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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.