The two circuits below illustrate
generating low frequency sinewaves by shifting the phase of the signal
through an RC network so that oscillation occurs where the total phase
shift is 360 degrees. The transistor circuit on the right produces a
reasonable sinewave at the collector of the 3904 which is buffered by
the JFET to yield a low impedance output. The circuit gain is critical
for low distortion and you may need to adjust the 500 ohm resistor to
achieve a stable waveform with minimum distortion. The transistor
circuit is not recommended for practical applications due to the
critical adjustments needed.
The op-amp based phase shift oscillator
is much more stable than the single transistor version since the gain
can be set higher than needed to sustain oscillation and the output is
taken from the RC network which filters out most of the harmonic
distortion. The sinewave output from the RC network is buffered and the
amplitude restored by the second (top) op-amp which has gain of around
28dB. Frequency is around 600 Hz for RC values shown (7.5K and 0.1uF)
and can be reduced by proportionally increasing the network resistors
(7.5K). The 7.5K value at pin 2 of the op-amp controls the oscillator
circuit gain and is selected so that the output at pin 1 is slightly
clipped at the positive and negative peaks. The sinewave output at pin 7
is about 5 volts p-p using a 12 volt supply and appears very clean on a
scope since the RC network filters out most all distortion occurring at
pin 1.
Last Updated : February 15, 2002 
