3-28
Figure 3-19.—Closed-loop frequency-response curve for gain of 10.
The relationship between circuit gain and bandwidth in an operational-amplifier circuit can be
expressed by the GAIN-BANDWIDTH PRODUCT (GAIN BANDWIDTH = UNITY GAIN POINT).
In other words, for operational-amplifier circuits, the gain times the bandwidth for one configuration of an
operational amplifier will equal the gain times the bandwidth for any other configuration of the same
operational amplifier. In other words, when the gain of an operational-amplifier circuit is changed (by
changing the value of feedback or input resistors), the bandwidth also changes. But the gain times the
bandwidth of the first configuration will equal the gain times the bandwidth of the second configuration.
The following example should help you to understand this concept.
The frequency-response curves shown in figures 3-17, 3-18, and 3-19 have a gain-bandwidth product
of 1,000,000. In figure 3-17, the gain is 100,000 and the bandwidth is 10 hertz. The gain-bandwidth
product is 100,000 times 10 (Hz), or 1,000,000. In figure 3-18, the gain has been reduced to 100 and the
bandwidth increases to 10 kilohertz. The gain-bandwidth product is 100 times 10,000 (Hz) which is also
equal to 1,000,000. In figure 3-19 the gain has been reduced to 10 and the bandwidth is 100 kilohertz. The
gain-bandwidth product is 10 times 100,000 (Hz), which is 1,000,000. If the gain were reduced to 1, the
bandwidth would be 1 megahertz (which is shown on the frequency-response curve as the unity-gain
point) and the gain-bandwidth product would still be 1,000,000.
Q-19.
What does the term "closed-loop" mean in the closed-loop configuration of an operational
amplifier?
In answering Q20, Q21, and Q23, select the correct response from the choices given in the
parentheses.
