Op-amp variable gain amplifier circuit

- a variable gain circuit using an operational amplifier

A variety of operational amplifier, op-amp circuits including:
[1] Operational amplifier basics
[2] Op-amp circuit gain summary
[3] Inverting op-amp
[4] Inverting op-amp
[5] Op-amp high pass filter
[6] Op-amp low pass filter
[7] Op-amp bandpass filter
[8] Op-amp variable gain amplifier
[9] Op-amp notch filter circuits
[9] Operational amplifier multivibrator
[11] Op-amp bistable
[12] Op-amp comparator
[13] Op-amp Schmitt trigger

Most operational amplifier or op-amp circuits have a fixed level of gain. However it is often useful to be able to vary the gain. This can be done simply by using a potentiometer on the output of a fixed gain op-amp circuit, but sometimes it may be more useful to vary the actual gain of the amplifier circuit itself. This can be achieved very simply by using the variable gain operational amplifier circuit shown below.

The circuit is very simple, and only uses one additional component over that of a basic operational amplifier circuit. The circuit simply uses a single variable gain amplifier. The circuit uses a single operational amplifier, two resistors and a variable resistor. Additionally not only is the gain varied but also the sign.

Using this circuit the gain can be calculated from the formula given above. In this the variable "a" represents the percentage of travel of the potentiometer, and it varies between "0" and "1". It is also worth noting that the input impedance is practically independent of the position of the potentiometer, and hence the gain