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).