Op-Amp Variable Gain Amplifier Circuit

A simple and effective variable gain op amp circuit using a minimum number of components

Op-amp Circuits Include:
Introduction Circuits summary Circuit design hints & tips Inverting amplifier Summing amplifier Non-inverting amplifier Inverting vs non-inverting circuits Variable gain amplifier High pass active filter Low pass active filter Bandpass filter Notch filter Comparator Schmitt trigger Multivibrator Bistable Integrator Differentiator Wien bridge oscillator Phase shift oscillator

There are many instances when a variable gain op amp circuit may be required.

Although it is possible to place a potentiometer on the output of a circuit, there are more elegant ways in which this can be achieved, maintaining levels of impedance more accurately.

The op amp variable gain circuit below maintains input impedance over the range of the gain, providing a distinct advantage in some instances.

Op amp variable gain circuit

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.

Variable gain op amp amplifier — Variable gain op amp circuit

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".

The gain of the circuit can be seen from the formula below:

Voltage gain = 2 a - 1

It is also worth noting that the input impedance is practically independent of the position of the potentiometer, and hence the gain

The op amp variable gain circuit provides a very simple yet effective solution to providing a variable level of gain whilst also maintaining the impedance leves.