A transconductance amplifier is basically a voltage-controlled current source. As shown in Figure 15-28. the amplifier typically has a differential input and singleended output. Recall that transconductance is defined to be output current divided by input voltage. The transconductance of the amplifier.
A programmable transconductance amplifier is one whose value of gm is determined by the value of an externally supplied control current or voltage. In the programmable transconductance amplifier shown in Figure 15-29, the value of gm is controlled by t:1C value of amplifier bias current, IARc• A programmable transconductance amplifier can be used to construct a programmable voltage amplifier, that is. an amplifier whose voltage gain can be controlled electronically. The next example illustrates this application.
The transconductance of the programmable transconductance amplifier depends on IARC according to s; = 20/ABC
1. Determine the voltage gain of the amplifier as a function of IABC.
2. For what value of IARC will the voltage gain of the amplifier equal 15?
IMG
Another application of a programmable transconductance amplifier is its use as a programmable resistor: a resistor whose resistance is determined by an external control voltage or current. Figure 15-31 illustrates this perspective. Suppose that the relationship between.
Then, since the inverting input is grounded,
The 3080 Programmable Transconductance Amplifier
An example of an integrated-circuit programmable transconductance amplifier is the 3080, available from RCA as the CA3080 and National Semiconductor as the LM 3080. Figure 15-32 shows a pin diagram of the amplifier. Manufacturers’ specifications show that a typical room-temperature value for gIll is 9.6 mS when 1,tI,C = 500 J-LA. Thus, a typical value for k (in equation 15-30)
However, considerable variation is possible, since gill has a specified range of 6.7 mS to 13 mS when 1,\8c == 500 }.LA. 11I8c can be supplied using anyone of the three methods shown in Figure 15-33. If resistor RII8c is connected between pin 5 and ground, as shown in Figure 15-33(a}, then
With this method of control, the previously described applications of programmable gain and programmable resistance can be implemented using control voltage Finally, (‘ can be supplied directly from an external constant-current source,
