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Photodiode/Phototransistor Application Circuit

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Photodiode/Phototransistor Application Circuit Application Note Optoelectronics Photodiode/Phototransistor Application Circuit FUNDAMENTAL PHOTODIODE CIRCUITS the VOUT is given as VOUT = IP × RL. An output voltage Figures 1 and 2 show the fundamental photodiode proportional to the amount of incident light is obtained. circuits. The proportional region is expanded by the amount of VCC {proportional region: VOUT < (VOC + VCC)}. On the The circuit shown in Figure 1 transforms a photo- other hand, application of reverse bias to the photo- current produced by a photodiode without bias into a diode causes the dark current (I ) to increase, leaving voltage. The output voltage (V ) is given as d OUT a voltage of Id × R when the light is interrupted, and V =1 ×R . It is more or less proportional to the L OUT P L this point should be noted in designing the circuit. amount of incident light when VOUT < VOC. It can also be compressed logarithmically relative to the amount of Figure 2 (B) shows the operating point for a load incident light when VOUT is near VOC. (VOC is the open- resistor RL with reverse bias applied to the photodiode. terminal voltage of a photodiode). Features of a circuit used with a reverse-biased pho- Figure 1 (B) shows the operating point for a load resis- todiode are: tor (R ) without application of bias to the photodiode. L • High-speed response Figure 2 shows a circuit in which the photodiode is • Wide-proportional-range of output reverse-biased by VCC and a photocurrent (IP) is trans- formed into an output voltage. Also in this arrangement, Therefore, this circuit is generally used. EV1 < EV2 < EV3 I P I V EV1 E R V L VOUT EV2 EV3 RL VOUT (A) (B) OP1-16 Figure 1. Fundamental Circuit of Photodiode (Without Bias) VCC EV1 < EV2 < EV3 I P VCC I EV V EV1 EV2 RL VOUT EV3 VOUT RL (A) (B) OP1-17 Figure 2. Fundamental Circuit of Photodiode (With Bias) Application Note 1 Optoelectronics Photodiode/Phototransistor . The response time is inversely proportional to the In the circuit of Figure 4 (A), the output (VOUT) is reverse bias voltage and is expressed as follows: given as: r = Cj × RL VOUT = IP × R1 + IB × R1 + VBE ()1 This arrangement provides a large output and rela- Cj = AVD – VR – --- n tively fast response. Cj: junction capacitance of the photodiode The circuit of Figure 4 (B) has an additional transis- RL: load resistor tor (Tr2) to provide a larger output current. VD: diffusion potential (0.5 V - 0.9 V) V : Reverse bias voltage (negative value) R VCC VCC n: 2 - 3 V I R R BE PHOTOCURRENT AMPLIFIER CIRCUIT P L BE USING THE TRANSISTOR OF V PHOTODIODE OUT Tr1 Figures 3 and 4 show photocurrent amplifiers using Tr V transistors. 1 OUT VBE I The circuit shown in Figure 3 are most basic combina- P tions of a photodiode and an amplifying transistor. In the arrangement of Figure 3 (A), the photocurrent produced RBE RL by the photodiode causes the transistor (Tr1) to decrease its output (VOUT) from high to low. In the arrangement of Figure 3 (B), the photocurrent causes the VOUT to increase from low to high. Resistor RBE in the circuit is effective for suppressing the influence of dard current (Id) and is chosen to meet the following conditions: (A) (B) OP1-18 RBE < VBD/Id Figure 3. Photocurrent Amplifier Circuit RBE > VBE/{IP - VCC/(RL × hFE)} using Transistor Figure 4 shows simple amplifiers utilizing negative feedback. VCC VCC R 2 R3 Tr2 VOUT R1 VOUT IB Tr1 R1 VBE Tr1 R2 IP (A) (B) OP1-19 Figure 4. Photocurrent Amplifier Circuit with Negative Feedback 2 Application Note Photodiode/Phototransistor . Optoelectronics AMPLIFIER CIRCUIT USING a saturation of output because of the limited linear OPERATIONAL AMPLIFIER region of the operational amplifier, whereas logarithmic Figure 6 shows a photocurrent-voltage conversion compression of the photocurrent prevents the satura- circuit using an operational amplifier. The output volt- tion of output. With its wide measurement range, the ≅ logarithmic photocurrent amplifier is used for the expo- age (VOUT) is given as VOUT = IF × R1 (IP ISC). The arrangement utilizes the characteristics of an opera- sure meter of cameras. tional amplifier with two input terminals at about zero voltage to operate the photodiode without bias. The cir- LOG-DIODE (IS002) cuit provides an ideal short-circuit current (ISC) in a wide operating range. Figure 6 (B) shows the output voltage vs. radiant intensity characteristics. An arrangement with no bias +VCC and high impedance loading to the photodiode pro- vides the following features: OP V • Less influence by dark current AMP OUT + • Wide linear range of the photocurrent relative to the radiant intensity. VCC Figure 5 shows a logarithmic photocurrent amplifier OP1-21 using an operating amplifier. The circuit uses a logarith- mic diode for the logarithmic conversion of photocur- Figure 5. Logarithmic Photocurrent Amplifier rent into an output voltage. In dealing with a very wide using an Operational Amplifier irradiation intensity range, linear amplification results in R1 I ≅ P V (IP ISC) OUT . +VCC IP R1 OP V AMP OUT + EV VCC (A) (B) OP1-20 Figure 6. Photocurrent Amplifier using an Operational Amplifier (Without Bias) Application Note 3 Optoelectronics Photodiode/Phototransistor . LIGHT DETECTING CIRCUIT FOR Where A is the gain of the differential amplifier. The MODULATED LIGHT INPUT gain becomes A = R2/R1 when R1 = R3 and R2 = R4, then: Figure 7 shows a light detecting circuit which uses I R kT × SC2 × 2 an optical remote control to operate a television set, air VOUT = ------- log ----------- ------- q ISC1 R1 conditioner, or other devices. Usually, the optical remote control is used in the sunlight or the illumination The output signal of the semiconductor color sensor of a fluorescent lamp. To alleviate the influence of such is extremely low level. Therefore, great care must be a disturbing light, the circuit deals with pulse-modula- taken in dealing with the signal. For example, low- tion signals. biased, low-drift operational amplifiers must be used, and possible current leaks of the surface of P.W.B. The circuit shown in Figure 7 detects the light input must be taken into account. by differentiating the rising and falling edges of a pulse signal. To amplify a very small input signal, an FET pro- viding a high input impedance is used. VCC R PIN 5 COLOR SENSOR AMPLIFIER CIRCUIT PHOTODIODE R4 C2 Figure 8 shows a color sensor amplifier using a C V semiconductor color sensor. Two short circuit currents 1 OUT Tr1 (ISC1, ISC2) conducted by two photodiodes having dif- ferent spectral sensitivities are compressed logarithmi- + C3 cally and applied to a subtraction circuit which produces a differential output (VOUT). The output volt- R1 R2 age (VOUT) is formulated as follows: + R3 C I 4 kT × SC2 × VOUT = ------- log ----------- A q ISC1 OP1-22 Figure 7. Light Detecting Circuit for Modulated Light Input PIN Photodiode D1 (LOG-DIODE) C1 +VCC R1 R2 OP AMP + -V +VCC CC D2 (LOG-DIODE) C2 I OP V SC1 AMP OUT + ISC2 -VCC +VCC R3 OP AMP + R4 -VCC OP1-23 Figure 8. Color Sensor Amplifier Circuit 4 Application Note Photodiode/Phototransistor . Optoelectronics FUNDAMENTAL PHOTOTRANSISTOR CIRCUITS VCC VCC Figures 9 and 10 show the fundamental phototrans- istor circuits. The circuit shown in Figure 9 (A) is a com- mon-emitter amplifier. Light input at the base causes RL RL the output (VOUT) to decrease from high to low. The cir- cuit shown in Figure 9 (B) is a common-collector ampli- VOUT VOUT fier with an output (VOUT) increasing from low to high in response to light input. For the circuits in Figure 9 to operate in the switching mode, the load resistor (RL) Tr1 should be set in relation with the collector current (IC) as VCC < RL × IC. The circuit shown Figure 10 (A) uses a phototrans- istor with a base terminal. A RBE resistor connected RBE between the base and emitter alleviates the influence of a dark current when operating at a high temperature. The circuit shown in Figure 10 (B) features a cascade connection of the grounded-base transistor (Tr1) so (A) (B) that the phototransistor is virtually less loaded, thereby OP1-25 improving the response. Figure 10. Fundamental Phototransistor Circuit (II) AMPLIFIER CIRCUIT USING TRANSISTOR Figure 11 shows the transistor amplifiers used to V V amplify the collector current of the phototransistor CC CC using a transistor (Tr1). The circuit in Figure 11 (A) R2 R2 increases the output from high to low in response to a I light input. The value of resistor R depends on the C 1 R input light intensity, ambient temperature, response 1 V speed, etc., to meet the following conditions: VOUT OUT R1 < VBE/ICEO,R1 > VBE/IC Tr Tr Where ICBO is the dark current of phototransistor 1 1 VBE V and IC is the collector current. BE ≅ VCC VCC IE IC R1 RL (A) (B) OP1-26 VOUT Figure 11. Amplifier Circuit using Transistor VOUT IC RL (A) (B) OP1-24 Figure 9. Fundamental Phototransistor Circuit (I) Application Note 5 Optoelectronics Photodiode/Phototransistor . MODULATED SIGNAL DETECTION CIRCUIT When the switch is in the ON-state, the SCR2 and Figure 12 shows the circuits used to detect a modu- SCR3 turn on to discharge capacitor C4 so that the lated signal such as an AC or pulse signal. The pho- xenon lamp is energized to emit light. The anode of the totransistor has a base terminal with a fixed bias through SCR2 is then reverse-biased, causing it to turn off and light emission of the xenon lamp ceases.
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