Gain Compensation

When the closed-loop gain of an op amp circuit is related to the loop gain, as it is in voltage feedback op amps, the closed-loop gain can be used to stabilize the circuit. This type of compensation can not be used in current-feedback op amps because the mathematical relationship between the loop gain and ideal closed-loop gain does not exist. The loop gain equation is repeated as Equation 7–11. Notice that the closed-loop gain parameters ZG and ZF are contained in Equation 7–11, hence the stability can be controlled by manipulating the closed-loop gain parameters.

The original loop gain curve for a closed-loop gain of one is shown in Figure 7–12, and it is or comes very close to being unstable. If the closed-loop noninverting gain is changed to 9, then K changes from K/2 to K/10. The loop gain intercept on the Bode plot (Figure 7–12) moves down 14 dB, and the circuit is stabilized.

Figure 7–12. Gain Compensation

Gain compensation works for inverting or noninverting op amp circuits because the loop gain equation contains the closed-loop gain parameters in both cases. When the closed loop gain is increased, the accuracy and the bandwidth decrease. As long as the application can stand the higher gain, gain compensation is the best type of compensation to use.

Uncompensated versions of normally internally compensated op amps are offered for sale as stable op amps with minimum gain restrictions. As long as gain in the circuit you design exceeds the gain specified on the data sheet, this is economical and a safe mode of operation.