**Load Capacitance Effect:**

Capacitance connected at the output of an operational amplifier is termed Load Capacitance Effect (C_{L}). Figure 15-20 shows that C_{L} is in series with the op-amp output resistance (r_{o}), so C_{L} and r_{o} constitute a phase-lag circuit in the feedback network. As in the case of stray capacitance, another 10° of phase lag introduced by C_{L} and r_{o} could cause circuit instability where the phase margin is already small. The equation for calculating the Load Capacitance Effect that might cause instability is similar to that for stray capacitance;

In Eq. 15-6 f is the frequency at which A_{v}B = A_{CL}. If r_{o} is reduced, Eq 15-6 gives a larger C_{L} value. Thus, an op-amp with a low output resistance can tolerate more Load Capacitance Effect than one with a higher output resistance.

One method often used to counter instability caused by load capacitance is shown in Fig. 15-21(a). A resistor (R_{x}) usually ranging from 12 Ω to 400 Ω, is connected in series with the Load Capacitance Effect. The presence of R_{x} (with R_{2} connected at the op-amp output) can severely reduce the phase lag produced by r_{o} and C_{L}. However, R_{x} also has the undesirable effect of increasing the circuit output impedance to approximately the resistance of R_{x}.

A Miller-effect capacitor (C_{2}) connected across feedback resistor R_{2} may be used to compensate for the load capacitance, [see Fig. 15-21(b)]. In this case, C_{2} introduces some phase-lead in the feedback network to counter the phase-lag. The equation for calculating a suitable capacitance for C_{2} is, once again, similar to that for stray capacitance;

A modified form of Miller-effect compensation for load capacitance is shown in Fig. 15-21(c). An additional resistor (R_{x}) is included in series with C_{L} to reduce the phase lag, as discussed. But now, R_{2} is connected at the junction of R_{x} and C_{L}, so that (because of feedback) R_{x} has no significant effect on the circuit output impedance. Also, C_{2} is connected from the op-amp output terminal to the inverting input. With this arrangement, Eq. 15-7 is modified to,

It should be noted from Equations 15-7 and 15-8 that, as for stray capacitance, smaller resistance values for R_{2} give larger, more convenient, compensating capacitor values.