Capacitive loads often give rise to problems, in part because they can reduce the output bandwidth and slew rate, but mainly because the phase lag they produce in the op amp’s feedback loop can cause instability. Although some capacitive loading is inevitable, amplifiers are often subjected to sufficient capacitive loading to cause overshoots, ringing, and even oscillation. Among the parameters cited above, the amplifier output impedance, represented by the output resistance, RO, is the one factor that most affects performance with capacitive loads. 0 will drive any capacitive load without phase degradation.
To avoid sacrificing performance with light loads, most amplifiers are not heavily compensated internally for substantial capacitive loads, so external compensation techniques must be used to optimize those applications in which a large capacitive load at the output of the op amp must be handled. Typical applications include sample-and-hold amplifiers, peak detectors, and driving unterminated coaxial cables. Capacitive loading, as shown in Figures 1 and 2, affects the open-loop gain in the same way, regardless of whether the active input is at the noninverting or the inverting terminal: the load capacitance, CL, forms a pole with the open-loop output resistance, RO. A is the unloaded open-loop gain of the amplifier.
40 dB per decade, which, in turn, causes instability. This note discusses typical questions about the effects of capacitive loads on the performance of some amplifier circuits, and suggests techniques to solve the instability problems they raise. A simple op amp circuit with capacitive load. You’ll choose the compensation technique that best suits your design. For example, here’s a compensation technique that has the added benefit of filtering the op amp’s noise via an RC feedback circuit. Figure 3 shows a commonly used compensation technique, often dubbed in-the-loop compensation. To better understand this technique, consider the redrawn feedback portion of the circuit shown in Figure 4.
VB is connected to the amplifier’s minus input. Think of the capacitors, Cf and CL, as open circuits at dc, and shorts at high frequencies. With Cf shorted, Rx Rf, and Ro Rin, the pole and zero are functions of CL, Ro, and Rx. With CL open, the pole and zero are a function of Cf. Although this method helps prevents oscillation when heavy capacitive loads are used, it reduces the closed-loop circuit bandwidth drastically. Caution: The behaviors discussed here are typically experienced with the commonly used voltage-feedback amplifiers.
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