Most op amps have a low output impedance at frequencies well below their closed-loop bandwidth, but as frequency increases, so does the output impedance. When the op amp is used in a Sallen-Key low-pass filter, this increase in output impedance can reduce the filter's stopband rejection.

However, a simple emitter-follower buffer amplifier at the op amp's output can reduce the effects of the higher output impedance and improve the stop-band rejection.

The effect of increased output impedance can be seen in the basic Sallen-Key low-pass filter (including the dashed line for feedback in Fig. 1). When frequency increases, capacitor C2 (connected between the input and output) along with input resistor R4 form a voltage divider with the op amp's output impedance. This division ratio is the stop-band rejection of the filter, and the higher the output impedance, the lower the stop-band rejection.

Stop-band rejection can be improved by breaking the feedback loop and adding a buffer amplifier, which lowers the amplifier's output impedance. Transistor Q1 and resistors R1 and R2 form an emitter follower that buffers the op amp output and provides a low output impedance beyond the bandwidth of the op amp. Because transistor Q1 is inside the feedback loop, degradation in the filter dc and pass-band response are avoided.

Spice simulations indicate a 20-dB improvement in stop-band rejection for the filter (Fig. 2).