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Opamp bandwidth

For most applications, the bandwidth of a circuit is the frequency at which the gain drops from its low frequency value by 3dB. Video amplifiers are the exception, as in these circuits the bandwidth of interest may be the frequency where the gain falls by 0.1dB. The opamp chooser does not include the 0.1dB values.  The small signal bandwidth will be greater than the full power bandwidth due to an amplifier's slew rate limitations. When analysing small signal bandwidth, opamps fall into three basic categories.

Voltage feedback, unity gain stable, amplifiers

The voltage feedback, unity gain stable amplifier is the classic opamp. Voltage feedback amplifiers usually have a fixed gain-bandwidth product, which means that that the bandwidth can accurately be estimated by taking the gain-bandwidth product and dividing it by the closed loop gain.  A resistive feedback network can be used to set any reasonable closed loop gain, and the amplifier should be stable.  A unity gain stable amplifier is sometimes called a compensated amplifier.

Voltage feedback, non unity gain stable, amplifiers

The voltage feedback, non unity gain stable amplifier is similar to the unity gain stable opamp, provided that the closed loop gain exceeds a minimum value. As a voltage feedback amplifiers it also has a fixed gain-bandwidth product, and the bandwidth can accurately be estimated by taking the gain-bandwidth product and dividing it by the closed loop gain.  The payback for the lack of stability is a higher gain-bandwidth product without increased current consumption.  These amplifiers are sometimes called decompensated amplifiers, and some of them can be made unity gain stable by the addition of a compensation capacitor. The common minimum closed loop gains for stability are 2, 5 and 10.

Current feedback amplifiers

A current feedback amplifier uses a different circuit for the inverting input, which means the input ends up with a comparitively low impedance. The payback for this change is that the bandwidth does not fall dramatically with higher closed loop gains.

Comparing devices using the chooser

As different classes of amplifier behave differently with closed loop gain, no single number can sum up the bandwidth of an opamp. The chooser therefore holds data for the bandwidth at gains of 1, 2, 5 and 10. The drop down selector allows the user to select which data set is used. If an opamp isn't stable at the selected gain, then its table entry will be blank. Using the closed loop gain of 1 data is a suitable staring point for most users. Applications where bandwidth and bandwidth per mW are significant issues, the user should select the appropriate gain data. This will show uncompensated and current feedback opamps a more realistic light.

It is also important to note that the bandwidth of many devices varies significantly with supply voltage and temperature.