Free Op-Amp Stability & Phase Margin Calculator

Why Capacitive Loads Cause Op-Amp Oscillation

Every real operational amplifier has a finite open-loop output impedance ($R_o$, typically 20 to 100 Ω). When you connect a capacitive load ($C_L$) to the op-amp output, $R_o$ and $C_L$ form a low-pass RC filter inside the feedback loop.

1. The Phase Lag Catastrophe

This RC filter introduces an additional pole into the loop gain at:

An op-amp's internal dominant pole already provides -90° of phase lag. If the capacitive pole $f_p$ occurs before the loop crossover frequency $f_c$, it introduces up to another -90° of phase lag, driving total loop phase shift to -180°. At this point, negative feedback turns into positive feedback, and the amplifier bursts into uncontrolled oscillation.

2. The Out-of-the-Loop R_iso Solution

The simplest fix is inserting a small isolation resistor ($R_{iso}$) directly between the op-amp output pin and the capacitive load, taking the feedback from the op-amp pin before $R_{iso}$. This isolates $C_L$ from the feedback loop:

3. Dual Feedback (Zero DC Error)

While $R_{iso}$ fixes stability, DC current flowing through $R_{iso}$ causes a voltage drop ($I_{load} imes R_{iso}$). To preserve DC precision, add a dual-feedback network: take DC feedback after $R_{iso}$ via $R_f$, and add a high-frequency bypass capacitor ($C_f$) from the op-amp output pin back to the inverting input.