Gain-Bandwidth Product

The Gain Bandwidth Product describes how op amp gain behaviour relates to frequency. Manufacturers insert a dominant pole into the op amp’s frequency response, so that output voltage vs. frequency is predictable. This is because op amps have many active components, each with their own cut-off frequency and frequency response. This would make the overall op amp frequency response random, with poles and zeroes that differed from device to device even within the same family.

Manufacturers’ response to this has been to introduce a dominant pole to make the op amp frequency response more predictable, stable and ‘standardised’. The dominant pole makes the op amp behave like a single-pole system, with a drop of 20 dB for every decade of frequency, starting with the cut-off frequency.

Within such circuits, the gain bandwidth product shows that the product of the op amp gain and frequency at any point of the frequency response, is a constant. It can be calculated using the following formula:

In the case of Analog Devices’ ADA4004 precision amplifier, the gain bandwidth product is 12 MHz. Therefore, at a gain of one, the bandwidth is 12 MHz, and at the maximum open-loop gain of 500,000, the bandwidth is 12 MHz divided by 500,000, which is 24 Hz. This is the op amp open loop cut-off frequency.