_____________________________________________________________________ Special Amplifiers
BANDWIDTH LIMITATIONS
7-74. As with most amplifiers, the gain of an operational amplifier varies with
frequency. The specification sheets for operational amplifiers will usually state the open-
loop (no feedback) gain for DC (or 0 Hz). At higher frequencies, the gain is much lower. In
fact, for an operational amplifier, the gain decreases quite rapidly as frequency increases.
7-75. Figure 7-16 shows the open-loop (no feedback) frequency-response curve for a
typical operational amplifier. Remember, bandwidth is measured to the half-power points
of a frequency-response curve. The frequency-response curve shows that the bandwidth is
only 10 Hz with this configuration. The unity gain point, where the signal out will have the
same amplitude as the signal in (the point at which the gain of the amplifier is 1), is
1 MHz for the amplifier. As you can see, the frequency response of this amplifier drops off
quite rapidly.
7-76. Remember, most operational amplifiers are used in a closed-loop configuration.
When you look at the frequency-response curve for a closed-loop configuration, one of the
most interesting and important aspects of the operational amplifier is that the use of
degenerative feedback increases the bandwidth of an operational amplifier circuit. This is
just another example of the difference between the operational amplifier and the
operational-amplifier circuit (which includes the components in addition to the operational
amplifier). You should also be able to see that the external resistors not only affect the gain
of the circuit, but the bandwidth as well.
Figure 7-16. Open-loop Frequency-response Curve
7-77. Figure 7-17 should help to show you how the gain and bandwidth of a closed-loop,
operational-amplifier circuit are related. Figure 7-17 shows the frequency-response curve
that is for a circuit in which degenerative feedback has been used to decrease the circuit
gain to 100 (from 100,000 for the operational amplifier). Notice that the half-power point
of this curve is just slightly above 10 KHz.
23 June 2005
TC 9-62
7-23
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