Electronic Design

Gimmick Capacitor Stabilizes High-Frequency Op Amps

The stability of high-unity-gain-bandwidth op amps is susceptible to stray capacitances. Instability doesn't first show up as oscillation, but rather pronounced overshoot and ringing. Op amps such as the TLV278X, TLC08X, and TLC07X have unity-gain-crossover frequencies of 8 and 10 MHz. With any significant loading capacitor, adding any stray input capacitance (CI in the figure) causes pronounced ringing. CI works with RG to introduce a pole in the loop gain (Aβ) at the frequency:

F = 1/(2πRGCI)

When RG = 10 kΩ and CI = 3 pF, the pole frequency is 5.3 MHz. The amplifier gain does not decrease to unity gain before 8 or 10 MHz, so the pole contributes 56.5° or 62° negative phase shift, and the phase margin decreases considerably.

Connecting a feedback capacitor (CF) across RF adds a zero in the loop gain. When the time constants RFCF and RGCI are equal, the pole from RGCI is cancelled, restoring the circuit to normal operation. It's extremely hard to compute a value of CF that yields complete pole cancellation because CI is a stray capacitor dependent on the pc-board layout. However, implementing CF as a gimmick capacitor will solve the problem.

During layout, run a wide topside copper trace from the op-amp output lead under the feedback resistor so that it almost connects to the inverting input lead. Run the same-width bottomside copper trace from the inverting input lead so that it almost connects to the output lead. These two traces form the plates of a capacitor that's made of the same material as the stray capacitor and is a larger value than the stray capacitor.

Then, test the circuit board with a square-wave-input signal. Because CF is larger than CI, the output waveform is an over-damped square wave with no ringing. Use a small grinder or razor to trim the bottomside copper away until the circuit just starts to show slight overshoot. Measure the length of the bottomside trace, correct the artwork, and you've made a gimmick capacitor that compensates the op-amp circuit.

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