APPLICATION BULLETIN Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S

¨APPLICATION BULLETIN Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132

NEW ULTRA HIGH-SPEED CIRCUIT TECHNIQUES WITH ANALOG ICs

By Christian Henn, Burr-Brown International GmbH

With the increasing use of current-feedback amplifiers, the TECHNICAL PROCESS REQUIREMENTS Diamond Structure has come to play a key role in today’s FOR COMPLEMENTARY CIRCUIT TECHNIQUES analog circuit technology. Two new macro elements that Circuits implemented in push-pull arrangements, in which function in this structure are the Diamond Transistor and its both NPN and PNP transistors are located in the signal path, abridged version, the Diamond Buffer. These elements can be demand a particular high level of symmetry in the electrical used for both voltage and current control of analog signals up parameters of complementary transistors. See Figure 2. to several 100MHz. The OPA660 combines both of these elements in one package. Starting with a discussion of the The most important requirement is that the bandwidths be equal, since the slower transistor type determines the perfor- technical process requirements for complementary-bipolar circuit technology, we would like to focus on the basic and mance capability of the entire circuit. The bandwidth of an integrated bipolar transistor is dependent both upon the base functional circuits of the Diamond Transistor and Buffer. These circuits can be used in areas ranging from video signal transit time and upon various internal transistor resistances and p-n junction capacitances. processing and pulse processing in measurement technology to interface modules in fiber optic technology. Another important point is the DC performance, which can

be described best by the parameters saturation current IS, SYMBOLS AND TERMS current gain BF and early voltage. The Diamond Transistor and buffer are manufactured using a complicated process In technical literature, various symbols and terms are used with vertically structured NPN and PNP transistors. Table I to describe the same circuit structure, see Figure 1. Burr- shows the most important parameters of a transistor of size Brown has chosen the transistor symbol with opposed 111. Two metallization layers with a gold surface simplify emitter arrows. The symbol calls attention to the functional the connection between the circuit parts. similarity of the bipolar and Diamond Transistors, and the double arrows refer to the Diamond Transistor’s complementary construction and the ability to operate it in four APPLICATION EXAMPLES quadrants. Regardless of how it is depicted, this type of • Wide-bandwidth amplifiers structure has a high-impedance input, a low-impedance • Video signal processing input/output, high transconductance and a high-impedance • Pulse processing in radar technology current source output. The voltage is transferred with very • Ultrasonic technology low offset of +7mV from the high-impedance input to the • Optical electronics low-impedance input/output. • Test and communications equipment

VIN1 1B PNP Transistor NPN Transistor gm IOUT VIN2 BEB C BEBC

2E p - Epi p n - Epi n

Diamond Transistor Voltage-Controlled Current Source p - Substrate Transconductor Macro Transistor 3 + + Current Conveyor II+ n Doping p Doping Silicondioxide

1 nÐ Doping pÐ Doping Poly-Silicon (implanted) X Z CCII+ I Y OUT 2

FIGURE 1. Symbols and Terms. FIGURE 2. Complementary Bipolar Technique (CBip).

SBOA047 PARAMETER NPN PNP DIM The Operation Transconductance Amplifier section, or OTA, Current Gain 220 115 will be referred to as the Diamond Transistor in the follow- Early voltage 66 30 V ing. Its three pins are named base, emitter, and collector, like C 0.26 0.50 pF JS the pins of a bipolar transistor. This similarity in terms CJE 0.02 0.02 pF Ω points to the basic similarity in function of the two transis- RB 540 429 Ω RC 46 43 tors. Ideally, the voltage at the high-impedance base is Transit Frequency 3.5 2.7 GHz transferred to the emitter input/output with minimal offset • symmetric NPN/PNP pairs • n-epitaxy, p-collector implantation voltage and is available there in low-impedance form. If a • complex process sequence • complementary B/E structures • n-cube • isolation variations current flows at the emitter, then the current mirror reflects • p+ and n+ buried layer this current to the collector by a fixed ratio. The collector is thus a complementary current source, whose current flow is TABLE I. Process Parameter. determined by the product of the base-emitter voltage and the transconductance. Because of the PTAT (Proportional to SIMPLIFIED CIRCUIT DIAGRAM Absolute Temperature) power supply, the transconductance OF THE DIAMOND TRANSISTOR is independent of temperature and can be adjusted by an external resistor. The OPA660, Figure 3, is a new type of IC which can be used universally. It consists of a voltage-controlled current Besides these features, the Diamond Transistor and Buffer source (Diamond Transistor), a complementary offset-com- can process frequencies of up to several 100MHz with very pensated emitter follower (buffer amplifier, Diamond Buffer), low errors in the differential phase and gain. Thus, they are and a power supply. This new IC enables users to adjust the universal basic elements for the development of complex quiescent current, and through its temperature characteris- circuits designed to process fast analog signals. Current tics, it maintains a constant transconductance in the Dia- control, voltage control, and operation with or without mond Transistor and Buffer. The emitter follower functions feedback are all possible with the Diamond Transistor and without feedback. For this reason, its gain is somewhat less Buffer. See Table II. than one and is slightly dependent upon the load resistance. The main task of the emitter follower is to decouple signal PARAMETER UNIT processing stages. DT Transconductance 125mA/V It is distinguished by its extremely short delay time of 250ps Offset Voltage +7mV Offset Drift 50µV/°C and an excellent large-signal bandwidth/quiescent current Input Bias Current 2.1µA ratio. When comparing the Diamond Buffer with the Dia- Output Bias Current ±10µA mond Transistor, it becomes apparent that all aspects of the Input Resistance 1MΩ || 2.1pF Output Resistance 25kΩ || 4.2pF components are identical except for the current mirror. The Differential Gain 0.06% Diamond Buffer can thus be called an abridged version of Differential Phase 0.02% the Diamond Transistor. Quiescent Current 1-20mA TABLE II. Diamond Transistor Parameter.

(7) +5V

VIN VOUT B EC (5) (6) (3) (2) (8)

100Ω 50kΩ

RQ (1) Ð5V (4)

FIGURE 3. OPA660 Schematic.

2 PTAT POWER SUPPLY EQUIVALENT CIRCUIT DIAGRAM PTAT biasing-controlled current source with adjustable qui- The user can construct a simple equivalent circuit diagram, escent current, Figure 4. Figure 5, for the Diamond Transistor based on the previous Each individual transistor stage has a current source as the descriptions. The complementary emitter follower with an Ω load impedance. Thus, control of the current source allows input impedance of 1M || 2.1pF at the base pin can be adjustment of the quiescent current. The adjusted quiescent regarded as a controlled voltage source. Using the fact that currents and the transistors used determine the an emitter follower is, in principle, a voltage-controlled transconductance of the entire circuit. An external resistor, current source whose current flow is dependent upon the R , fixes the quiescent current. We will discuss the exact voltage difference between base and emitter, it is possible to Q determine the output resistance of the emitter. The output equations for the ratio of the quiescent currents RQ and transconductance to I in detail later in this paper. resistance can best be approximated as the reciprocal value Q of the transconductance. A quiescent current set at 20mA results in a transconductance of 125mA/V and a low-imped- Ω +VCC ance output resistance of 8 , which is adjustable but stable with temperature. The collector pin performs like a complementary current source, with output impedance 1 1 1 25kΩ || 4.2pF. The possible positive or negative current flow

V results from the product of the input voltage difference times I’ I’ = t In (10) Q Q transconductance times current mirror factor, which is fixed RQ at AI = 1 in the OPA660. The model presented here shows I’Q I’Q 2 In 10 the similarity to the small-signal behavior of the bipolar gm = RQ transistor. I’Q

10 1 1 OPA660 BASIC CONNECTIONS AND PINOUT CONFIGURATION

RQ For trouble-free operation of the OPA660, several basic ÐV CC components on the power supply, as well as those components which define function, are necessary. See Figure 6. FIGURE 4. PTAT Power Supply. The triple configuration of the supply decoupling capacitors at pins 4 and 7 guarantees a low-impedance supply up to 1GHz and supplies the IC during large-signal high-fre- ± ±VCC IC = AI ¥ gm ¥ VBE quency operation. The voltage supply is 5V, resulting in a C maximum rated output of ±4V. As already mentioned, the RC I = gm ¥ V CC external resistor R between pin 1 and –5V adjusts the IN BE 25kΩ Q B 4.2pF quiescent current. A resistance value of 250Ω results in a 2.1pF Ω 1 quiescent current of 20mA. Process variations can cause this VBE 1M gm ±VCC current to vary between 16mA and 26mA. The product data sheet illustrates the exact relation between R and I . As in 1 ~ 8Ω at I = 20mA Q Q gm Q E discrete HF (high-frequency) transistors, a low-impedance resistor damps oscillation that might arise at the inputs. The FIGURE 5. Equivalent Circuit. circuit consists of the pin capacitances and inductances of the bond wires. The resonant frequency is between 750MHz

Ω RQ = 250 1 8 C

E 2 7 +VCC

Ω RG = 80 to 250 3 +1 6 Out B 470pF 10nF 2.2µF

ÐVCC 4 5 In Ω RG = 80 to 250

2.2µF 10nF 470pF

FIGURE 6. OPA660 Basic Connection

3 and 950MHz, depending upon the package type and layout, BASIC CIRCUITS WITH THE OPA660 and is outside the operating range. The damping resistance Listed below are the basic circuits possible with the Dia- Ω Ω is between 50 and 500 , depending upon the application. mond Transistor: • Emitter Circuit TEST CONFIGURATION • Base Circuit FOR DETERMINING THE DYNAMIC FEATURES • Common Emitter OF THE DIAMOND TRANSISTOR • Common Emitter with Doubled Output Current Figure 7 shows the test configuration to determine the • Current-Feedback Amplifier dynamic features of the Diamond Transistor. The entire test • Direct-Feedback Amplifier Ω system functions as a 50 transmission system to avoid As already mentioned, the signal transmission of the Dia- reflections from the input resistances. Various signal genera- mond Transistor is the inverse of that of the bipolar transistors and indicators can be used depending upon the measure- tor. The emitter circuit functions in non-inverting mode and ment task. The layout of the demo boards used here is the base circuit in inverting mode. designed for minimum line length and stray capacitance and uses the three-level combination of supply decoupling ca- The emitter-collector connection enables the user to increase pacitors and 50Ω HF-connectors. Burr-Brown offers these the output current of the emitter follower. Since both cur- demo boards to support design engineers during the test rents flow in the same direction and the current loop factor phase. AI of the OPA660 equals 1, the output current doubles to ±30mA and the output resistance halves. See Figure 9. FUNCTION DIAGRAMS In many applications, the high-impedance collector output is a disadvantage. One possible solution to this problem is to The diagrams introduce two important characteristics that insert the complementary emitter follower between the col- help engineers to understand how the Diamond Transistor lector and the output. The emitter follower then ensures that functions as a voltage-controlled current source with adjust- the load resistance of the collector pin is high and that the able quiescent current. output of the circuit can drive low-impedance loads. See

Figure 8a shows the transfer curve IO/VBE with the quiescent Figure 10. current as a parameter. The transconductance increases with The inverting base circuit has a low-impedance input. This increasing quiescent current. current input has clear advantages in amplifying outputs of Figure 8b illustrates the transconductance dependency upon sensors which deliver currents instead of voltages. the input voltage. It is clear from this diagram that the transconductance of the Diamond Transistor remains more Common Collector Emitter Follower stable over the whole input voltage range than that of a with doubled bipolar transistor. Current Output

R2 VIN VIN

50Ω R1 VOUT VOUT 50Ω RE RE

Ω 50 R4 R3 Generator Network Analyzer FIGURE 9. Emitter-Collection Connection.

FIGURE 7. Test Circuit OTA. Common Base Common Emitter

(B) TRANSCONDUCTANCE R R (A) DT CHARACTERISTIC vs INPUT VOLTAGE C C 10 200 VOUT VOUT

VIN

0 100 I gm O R R mA mA E E V Ð10 0 V Ð40 0mV 40 Ð40 0mV 40 IN V V BE BE FIGURE 10. Emitter Follower. FIGURE 8. OTA Transfer Characteristics.

4 VOUT f–3dB Figures 11 through 13 show the frequency response attained ±100mV 351MHz with a gain of 3.85 and the pulse response achieved with an ±300mV 374MHz input pulse rise time of 1.3ns of the open-loop amplifier ±700mV 435MHz illustrated in the diagram below. We call this open-loop ±1.4V 460MHz ±2.5V 443MHz amplifier a “straight forward amplifier.” TABLE III. –3dB Bandwidth vs Output Voltage. With a quiescent current of 20mA and the applied component values, the –3dB bandwidth of the open-loop amplifier is between 350MHz at ±100mV and 460MHz at ±1.4V, Ω 180 depending upon the output voltage. See Table III. VOUT The rise/fall time at the output is 1.4ns, and the maximum 56Ω 8 overshoot is under 10% and settles to less than 1% after 5ns. 100Ω 3 VIN The settling time at 0.1%/10-bit is 25ns.

2 CURRENT-FEEDBACK AMPLIFIER Ω Ω 75 51 Advantages:

5.6pF • Fewer transistor stages (signal delay time). • Shorter signal delay time = larger bandwidth. FIGURE 11. Straight Forward Amplifier. • Small-signal bandwidth independent of gain compensation of the frequency response possible with feedback resistance instead of capacitance. OPA660 OTA • Complementary-symmetric circuit technique improves 20 large-signal performance. 2.5Vpo 15 Disadvantages: 10 1.4Vpo 5 0.7Vpo • Low-impedance inverting input. • Asymmetric differential inputs. 0 0.3Vpo • Low common-mode rejection ratio.

dB –5 • Relatively high input offset voltage. –10 0.1Vpo –15 –20 ADVANTAGES A CF-AMPLIFIER –25 HAS WITH THE OPA660 Ð30 The –3dB bandwidth stays constant over the entire modula- 100k 1M 10M 100M 1G tion range up to ±2.5V and gains up to 12. Quiescent current Frequency (Hz) control guarantees an excellent bandwidth /quiescent current , , IQ = 20mA, R1 = 100 R4 = 51 R2 = 180, ratio. See Figure 16. , R3 = 56, R4p = 75 C4p = 5.6pF RQ varies the quiescent current to produce the necessary FIGURE 12. Straight-Forward Amplifier Frequency bandwidth. Feedback resistances can optimize frequency Response. response over a broad range. This configuration also pro- vides excellent pulse behavior, even up to large pulse ampli- tudes. Burr-Brown offers the Current-Feedback Amplifier OTA PULSE RESPONSE completely assembled as a small demo board under the part number DEM-OPA660-2GC.

C1 R3 R2 47Ω (V)

O 5 6 V 0V DB VOUT 8 R FPO 1 V 3 IN R4 2

Output Voltage = 5Vp-p R5

FIGURE 13. Pulse Behavior of a Straight-Forward Amplifier with Compensation. FIGURE 14. Current-Feedback Amplifier.

5 DIRECT-FEEDBACK AMPLIFIER OPA660 CURRENT FEEDBACK Another interesting basic circuit with the OPA660 is the so- 20 2.5Vpo called Direct-Feedback Amplifier, Figure 15. The idea of 15 using voltage feedback from the collector to the emitter for 10 1.4Vpo Current-Conveyor structures was suggested for the first time 5 0.7Vpo 0 a few years ago, and even in test configurations with simple 0.3Vpo

Current-Conveyor structures, this design demonstrated ex- dB –5 cellent RF features. We named this structure the Direct- –10 0.1Vpo Feedback Amplifier, due to its short feedback loop across –15 the complementary current mirror. As shown in detail with –20 the Current-Feedback Amplifier, the open-loop gain of the Ð25 Direct-Feedback Amplifier varies according to the closed- Ð30 loop gain. This relation causes the product of the open-loop 100k 1M 10M 100M 1G Frequency (Hz) gain, VO, and feedback factor, kO, to stay constant, while the bandwidth also remains independent of the adjusted total Ω, Ω, Ω, Ω, IQ = 20mA, R1 = 47 R2 = 56 R4 = 200 R5 = 22 Gain = 10 gain. The currents at the emitter and collector always flow in the same direction. The current from the collector across R3 FIGURE 16. Current-Feedback Amplifier Frequency causes an additional voltage drop in XE and counteracts the Response. base-emitter voltage. The reduced voltage difference, however, causes reduced current flow at the emitter and across OPA660 DIRECT FEEDBACK the current mirror at the collector. It functions like double 20 2.5Vpo feedback and is adjusted by the ratio between R3 and XE. The 15 Diamond Buffer decouples the high-impedance output. 10 1.4Vpo Burr-Brown offers the Direct-Feedback Amplifier completely 5 1.7Vpo 0 assembled as a small demo board under the part number 0.3Vpo DEM-OPA660-3GC. –5 dB –10 Figures 17 and 18 show the excellent test results with the 0.1Vpo Direct-Feedback Amplifier. –15 –20 Using a quiescent current of 20mA and the given component values and compensation at the emitter, it is possible to –25 ± ± –30 attain 330MHz at 100mV and max 550MHz at 1.4V 100k 1M 10M 100M 1G bandwidth. The frequency response curve is extremely flat Frequency (Hz) and shows peaking of 1dB only with output signals of R = 100Ω, R = 120Ω, R = 390Ω, R = 200Ω, ± 1 2 3 4 2.5V. The voltage gain G is 3. The pulse diagrams shown R = 68Ω, I = 20mA, R = 80Ω, C = 6.4p here for small-signal modulation illustrate the excellent 6 Q p p pulse response. There is no difference in pulse response FIGURE 17. Direct-Feedback Amplifier Frequency between 300mVp-p and 5Vp-p. Response. The calculated slew rate is 2500V/µs during 5Vp-p signals. Previously, this slew rate could only be achieved using hybrid circuits with a quiescent current between 20mA and Gain = 3, tr = tf = 2ns, VI = 100mVp–p 500mA and a voltage supply of ±15V for ±2.5V signals. See Table IV.

FUNCTIONAL CIRCUITS WITH THE OPA660 Ω 120 5 6 DB VOUT (V)

8 O

Ω V 100 V 3 R3 IN 390Ω 2

XE 100Ω 82 6.4pF FIGURE 18. Pulse Behavior of the Direct-Feedback FIGURE 15. Direct-Feedback Amplifier. Amplifier.

6 VIDEO RECORD AMPLIFIER VOUT f–3dB ±100mV 331MHz A good way to see the advantages of current control over ±300mV 362MHz voltage control is to compare them when driving magnetic ±700mV 520MHz ±1.4V 552MHz heads in video technology. Analog recording requires high ±2.5V 490MHz linearity, while digital recording demands sharp edges and low phase distortion, since the zero crossing point contains TABLE IV. –3dB Bandwidth vs Output Voltage. the relevant information. A special recording amplifier is necessary to drive the • Driver Circuits for Diodes Capacitive/Inductive Loads rotating video heads. This amplifier, Figure 19, delivers the • Operational Amplifiers current to magnetize the tape. The recording current can be between 1mA and 60mA, depending upon the amplitude, • Line Drivers type of recording, and type of tape used. The current flowing • Integrators/Rectifier Circuits through the video heads must be independent of the fre- • Receiver Amplifier for Pin Diodes quency and load. Current source control can deliver current through the load up to the rated output limit, independent of • Active Filters the voltage drop. In addition, the recording current is di- The new circuit technology really comes into full use, rectly proportional to the magnetic field intensity and flux however, in applications in which the current is the actual density. The record drive amplifier for digital signals shown signal. Such applications include active filters with Current- here functions in a bridge configuration, in which the invert- Conveyor structures, control of LED and laser diodes, as ing and non-inverting digital data streams control the signal well as control of tuning coils, driver transformers, and differentially. Bridge operation, and thus a doubled voltage magnetic heads for analog and digital video recording. range, is necessary because the voltage drop across the load inductance exceeds the voltage range of the Diamond Tran- sistor at the 30MHz recording rate and maximum record current. The common emitter resistor allows simple adjust- +V +V CC CC OUT ment of the transconductance. RQ ÐVCC In previous amplifiers, relays separated the replay and record

R amplifiers when switching from recording to replay. Using B the OPA660 or the OPA2662, which contains two Diamond Transistors with high-current output stages, the I (OPA660) ROG Q EN Data 1 or EN inputs (OPA2662) can switch the record drive ampli- EQ EN2 fier into high-impedance mode. The gate in front of the Playback output stage stops the digital data stream. In high-impedance Amplifier mode, the output stage requires very little current. RB TTL REC DRIVER AMPLIFIER FOR LED TRANSMISSION DIODES ÐVCC ÐVCC OUT The advantages of current control also become apparent FIGURE 19. Video Record Amplifier. when driving light-emitting diodes and laser diodes in analog/digital telecommunications and in test procedures with +5V +5V modulated laser light. Using the OPA2662, it will be possible to control laser diodes by a complementary-bipolar 100Ω 22Ω 22Ω current source; using the OPA660, it is already possible to 2 control LEDs with ±30mA drive current. Figure 20 shows 220Ω 3 Q1 Q2 the circuit implementation. The quiescent current is 20mA max when R = 220Ω, and the inputs of both emitter 8 Q V IN 8 followers, which are not illustrated in the Figure, are grounded 3 through 220Ω resistances, since these inputs are not neces- 4148 270Ω Ω sary in this application. The current mirror consisting of Q 220 LED 1 2 and Q2 sets the quiescent current for the LED, which can PBIAS 100Ω then be adjusted by PBIAS. Two Diamond Transistors wired 200Ω parallel to each other deliver the signal current. Diamond Transistors can be connected to each other at the collector Ð5V output to increase the output current, which has already increased to ±30mA in this configuration. The diode 4148 FIGURE 20. Wideband LED Transmitter. protects the transmitter diodes against excessive reverse voltages.

7 inverting operations are possible. The ratio of the feedback +VCC resistances determines the closed-loop gain, and the user can attain optimum frequency response by adjusting the open-

loop gain externally with ROG. The frequency response of the C differential amplifier is equivalent to that of a 2nd order low- B 1 pass Butterworth filter with gain. Due to the additional delay V R2 IN time in the control loop caused by the feedback buffer, the B2 V ROG OUT frequency response is poorer than the current feedback by 30%. The OPA622, which was recently introduced, contains R1 a Diamond Transistor and two buffers. With the output current capability of ±100mA, this IC can drive several low- impedance outputs. The output buffer has its own supply ÐVCC voltage pins to decouple the output stage from differential stage and to enable external current limitation. Because of FIGURE 21. Voltage-Feedback Amplifier. the identical high-impedance inputs, the typical offset voltage at the output is ±1mV, and the common-mode rejection OPERATIONAL AMPLIFIER WITH ratio is over 70dB. These values are excellent results for RF VOLTAGE FEEDBACK IN DIAMOND STRUCTURE amplifiers. The disadvantages of the Current-Feedback Amplifier listed above are unbalanced inputs, low-impedance inverting in- DRIVER AMPLIFIER FOR put, poor common-mode rejection ratio, and size of the input LOW-IMPEDANCE TRANSMISSION LINES offset voltage. Now, we would like to present a concept The ability of the Current-Feedback Amplifier to deliver which integrates the Diamond structure with voltage feed- ±15mA output current makes it a good choice as a driver back in one circuit. An additional buffer transforms the amplifier for low-impedance (50Ω/75Ω) coaxial transmis- current feedback of the Current Feedback Amplifier into sion lines. To transmit the pulse free of reflections, the voltage feedback. Figures 21 and 22 illustrate the circuit transmission line must be terminated on both sides by the diagram and the extended Voltage-Feedback Amplifier. The characteristic impedance of the line. A resistance in series to feedback buffer is identical to the input section of the the output resistance of the driver amplifier, Figure 23, Diamond Transistor and forms one side of the differential matches the output of the amplifier to the line. The total amplifier, while the Diamond Transistor is the other side. resistance of the output and series resistors should be equal Both buffer outputs are connected to R , which determines OG to the characteristic impedance. The output resistance of the open-loop gain and corresponds to the emitter degenera- operational amplifiers rises with increasing frequency. Thus, tion resistor of a conventional differential stage. the impedances are no longer matched and reflections arise The output of this differential stage is the collector of the due to high-frequency components in the signal. The output Diamond Transistor, which is driven in quasi open-loop resistance of Current-Feedback Amplifiers rises, for ex- mode due to the output buffer. Both inverting and non- ample, up to 25Ω at 50MHz.

+VCC +VEE

ROG

VIN VOUT

ÐVCC ÐVEE

R1 R2

FIGURE 22. Extended Voltage-Feedback Amplifier. 8 DIFFERENTIAL OUTPUT R The circuit in Figure 24 is well suited to applications with O 56Ω 50Ω Z 6 O larger dynamic ranges, which require a differential output to DB 5 drive triax lines. A signal amplitude of ±5V is provided to 8 Ω V 47 drive a load which is not grounded. The load could be the IN 3 RIN 50Ω input resistance of an RF device in an EMC contaminated Ω 2 200 environment. Resistances in series to each amplifier output R match the output to the line. These resistances are selected IN 200Ω at somewhat less than half of the characteristic impedance. While the rise/fall time and bandwidth do not change, the slew rate doubles. FIGURE 23. 50Ω Driver Amplifier.

Ω MONOCHROMATIC MATRIX OR 200 B/W HARDCOPY OUTPUT AMPLIFIER 100Ω ~ZO /2 The inverting amplifier in Figure 25 amplifies the three Z input voltages, which correspond to the luminance section of O 47Ω the RGB color signal. Different feedback resistances weight RL R 47Ω the voltages differently, resulting in an output voltage con- O ~ZO /2 sisting of 30% of the red, 59% of the green, and 11% of the blue section of the input voltage. The way in which the 100Ω signal is weighted corresponds to the transformation equa- 200Ω tion for converting RGB pictures into B/W pictures. The 200Ω output signal is the black/white replay. It might drive a monochrome control monitor or an analog printer (hardcopy output). FIGURE 24. Balanced Driver.

OPERATIONAL 6 DB V TRANSCONDUCTANCE AMPLIFIER (OTA) 5 LUMINANCE 8 The Diamond Transistor and Diamond Buffer form a differ- 3 ential amplifier with two symmetric high-impedance inputs with current output. This amplifier is also known as the 2 Ω Ω Operational Transconductance Amplifier, Figure 26. In this 665 200 VRED application, RE sets the open-loop gain. The bipolar current Ω output can be connected to a discrete cascode transistor, 340 V which enables wideband and high voltage outputs. GREEN Ω 1820 VBLUE NANOSECOND INTEGRATOR One very interesting application using the OPA660 in physi- FIGURE 25. Monochrome Amplifier. cal measurement technology is a non-feedback ns-integrator, Figures 27 and 28, which can process pulses with an amplitude +15V of ±2.5V, have a rise/fall time of as little as 2ns, and pulse width of more than 8ns. The voltage-controlled current source 1kΩ charges the integration capacitor linearly according to the following equation: VOUT

VC = VBE • gm • t/C +VB BFQ262 VC = Voltage At Pin 8

VBE = Base-Emitter Voltage gm = Transconductance 8 t = Time 3 C = Integration Capacitance +VIN

The output voltage is the time integral of the input voltage. 2 It can be calculated from the following equation: RE

T 6 ÐVIN DB gm V = Output Voltage 5 I = gm ¥ (V Ð V ) V = ∫ V dt O OUT IN+ INÐ O C BE T = Integration Time O C = Integration Capacitance FIGURE 26. Operational Transconductance Amplifier. 9 R3 200Ω 6 Sample 10 DB & ADC R 5 2 8 Ω C Hold 780 1 Preamp 3 27pF DT R R R 5 6 1 2 Ω 50 Ω Ω 620 820 +5V Ð5V C R7 2 Ω 1µF 50k RQC 470pF 470pF Ω 470 +5V Ð5V

R 1kΩ Offset 10nF 10nF 8 Trigger Circuit 2.2µF 2.2µF 1nF

Pin 7 Pin 4 Pin 1

FIGURE 27. Nanosecond Integrator.

Channel 1 Input

2V/DIV

Channel 2 Output

500mV/DIV 10ns/DIV 200mV/DIV 200ns/DIV

Trigger

FIGURE 28. Integrator Performance.

10 +5V Cd8 0.5...2.5p

R P 1 2 47kΩ Rd8 10kΩ 27kΩ +5V +5V Ð5V C3 1 R3 7 C3 100Ω R R R 3 2 OUT i1 150Ω 47Ω 100Ω RC5 8 4 8 150Ω DT BUF600 V 5 6 2 OUT VIN DB 0Ω R 4 RQC 5 i2 Ω 100Ω 220

CB C3 500Ω P1 Propagation Delay Time = 6ns Ð5V Ð5V Rise Time = 2.5ns D1

HP2711/DMF3068A

INPUT/OUTPUT VOLTAGE OF THE COMPARATOR 200 VOUT = 200mVp-p fIN = 10MHz 150

100

0 Voltage (V) –50 Input –100

–150 Output Ð200 0 5 10 15 20 25 30 35 40 45 50 55 60 Time (ns)

FIGURE 29. Comparator (Low Jitter).

11 COMPARATOR The current source compensates for different voltage drops An interesting and also cost effective circuit solution using across the diodes up to its maximum rated voltage. It is the OPA660 as a low jitter comparator is illustrated in Figure possible to extend this circuit to a full-wave rectifier by 29. This circuit uses, at the same time, a positive and connecting the second diode, instead to GND, over a resistor negative feedback. The input is connected to the inverting E- to GND, to rectify the negative half of the input signal. input. The output signal is applied in a direct feedback over the two antiparallel connected gallium-arsenide diodes back CONTROLLING THE GAIN to the emitter. A second feedback path over the RC combi- BY ADJUSTING THE BIAS CURRENT nation to the base, which is a positive feedback, accelerates The transfer curve of the Diamond Transistor demonstrates the output voltage change when the input voltage crosses the that the transconductance varies according to the quiescent threshold voltage. The output voltage is limited to the current. The circuit, Figure 31, described here uses this threshold voltage of the antiparallel diodes. The diagram on relation to control the gain. As measurements have shown, the right side of Figure 29 demonstrates the low jitter it is possible to produce a gain range of 20dB, but the performance of the presented comparator circuit. minimum quiescent current should not fall short of 1mA. Quiescent currents smaller than 0.5mA increase the non- RECTIFIER FOR RF SIGNAL IN THE mV RANGE linearities to a value which can no longer be tolerated. A

Previously, rectifier diodes were included in the feedback positive current flowing into the IQ-adjust (pin 1) disables loop of operational amplifier circuits to form ideal diodes for the OPA660, the output of which goes into high-impedance accurate detection of small signals in the mV range. In this state. The switch-on period lasts only a few ns, while the µ configuration, the slew rate of the operational amplifier fixes switch-off time is several s. The internal capacitances are the maximum frequency which can be rectified. The circuit discharged at different speeds according to the load. The in Figure 30 illustrates a new method of rectifying RF possibility of modulating the bias current dynamically has signals. The diodes at the current source output direct the not yet been investigated. But based on the internal configu- current either into the load resistance or toward ground. The ration, modulation frequencies up to several kHz should be output current is zero even during zero crossing, resulting in possible. a very soft transfer from one diode to the next. PIN DIODE RECEIVER

1SS83 Figure 32 illustrates a preamplifier which can recover both 5 6 DB VOUT analog and digital signals for a fiber optic receiver. This preamplifier can amplify weak and noisy signal currents and 8 Ω 220 convert them into voltage. In this arrangement, the Diamond 3 VIN Transistor operates in the inverting base configuration, which 500Ω functions excellently in this application due to its low- 2 50Ω impedance current input. In the ideal case, the voltage set at 25Ω the base by the voltage divider appears at the low-impedance emitter free of offset errors. The voltage drop above the

FIGURE 30. RF Rectifier.

+5V Ð5V

4.7kΩ 475Ω

+5V Ð5V 2N3906 100Ω 75Ω 5 6 DB VOUT

1 8 2.1kΩ 3 VIN

2 Ω Ω Ω Ω 75 100 25 500

FIGURE 31. Controlled Amplifier.

12 diode is adjusted to zero volts. During exposure to light, the adjoint network concept. A network is reversible or recipro- pin diode functions as a high-impedance current source and cal when the transfer function does not change even when either delivers current to the emitter or removes current. The the input and output have been exchanged. Most networks, resulting voltage difference between the base and emitter of course, are nonreciprocal. The networks, Figure 34, controls the collector current. The current gain error is perform interreciprocally when the input and output are dependent both upon the dynamic output resistance of the exchanged, while the original network, N, is exchanged for pin diode and upon the transconductance of the Diamond a new network NΑ. In this case, the transfer function remains

Transistor. It is possible to achieve current gain factors of the same, and NA is the adjoint network. It is easy to 200 to 400, depending upon the diode and quiescent current construct an adjoint network for any given circuit, and these used. Advantages of this circuit structure include the follow- networks are the base for circuits in Current-Conveyor ing points: structure. Individual elements can be interchanged accord- • The transconductance and speed of the Diamond ing to the list in Figure 35. Voltage sources at the input Transistor keep the voltage drop across the diode low, become short circuits, and the current flowing there be- preventing the diode capacitance from increasing with comes the output variable. In contrast, the voltage output the modulation. becomes the input, which is excitated by a current source. The following equation describes the interreciprocal fea- • A fixed voltage across the diode improves the linearity, tures of the circuit:VOUT/VIN = IOUT/IIN. Resistances and since the sensitivity of the diode varies with diode capacitances remain unchanged. In the final step, the opera- voltage. tional amplifier with infinite input impedance and 0Ω output • The capacitance at the emitter is only 2pF. impedance is transformed into a current amplifier with 0Ω • The signal path is short, resulting in a very wide input impedance and infinite output impedance. A Diamond bandwidth. Transistor with the base at ground comes quite close to an ideal current amplifier. The well-known Sallen-Key low- pass filter with positive feedback, Figure 36, is an example ACTIVE FILTERS USING THE OPA660 of conversion into Current-Conveyor structure. The positive IN CURRENT CONVEYOR STRUCTURE gain of the operational amplifier becomes a negative second One further example of the versatility of the Diamond type of Current Conveyor (CCII), Figure 37. Both arrange- Transistor and Buffer is the construction of active filters for ments have identical transfer functions and the same level of the MHz range. Here, the Current Conveyor structure, Fig- sensitivity to deviations. The most recent implementation ure 33, is used with the Diamond Transistor as a Current of active filters in a Current-Conveyor structure produced a Conveyor. second-order Bi-Quad filter. The value of the resistance in The method of converting RC circuit loops with operational the emitter of the Diamond Transistor controls the filter amplifiers in Current Conveyor structures is based upon the characteristic.

+5V 47Ω 220Ω 47Ω 5 6 DB VOUT Ω 8 10k 150Ω 3

R 2 C2

Ð5V 10nF

FIGURE 32. Preamplifier.

I VOUT E OUT C +1 CCIIÐ B

C C VIN R R

R R IIN

C/2 C/2

2 2 2 VOUT IOUT 4KQ /R C T(s) = = = 2 2 2 2 VIN IIN s + 2/RC[2Q(1Ð K) + 1]s + 4KQ /R C FIGURE 33. Current Conveyor.

13 Reciprocal Networks

VIN N VOUT IOUT N IIN

V I OUT = OUT VIN IIN

Interreciprocal Networks

V V I I IN N OUT OUT NA IN

FIGURE 34. Networks.

TRANSFER FUNCTION Element Adjoint

V I IN OUT 2 R R 1 2 1 2 S C R 2M +sC 1M + 1 Signal 1 1M 1 I V R R R Sources IN = OUT = 3 2 1 1 ÐVOUT +2 1 2 F(p) V 2 R R IN S C C R 2M +sC 1M + 1 RR 1 2 1M R 11 R R 1 2 1 2 3S 2S 1S Passive Elements CC 1 2 1 2 FILTER CHARACTERISTICS ∞ Low-pass filter: R2 = R3 = 1 3 3 High-pass filter: R = R = ∞ Controlled + 1 2 V µV µI I ∞ Sources Ð Bandpass filter: R1 = R3 = 1 4 4 ∞ Band rejection filter: R2 = ; R1 = R3 All-pass filter: R = R , R = R , R = R FIGURE 35. Individual Elements in the Current Conveyor. 1 1S 2 2S 3 3S

R3 R2

VIN VOUT

C C R1 1 R1M 2 R2M

RB1 RB2 RB3 R1S R2S R3S

FIGURE 36. Universal Active Filter.

14 The design of a low-pass filter with a corner frequency of lower curve in the diagram on the right half of Figure 38 30MHz results in the following values: shows the behavior of the output impedance vs frequency. Ω For some applications, like the integrator for ns-pulses of R1M = R2M = 91 ; C1 = C2 = 100pF R = 142Ω; R = 161Ω; R = 140Ω; R = 426Ω Figure 27, the relatively low output impedance is a real 1 1S 2S 3S disadvantage. The fast discharge of the integration capacitor Figure 37 illustrates the frequency response and phase char- after (Figure 28) the pulse is over demonstrates this behav- acteristics of the filter. Advantages of active filters in a ior. An easy way to improve the output impedance is a Current Conveyor structure: positive feedback path formed by the resistor divider from • The increase in output resistance of operational ampli- the collector to the base and the GND. The ratio of the two fiers at high frequencies makes it difficult to construct resistors determines the final output impedance, which can feedback filter structures (decrease in stop-band attenu- even be made negative. The capacitor between C and B ation). supports the improvement vs frequency, which is illustrated • All filter coefficients are represented by resistances, in the diagram of Figure 38. The positive feedback results in making it possible to adjust the filter frequency a dynamic increase of the open loop gain, which can be response without affecting the filter coefficients. made higher than 110dB. • The capacitors which determine the frequency are located between the ground and the current source DIFFERENTIATOR FOR WEAK AND outputs and are thus grounded on one side. There- DISTURBED DIGITIZED SIGNALS fore, all parasitic capacitances can be viewed as part of As it is shown in Figure 39 a RC network can be connected these capacitors, making them easier to comprehend. between the E-output of the OTA and buffer output. The • The features which determine the frequency character- proposed circuit improves the pulse shape of digitized signals coming from a magnetic tape or a hard disc drive. istics are currents, which charge the integration capacitors. This situation is similar to the transfer characteristic of the Diamond Transistor. CONTROL LOOP AMPLIFIER A new type of control loop amplifier for fast and precise OPTIMATION WITH DIAMOND STRUCTURE control circuits can be designed with the OPA660. The circuit of Figure 40 shows a series connection of two voltage • AGC Amplifier control current sources which have an integral and at higher • DC-Restored Amp frequencies a proportional behavior vs frequency. The con- • Analog Multiplexer trol loop amplifiers show an integrator behavior from DC to • PLL the frequency, represented by the RC time constant of the • Sample/Hold network from the C-output to GND. Above this frequency • Multiplier they operate as an amp with constant gain. The series • Oscillators connection increases the overall gain to about 110dB and • RF-Instrumentaion Amplifiers thus minimizes the control loop deviation. The differential configuration at the inputs enables one to apply the mea- DYNAMIC OUTPUT IMPEDANCE INCREASE sured output signal and the reference voltage to two identical As illustrated in Table II, the output impedance of the OTA high-impedance inputs. The output buffer decouples the C- at a quiescent current of ±20mA equals to 25kΩ || 4.2pF. The output of the second OTA in order to insure the AC- performance and to drive subsequent output stages.

7.0 180¡ 20

P h dB a dB s e

Ð10 Ð180¡ Ð60 300k Frequency (Hz)50M 1M Frequency (Hz) 400M

FIGURE 37. Current Conveyor LP.

15 Cd8

4pF RO1 ROUT = (VO/V8) Ð 1 Rd8

26kΩ 1MΩ +5V ROUT = V(VO)/V(V8) Ð 1

7 R3 3 13 14 R01 C01 Ω 15 DT 8 2 1 1E6 1F VO Ω R02 1E9 RO 1 10k…3GHz 4 Rqc DEC20 250Ω Ð5V C π • 02 COUT = 1/(2 f10MHz ROUT 10MHz) 02 Ω ROUT = 1.19M 1F COUT = 0.50pF

1E-6 G02 R2 1E9 C2 1F (PSpice¨ Simulation)

10M Rd8, R3

) R , R C

Ω d8 3, d8 100k

Without Feedback 1k Output Impedance (

10 1k10k 100k 1M 10M 100M 1G Frequency (Hz)

FIGURE 38. Transconductance Output Impedance (Dynamic increase with positive feedback).

220Ω 180Ω 5 BUF601 V 8 1 OUT 3 180Ω 75Ω 5.6pF 5 6 V +1 IN 2 180Ω Differentiator Network

FIGURE 39. Differentiator for Digitized Video Signals.

16 8 5 6 +1 VOUT

8 180Ω 2 V 10pF 10pF REF 3 2 10Ω 33Ω 10Ω 33Ω 180Ω 5 6 VIN +1

FIGURE 40. Control Loop Amplifier.

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