TC 9-62
other unwanted signals are amplified so much in open-loop operation that the operational
amplifier is usually not used in this way. Therefore, most operational amplifiers are used
with feedback (closed-loop operation).
7-56. Operational amplifiers are used with degenerative (or negative) feedback that
reduces the gain of the operational amplifier but greatly increases the stability of the
circuit. In the closed-loop configuration, the output signal is applied back to one of the
input terminals. This feedback is always degenerative (negative). In other words, the
feedback signal always opposes the effects of the original input signal. One result of
degenerative feedback is that the inverting and noninverting inputs to the operational
amplifier will be kept at the same potential. Closed-loop circuits can be of the inverting
configuration or noninverting configuration. However, the inverting configuration is used
more often than the noninverting configuration.
Inverting Configuration
7-57. Figure 7-12 shows an operational amplifier in a closed-loop, inverting
configuration. Resistor R2 is used to feed part of the output signal back to the input of the
operational amplifier.
Figure 7-12. Inverting Configuration
7-58. It is important to remember the difference between the entire circuit (or operational
circuit) and the operational amplifier. The operational amplifier is represented by the
triangle-like symbol. The operational circuit includes the resistors and any other
components, as well as the operational amplifier. In other words, the input to the circuit is
shown in Figure 7-12. However, the signal at the inverting input of the operational
amplifier is determined by the feedback signal as well as by the circuit input signal.
7-59. As you can see in Figure 7-12, the output signal is 180 degrees out of phase with
the input signal. The feedback signal is a portion of the output signal and, therefore, also
180 degrees out of phase with the input signal. Whenever the input signal goes positive,
the output signal and the feedback signal goes negative. The result of this is that the
inverting input to the operational amplifier is always very close to 0 volts with this
configuration. In fact, with the noninverting input grounded, the voltage at the inverting
input to the operational amplifier is so small compared to other voltages in the circuit that it
7-16
TC 9-62
23 June 2005
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