Answer
4—C1
works with the input voltage divider to form a simple
low-pass filter that helps eliminate transients and noise.
100 nF, in conjunction with the divider’s effective impedance
of 62.5 k W, makes
the cutoff frequency approximately 25 Hz.
D1
makes sure that the reverse breakdown voltage of Q1’s
B-E junction is never reached by conducting any reverse
currents around the transistor. The base of Q1 will never
go below –0.7 V, and VBE will never go below
–3.7 V.
Contributor: David Tweed
Published
January 2004
