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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.
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:
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.
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