shared by each of bandwidth of the output amplifier. The typical common mode the servo amplifiers. This bias current is divided evenly rejection for the IL300 is shown in figure 27. between these two servo amplifiers when the input voltage is equal to zero. This division of current creates a differential signal at the output photodiodes of U2 and U6. The transfer - 60 gain, Vout/ Vin, for this amplifier is given in equation 34. TA = 25 °C - 70 Vout R4 R2 K3 U5 R3 R1 K3 U2 ------= ------⋅⋅()+ ⋅⋅()- (34) V 2R1R2⋅⋅ - 80 in The offset independent of the operational amplifiers is given - 90 in equation 35. Is⋅ [] R1⋅⋅ R3 K3() U2 – R2⋅⋅ R4 K3() U5 - 100 V = ------offset R1+ R2 - 110 (35) Equation 35 shows that the resistors, when selected to - 120 CMRR - Rejection Ratio (dB) produce equal differential gain, will minimize the offset - 130 voltage, Voffset. Figure 29 illustrates the voltage transfer 10 100 1000 10000 100000 1000000 characteristics of the prototype amplifier. The data indicates 17778 F-F the offset at the output is - 500 µV when using 1 kΩ 1 % resistors. Fig. 27 - Common mode rejection The CMRR of the isolation amplifier can be greatly enhanced by using the CMRR of the output stage to its fullest extent. This is accomplished by using a differential amplifier at the

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1015 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

7 VCC U2

3 + Ω IL300 U1 6 2.2 k 1 8 OP-07 2 2 K2 7 Ω Inverting 10 kΩ – 4 - VCC 470 K1 100 pF 2N3904 V VCC 3 6 CC

4 5 IP1 IP2 Gain

1 kΩ 1 % 2 kΩ

7 V 1.2 V CC 6.8 kΩ 7 VCC 2N3904 + 3 2 – 1 kΩ 6 U4 U3 0.01 µF 6 OP-07 OP-07 2 4 - V – 3 Output CC + Common 100 µA current sink 4-VCC LM313 1 kΩ 1 % 12 kΩ

- VCC Zero adjust Ω 7 VCC U6 2 k

3 + 6 U5 2.2 kΩ 1 IL300 8 OP-07 10 kΩ 2 2 K2 7 Noninverting Ω – 4 - VCC 470 K1 100 pF 2N3904 V VCC 3 6 CC

4 5 IP1 IP2

17779 Fig. 28 - Differential Photoconductive Isolation Amplifier

0.6 46

0.5 Ip2 = 74.216 µA - 6.472 (µA/V) x Vin Vout = - 0.4657 mV - 5.0017 x Vin ) 0.4 V T = 25 °C 0.3 A 44 0.2 TA = 25 °C oltage (

0.1 rrent (µA) V u t

u 0.0 42 t C tp u

u - 0.1 tp u

- O - 0.2 t u - O o - 0.3 40 V p2 - 0.4 I - 0.5 - 0.6 38 - 0.15 - 0.10 - 0.05 - 0.00 0.05 0.10 0.15 4.4 4.6 4.8 5.0 5.2 5.4 5.6

17780 V - Input Voltage (V) in 17810 Vin - Input Voltage (V) Fig. 29 - Differential Photoconductive Isolation Amplifier Fig. 30 - Transistor Unipolar Photoconductive Isolation Transfer Characteristics Amplifier Transfer Characteristics

DISCRETE ISOLATION AMPLIFIER A unipolar photoconductive isolation amplifier can be constructed using two discrete transistors. Figure 32 shows such a circuit. The servo node, Va, sums the current from the photodiode and the input signal source. This control loop keeps Va constant. This amplifier was designed as a feedback control element for a DC power supply. The DC and AC transfer characteristics of this amplifier are shown in figures 30 and 31. www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1016 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler

CONCLUSION 5 45 The analog design engineer now has a new circuit element Phase response reference that will make the design of isolation amplifiers easier. The to amplifier gain of - 1; 0 ° = 180 ° preceding circuits and analysis illustrate the variety of 0 0 isolation amplifiers that can be designed. As a guide, when dB highest stability of gain and offset is needed, consider the PHASE - 5 - 45 photovoltaic amplifier. Widest bandwidth is achieved with the photoconductive amplifier. Lastly, the overall performance of de Response (dB)

u the isolation amplifier is greatly influenced by the operational - 10 - 90 amplifier selected. Noise and drift are directly dependent on Ø - Phase Responce (°C) Amplit the servo amplifier. The IL300 also can be used in the digital

- 15 - 135 environment. The pulse response of the IL300 is constant 102 103 104 105 106 over time and temperature. In digital designs where LED

17781 degradation and pulse distortion can cause system failure, F - Frequency (Hz) the IL300 will eliminate this failure mode. Fig. 31 - Transistor Unipolar Photoconductive Isolation Amplifier Frequency and Phase Response

6.2 kΩ 5 VVCC

IL300 1 8 MPSA10

2 K2 7 K1 Va 3 6 5 V V MPSA10 CC 100 kΩ 4 5 IP1 IP2 Vin Vout + 5 V Ω 1.1 kΩ Ω 15 k 200 Ω 10 k

GND1 GND2 17782

Fig. 32 - Unipolar Photoconductive Isolation Amplifier with Discrete Transistors SUPPLEMENTAL INFORMATION shown in figure 33. The graph shows that the photodiode will generate photocurrent in either forward biased PHOTODETECTOR OPERATION TUTORIAL (photovoltaic), or reversed biased (photoconductive) mode. In the forward biased mode the device functions as a PHOTODIODE OPERATION AND photovoltaic, voltage generator. If the device is connected to CHARACTERISTICS a small resistance, corresponding to the vertical load line, the The photodiodes in the IL300 are PIN (P-material - Intrinsic current output is linear with increases in incident flux. As RL material - N-material) diodes. These photodiodes convert the increases, operation becomes nonlinear until the open circuit LED’s incident optical flux into a photocurrent. The (load line horizontal) condition is obtained. At this point the magnitude of the photocurrent is linearly proportional to the open circuit voltage is proportional to the logarithm of the incident flux. The photocurrent is the product of the diode’s incident flux. 2 responsivity, Sl, (A/ W), the incident flux, Ee (W/mm ), and In the reverse-biased (photoconductive) mode, the 2 the detector area AD (mm ). This relationship is shown photodiode generates a current that is linearly proportional to below: the incident flux. Figure 33 illustrates this point with the equally spaced current lines resulting from linear increase of IP = SI ⋅⋅Ee AD (1a) Ee. PHOTODIODE I/V CHARACTERISTICS The photocurrent is converted to a voltage by the load Reviewing the photodiode’s current/voltage characteristics resistor RL. Figure 33 also shows that when the incident flux aids in understanding the operation of the photodiode, when is zero (E = 0), a small leakage current, or dark current (ID) connected to an external load. The I-V characteristics are will flow.

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1017 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

Photoconductive 10-4 Vd/RL load line Ee-5 10-5 RL (small) Ee-4 Photovoltaic 10-6

load line rrent (A)

Ee-3 u 10-7

Ee-2 ard C w 10-8 Ee-2 - For F RL (large) I 10-9 Ee-1 Id 10-10 Forward bias Reverse bias 0.0 0.1 0.2 0.3 0.4 0.5 0.6 17783 17785 Vf - Forward Voltage (V) Fig. 33 - Photodiode I/V Characteristics Fig. 35 - Photodiode Forward Voltage vs. Forward Current Inserting the diode equation 2a into the two nodal equations PHOTOVOLTAIC OPERATION gives the following DC solution for the photovoltaic operation Photodiodes, operated in the photovoltaic mode, generate a (equation 3a): load voltage determined by the load resistor, RL, and the 0I= P – IS ⋅ {}EXP[] VO()R + R ⁄ KR⋅ – 1 (3a) photocurrent, I . The equivalent circuit for the photovoltaic S L L P – V []()⁄ operation is shown in figure 34. The photodiode includes a O RS ++RL RP ()RP ⋅ RL current source (IP), a shunt diode (D), a shunt resistor (RP), Typical IL300 values: a series resistor (RS), and a parallel capacitor (CP). The -12 IS = 13.94 ·10 intrinsic region of the PIN diode offers a high shunt RS = 50 Ω resistance resulting in a low dark current, and reverse RP = 15 GΩ leakage current. K = 0.0288

By inspection, as RL approaches zero ohms the diode voltage, VF, also drops. This indicates a small diode current. I Anode + P R All of the photocurrent will flow through the diode series IF S RP I resistor and the external load resistor. Equation 3a was L R solved with a computer program designed to deal with CP L VO D nonlinear transcendental equations. Figure 36 illustrates the Cathode - solution.

17784 0.50 Fig. 34 - Equivalent Circuit - Photovoltaic Mode 100 The output voltage, Vo, can be determined through nodal 0.40 300 ) analysis. The circuit contains two nodes. The first node, VF, V 500 700 includes the photocurrent generator, IP, the shunt diode, D, 0.30 1 K shunt resistor (RP), and parallel capacitance, CP. The oltage ( V 3 K t second node, VO, includes: the series resistor, RS, and the u 0.20 5 K tp load resistor, RL. The diode, D, in the VF node is responsible u 7 K 10 K for the circuit’s nonlinearity. The diode’s current voltage - O

o 0.10 relationship is given in equation 2a. V 20 K 30 K 50 K IF = IS ⋅ []EXP() VF ⁄ K – 1 (2a) 0.00 This graphical solution of 2a for the IL300 is shown in 050 100 150 200 figure 35. 17786 Ip - Photocurrent (µA) Fig. 36 - Photovoltaic Output vs. Load Resistance and Photocurrent This curve shows a series of load lines, and the output voltage, Vo, caused by the photocurrent. Optimum linearity is obtained when the load is zero ohms. Reasonable linearity is obtained with load resistors up to 1000 Ω. For load resistances greater than 1000 Ω, the output voltage will respond logarithmically to the photocurrent. This response is due to the nonlinear characteristics of the intrinsic diode, D. Photovoltaic operation with a zero ohm load resistor offers www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1018 Rev. 1.4, 27-Jun-08 Application Note 50 Designing Linear Amplifiers Using the Vishay Semiconductors IL300 Optocoupler the best linearity and the lowest dark current, ID. This responsivity percentage change versus bias voltage. This operating mode also results in the lowest circuit noise. A zero graph is normalized to the performance at a reverse bias of load resistance can be created by connecting the photodiode 15 V. The responsivity is reduced by 4 % when the bias is between the inverting and non-inverting input of a trans reduced to 5 V. resistance operational amplifier, as shown in figure 37.

102 V = RI out p Ta 1 IL300 8 70 °C R 101 2 K2 7 K1 - 50 °C Ip

3 6 rrent (nA)

U u 0 IF 11

4 5 + Vout 25 °C IP1 IP2

- Dark C -1 d 10 I 17787 Fig. 37 - Photovoltaic Amplifier Configuration 10-2 0 5 10 15 20 25 30 35 PHOTOCONDUCTIVE OPERATION MODE 17789 V - Reverse Bias (V) Isolation amplifier circuit architectures often load the r photodiode with resistance greater than 0 Ω. With non-zero Fig. 39 - Dark Current vs. Reverse Bias loads, the best linearity is obtained by using the photodiode in the photoconductive or reverse bias mode. Figure 38 shows the photodiode operating in the photoconductive 2 mode. The output voltage, Vo, is the product of the photocurrent times the load resistor. 1

The reverse bias voltage causes a small leakage or dark ) 0 % current, ID, to flow through the diode. The output - 1 photocurrent and the dark current, sum the load resistor. - 2 This is shown in equation 4a. - 3 - 4 VL = RL ⋅ ()IP + ID (4a). - 5

Percent Difference ( - 6 I Cathode - 7 P ID I F RP C D P - 8 + 0 5 10 15 20

17790 Vr - Reverse Voltage (V) VD R S Anode VO Fig. 40 - Photoconductive Responsivity vs. Bias Voltage The photodiode operated in the photoconductive mode is 17788 IL RL easily connected to an operational amplifier. Figure 41 shows the diode connected to a trans resistance amplifier. The transfer function of this circuit is given in equation 5a.

Vout = RI⋅ ()P ⋅ Id (5a) Fig. 38 - Photoconductive Photodiode Model The dark current depends on the diode construction, reverse bias voltage and junction temperature. The dark current can double every 10 °C. The IL300 uses matched PIN photodiodes that offer extremely small dark currents, typically a few picoamps. The dark current will usually track one another, and their effect will cancel each other when a servo amplifier architecture is used. The typical dark current as a function of temperature and reverse voltage is shown in figure 39. The responsivity, S, of the photodiode is influenced by the potential of the reverse bias voltage. Figure 40 shows the

Document Number: 83708 For technical questions, please contact: [email protected] www.vishay.com Rev. 1.4, 27-Jun-08 1019 Application Note 50 Vishay Semiconductors Designing Linear Amplifiers Using the IL300 Optocoupler

BANDWIDTH CONSIDERATIONS The zero biased photovoltaic amplifier offers a 50 kHz to PIN photodiodes can respond very quickly to changes in 60 kHz usable bandwidth. When the detector is reverse incident flux. The IL300 detectors respond in tens of biased to - 15 V, the typical isolation amplifier response nanoseconds. The slew rate of the output current is related increases to 100 kHz to 150 kHz. The phase and frequency response for the IL300 is presented in figure 43. When to the diodes junction capacitance, Cj, and the load resistor, R. The product of these two elements set the maximum system bandwidth is desired, the reverse biased photo-response time constant. photoconductive amplifier configuration should be considered. τ = RCj⋅ (6a) This time constant can be minimized by reducing the load resistor, R, or the photodiode capacitance. This capacitance 5 45 IFq = 10 mA, MOD = 4 mA is reduced by depleting the photodiode’s intrinsic region, I, by Ω TA = 25 °C, RI = 50 applying a reverse bias. Figure 42 illustrates the effect of 0 0 photodiode reverse bias on junction capacitance. - 5 - 45 dB 1 IL300 8 PHASE - 10 - 90 de Response (dB)

7 u 2 K2 7 3 + I K1 Vout

- 15 - 135 Ø - Phase Response (°C) F Vcc 6 Amplit 3 6 U2 2 - 20 - 180 4 5 - 103 104 105 106 107 IP1 IP2 4

R 17793 IP2 F - Frequency (Hz)

17791 Fig. 43 - Phase and Frequency Response Fig. 41 - Photoconductive Amplifier

20

15

10 nction Capacitance (pF) u 5 - J J C

0 0 5 10 15 20 25 30

17792 Vr - Reverse Bias (V)

Fig. 42 - Photodiode Junction Capacitance vs. Reverse Voltage

www.vishay.com For technical questions, please contact: [email protected] Document Number: 83708 1020 Rev. 1.4, 27-Jun-08