Stability problems can arise in applications in which an amplifier must drive a reactive load. Most amplifiers can easily drive purely resistive loads with no stability problems. However, some reactive loads can cause an otherwise stable amplifier to ring severely or oscillate. A good example is an audio-power amplifier. The device’s load is a loudspeaker, which presents a reactive load consisting of an inductance and a series resistance. Because of the inductor, this load appears as an open circuit at high frequencies. The addition of a simple Zobel network can eliminate amplifier instability (Reference 1).
The simple Zobel network in Figure 1a comprises a series RC combination in parallel with a series RL combination. Examination of the input-impedance expression for this network reveals that proper component-value selection can make the input impedance purely resistive:
You can make ZIN frequency-independent and equal to R2 by equating the coefficents of s in the numerator and denominator. This operation yields the following two constraints:
Assume you want a power amplifier to "see" a purely resistive load of 8 ohms over all frequencies. If L=6.5 µH, R1=R2 and C=L/R22=0.1 µF (Figure 1b). Thus, the addition of a series 8 ohm, 0.1-µF network in parallel with the speaker completes the Zobel network (Figure 1c). You can also use a Zobel network to compensate for a capacitive reactance by adding an appropriate-valued series RL combination. Emitter-follower buffers, for example, are prone to oscillate when they drive large capacitive loads. You can stabilize the load by presenting a constant, purely resistive load to the emitter. In this case, solve for L instead of C in the above equation: L=CR22. (DI #1801)