Operational Amplifier

Published : 01-01-2015 by : Pawan Janorkar

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The frequency response of the op-amp is pretty straight forward. Basically, as the operating frequency of the op-amp increases, the voltage gain decreases. Actually, it is only after the cutoff frequency is reached that the attenuation of voltage gain starts happening. The cutoff frequency is defined as the frequency at which the open loop gain equals 70.7% of its maximum gain, or, equivalently, down 3 dB from the maximum gain. All frequencies lower than the cutoff frequency, even 0 Hz, see the max gain because the op-amp is a dc amplifier. Gain bandwidth product is a simple formula that relates closed loop gain (Acl), bandwidth (cutoff frequency, fco), and unity-gain frequency, as such: funity = (Acl)(fco)

Unity-gain frequency is the maximum frequency possible where the gain equals 1. Remember that a closed loop lowers the voltage gain, yet by lowering the voltage gain, higher operating frequencies are made available. So, depending on what is needed for the job, a certain degree of flexibility is available. A high gain, low frequency (or bandwidth) arrangement is possible, as is a low gain, high bandwidth configuration, as long as their product equals the unity-gain frequency. See the figure below to better understand op-amp frequency response.

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