An IC Operational Transconductance Amplifier ODUCT (OTA) With Power Capability OBSOLETE PR ACEMENT MENDED REPL NO RECOM Application NoteTERSIL October 2000 AN6077.3 ations 1-888-IN ll Central Applic Ca [email protected] or email: centa In 1969, the first triple operational transconductance [ /Title 7 V+ amplifier or OTA was introduced. The wide acceptance of (AN60 this new circuit concept prompted the development of the 77) single, highly linear operational transconductance amplifier, Y Z /Sub- the CA3080. Because of its extremely linear ject transconductance characteristics with respect to amplifier OUTPUT bias current, the CA3080 gained wide acceptance as a gain INVERTING NON-INVERTING (An IC 2 Q1 Q2 3 6 INPUT INPUT Opera- control block. The CA3094 improved on the performance of the CA3080 through the addition of a pair of transistors; AMPLIFIER tional these transistors extended the current carrying capability to BIAS 5 W X CURRENT Transc 300mA, peak. This new device, the CA3094, is useful in an IABC onduc- extremely broad range of circuits in consumer and industrial 4 V- tance applications; this paper describes only a few of the many Ampli- consumer applications. FIGURE 2. CURRENT MIRRORS W, X, Y AND Z USED IN THE OTA fier What Is an OTA? (OTA) The OTA, operational transconductance amplifier, concept is 1.0 With as basic as the transistor; once understood, it will broaden the 0.8 DIFFERENTIAL AMPLIFIER Power TRANSFER CHARACTERISTIC designer's horizons to new boundaries and make realizable 0.6 Capa- designs that were previously unobtainable. Figure 1 shows an 0.4 bility) equivalent diagram of the OTA. The differential input circuit is /Autho the same as that found on many modern operational 0.2 r () amplifiers. The remainder of the OTA is composed of current 0 mirrors as shown in Figure 2. The geometry of these mirrors is -0.2 /Key- such that the current gain is unity. Thus, by highly -0.4 words degenerating the current mirrors, the output current is -0.6 (Inter- precisely defined by the differential input amplifier. Figure 3 DEVIATION FROM STRAIGHT LINE DEVIATION FROM -0.8 sil shows the output current transfer characteristic of the NORMALIZED OUTPUTCURRENT AND -1.0 Corpo- amplifier. The shape of this characteristic remains constant -150 -100 -50 0 50 100 150 ration, and is independent of supply voltage. Only the maximum ΔVbe (mV) current is modified by the bias current. power FIGURE 3. THE OUTPUT CURRENT TRANSFER switch, CHARACTERISTIC OF THE OTA IS THE SAME 7 V+ AS THAT OF AN IDEALIZED DIFFERENTIAL power AMPLIFIER ampli- - 2 OTA The major controlling factor in the OTA is the input amplifier fier, bias current I ; as explained in Figure 1, the total output RIN ABC pro- 2RO ein current and gm are controlled by this current. In addition, the gram- gm ein 6 IOUT = gm (±ein) input bias current, input resistance, total supply current, and mable 2RO output resistance are all proportional to this amplifier bias 3 power + current. These factors provide the key to the performance of gm = 19.2 • IABC this most flexible device, an idealized differential amplifier, switch) (mS) (mA) ±IOUT ≈ IABC Max (mA) i.e., a circuit in which differential input to single ended output /Cre- RO ≈ 7.5/IABC 4 V- (mA) conversion can be realized. With this knowledge of the (MΩ)(mA) ator () 5 IABC basics of the OTA, it is possible to explore some of the applications of the device. FIGURE 1. EQUIVALENT DIAGRAM OF THE OTA DC Gain Control The methods of providing DC gain control functions are numerous. Each has its advantage: simplicity, low cost, high level control, low distortion. Many manufacturers who have nothing better to offer propose the use of a four quadrant 1 1-888-INTERSIL or 1-888-468-3774 | Copyright © Intersil Corporation 2000 Application Note 6077 multiplier. This is analogous to using an elephant to carry a the improvement in linearity of the transfer characteristic. twig. It may be elegant but it takes a lot to keep it going! Reduced input impedance does result from this shunt When operated in the gain control mode, one input of the connection. Similar techniques could be used on the OTA standard transconductance multiplier is offset so that only output, but then the output signal would be reduced and the one half of the differential input is used; thus, one half of the correction circuitry further removed from the source of non multiplier is being thrown away. linearity. It must be emphasized that the input circuitry is differential. The OTA, while providing excellent linear amplifier characteristics, does provide a simple means of gain control. 7 For this application the OTA may be considered the DIODE CURRENT = 0mA 6 realization of the ideal differential amplifier in which the full differential amplifier gm is converted to a single ended 5 100 output. Because the differential amplifier is ideal, its gm is IABC directly proportional to the operating current of the 4 500μA CA3080A 80 S/N RATIO differential amplifier; in the OTA the maximum output current 3 60 is equal to the amplifier bias current IABC. Thus, by varying 10μA (dB) S/N RATIO the amplifier bias current, the amplifier gain may be varied: THD (PERCENT) 2 40 A = gm RL where RL is the output load resistance. Figure 4 shows the basic configuration of the OTA DC gain control 1 20 THD circuit. 0 0 V +6V 0.1 1.0 10 100 1.0V X IO = gm VX AMPLITUDE SIGNAL 7 INPUT VOLTAGE (mV) INPUT MODULATED 2 - 51 OUTPUT FIGURE 5A. OTA IO CA3080A 6 7 DIODE CURRENT = 0.5mA 51 3 + 10K 6 4 5 -6V 5 100 IABC IABC 500μA CA3080A VM R 4 80 M S/N RATIO GAIN CONTROL 3 60 10μA S/N RATIO (dB) THD (PERCENT) 2 40 FIGURE 4. BASIC CONFIGURATION OF THE OTA DC GAIN THD CONTROL CIRCUIT 1 20 0 0 As long as the differential input signal to the OTA remains 1 10 100 1V 10V under 50mV peak-to-peak, the deviation from a linear INPUT VOLTAGE (mV) transfer will remain under 5 percent. Of course, the total FIGURE 5B. harmonic distortion will be considerably less than this value. 7 Signal excursions beyond this point only result in an DIODE CURRENT = 1mA undesired “compressed” output. The reason for this 6 compression can be seen in the transfer characteristic of the 5 100 differential amplifier in Figure 3. Also shown in Figure 3 is a curve depicting the departure from a linear line of this 4 80 500μA transfer characteristic. 3 60 The actual performance of the circuit shown in Figure 4 is IABC 10μA 2 40 CA3080A plotted in Figure 5. Both signal to noise ratio and total S/N RATIO (dB) THD (PERCENT) S/N THD harmonic distortion are shown as a function of signal input. 1 RATIO 20 Figures 5B and 5C show how the signal handling capability of 0 0 the circuit is extended through the connection of diodes on DISTORTION IS PRIMARILY the input as shown in Figure 6 [2]. Figure 7 shows total A FUNCTION OF SIGNAL INPUT system gain as a function of amplifier bias current for several 110 100 1V 10V values of diode current. Figure 8 shows an oscilloscope INPUT VOLTAGE (mV) reproduction of the CA3080 transfer characteristic as applied FIGURE 5C. to the circuit of Figure 4. The oscilloscope reproduction of FIGURE 5. PERFORMANCE CURVES FOR THE CIRCUIT OF Figure 9 was obtained with the circuit shown in Figure 6. Note FIGURES 4 AND 6 2 Application Note 6077 E IN 2K 100 0mA 1 10μA 2 2 E 51 O 100μA DIODE 3 CA3080A 6 10 CURRENT 0.5mA 4 10K 5 3 1mA 8 6 + 1.0 2K GAIN 7 14 DIODE 11 CURRENT 9 12 13 0.1 V- 0.01 Transistors from CA3046 array. 0 100 200 300 400 500 AGC System with extended input range. IABC (μA) FIGURE 6. A CIRCUIT SHOWING HOW THE SIGNAL FIGURE 7. TOTAL SYSTEM GAIN vs AMPLIFIER BIAS HANDLING CAPABILITY OF THE CIRCUIT OF CURRENT FOR SEVERAL VALUES OF DIODE FIGURE 4 CAN BE EXTENDED THROUGH THE CURRENT CONNECTION OF DIODES ON THE INPUT Horizontal: 25mV/Div. Vertical: 50μA/Div., I = 100μA ABC Horizontal: 0.5V/Div. Vertical: 50μA/Div., IABC = 100μA, Diode Current = 1mA FIGURE 8. CA3080 TRANSFER CHARACTERISTIC FOR THE FIGURE 9. CA3080 TRANSFER CHARACTERISTIC FOR THE CIRCUIT OF FIGURE 4 CIRCUIT OF FIGURE 6 Simplified Differential Input to Single Ended Output Conversion One of the more exacting configurations for operational common mode range is that of the CA3080 differential amplifiers is the differential to single-ended conversion amplifier. In addition, the gain characteristic follows the circuit. Figure 10 shows some of the basic circuits that are standard differential amplifier gm temperature coefficient of usually employed. The ratios of the resistors must be -0.3%/oC. Although the system of Figure 11 does not precisely matched to assure the desired common mode provide the precise gain control obtained with the standard rejection. Figure 11 shows another system using the operational amplifier approach, it does provide a good CA3080 to obtain this conversion without the use of simple compromise suitable for many differential transducer precision resistors. Differential input signals must be kept amplifier applications. under 126mV for better than 5 percent nonlinearity. The 3 Application Note 6077 R2 The CA3094 R1 The CA3094 offers a unique combination of characteristics DIFFERENTIAL - that suit it ideally for use as a programmable gain block for + OUTPUT INPUT audio power amplifiers. It is a transconductance amplifier in R3 R1 R3 R4 = which gain and open-loop bandwidth can be controlled R2 R4 between wide limits.