May/June 2018 85
OPEN COMMUNICATION n BY LOU FRENZEL W5LEF
Wien Bridge Oscillator
A popular low frequency (audio, and up to about
100 kHz or so) sine wave oscillator is the Wien bridge
shown in Figure 1. It uses an RC network that produces
a zero degree phase shift from output back to the input,
producing positive feedback that, in turn, produces
oscillation. An op-amp is used to produce a gain of three
that offsets the attenuation of the RC network. With a net
closed loop gain of one, the circuit oscillates at a frequency
determined by the values of the RC network:
f = 1/2πRC
This circuit works great and produces a very clean
low distortion sine wave. Its problem is that instabilities
in the gain and phase can cause the circuit to go out of
oscillation completely, or go into saturation producing a
clipped sine wave or square wave. Some compensation
components are usually added to eliminate this problem.
A simple solution is to replace R1 with a small
incandescent bulb whose resistance changes with current.
As the output goes
up, the bulb current
reduces the gain
to compensate. If
the output goes down, the current decreases, lowering
the resistance and increasing the gain to keep the output
constant. One working example is to make R2 390 ohms
and R1 a type 327 bulb. Other more elaborate schemes
use an FET as a variable resistor to vary the gain.
This circuit works and has a frequency of about 1,592
Hz. Output amplitude depends on the power supply
A popular way to make a sine wave oscillator is to
use an RC network to produce a 180 degree phase shift
to use in the feedback path of an inverting amplifier.
Setting the gain of the amplifier to offset the RC network
attenuation will produce oscillation. There are multiple
variations of phase shifters, including a Twin-T RC network
and cascaded RC high pass sections that produce either
45 or 60 degree shifts in each stage. The amplifier can be a
single transistor, single op-amp, or multiple op-amps. Figure
2 shows one popular variation.
he sine wave is a naturally occurring signal shape in communications and other
Many electronic products use signals of the sine wave form. Audio, radio, and power
equipment usually generates or processes sine waves. As it turns out, there are literally
dozens of ways to generate a sine wave. This article presents some popular methods you
should be familiar with.
Seven Common Ways to
Generate a Sine Wave
Figure 1. The popular
Wien bridge oscillator.
An oldie but goodie.
Frequency can be varied
by using pots for R and
using different values of
C switched in.
Figure 2. A fixed frequency is a disadvantage, but for a single
frequency is good. The pure output needs to be buffered with
an op-amp follower if you are going to drive a load.