WEIN BRIDGE OSCILLATOR
An oscillator is a circuit that
converts a dc input to an ac output. This project investigates sinusoidal,
output oscillators. Sinusoidal oscillators consist of an amplifier with a
positive feedback loop of a frequency selective network. The amplifier can be a
transistor amplifier or an operational amplifier. The frequency of the
oscillator is determined by the frequency selective network.
The criteria for an oscillator to
produce sinusoidal oscillations are that
1. The magnitude of the loop
gain should be greater than or equal unity and
2. The total phase difference
between the output signal and the input provided through feedback should be
integral multiple of 3600 or zero.
It is one of the most popular type of oscillators used in audio and
sub-audio frequency ranges (20 – 20 kHz). This type of oscillator is simple in
design, compact in size, and remarkably stable in its frequency output.
Furthermore, its output is relatively free from distortion and its frequency
can be varied easily. However, the maximum frequency output of a typical Wien
bridge oscillator is only about 1 MHz. This is also, in fact, a phase-shift
oscillator. It employs two transistors, each producing a phase shift of 180°,
and thus producing a total phase-shift of 360° or 0°.
CIRCUIT
DIAGRAM:-
The
circuit diagram of Wien bridge oscillator is shown in the figure below.
It is essentially a two-stage amplifier with an R-C bridge
circuit. R-C bridge circuit (Wien bridge) is a lead-lag network. The
phase’-shift across the network lags with increasing frequency and leads with
decreasing frequency. By adding Wien-bridge feedback network, the oscillator
becomes sensitive to a signal of only one particular frequency. This particular
frequency is that at which Wien bridge is balanced and for which the phase
shift is 0°.If the Wien-bridge feedback network is not employed and output of
transistor Q2 is
fed back to transistor Q1 for providing regeneration required
for producing oscillations, the transistor Q1 will amplify signals over a wide range
of frequencies and thus direct coupling would result in poor frequency
stability.
Thus by employing
Wien-bridge feedback network frequency stability is increased.
In the bridge circuit R1 in series with C1, R3, R4 and R2 in parallel with C2 form
the four arms.
This bridge circuit can be used as feedback network for an
oscillator, provided that the phase shift through the amplifier is zero.
This requisite condition
is achieved by using a two stage amplifier, as illustrated in the figure. In
this arrangement the output of the second stage is supplied back to the
feedback network and the voltage across the parallel combination C2 R2 is
fed to the input of the first stage.
Transistor Q1 serves as an oscillator and amplifier
whereas the transistor Q2 as an inverter to cause a phase shift
of 180°. The circuit uses positive and negative feedbacks. The positive
feedback is through R1 C1 R2, C2 to
transistor Q1 and negative feedback is through the
voltage divider to the input of transistor Q1. Resistors R3 and R4 are
used to stabilize the amplitude of the output.
The two transistors Q1 and Q2 thus cause a total phase shift of 360°
and ensure proper positive feedback. The negative feedback is provided in the
circuit to ensure constant output over a range of frequencies. This is achieved
by taking resistor R4 in the form of a temperature
sensitive lamp, whose resistance increases with the increase in current. In
case the amplitude of the output tends to increase, more current would provide
more negative feedback. Thus the output would regain its original value. A
reverse action would take place in case the output tends to fall.
The amplifier voltage gain,
A = (R3 + R4) / R4 = (R3 / R4) + 1 = 3
{Since R3 = 2 R4}
The above corresponds with the feedback network
attenuation of 1/3. Thus, in this case, voltage gain A, must be equal to or
greater than 3, to sustain oscillations.
To have a voltage gain of 3 is not difficult.
On the other hand,to have a gain as low as 3 may be
difficult. For this reason also negative feedback is essential.
Wein Bridge Oscillator Using Op-Amp
IC 741
Wein bridge oscillator is an audio frequency sine wave oscillator of high stability and simplicity. Before that let us
see what is oscillator? An oscillator is a circuit that produces periodic
electric signals such as sine wave or square wave. The application of
oscillator includes:
sine wave generator, local oscillator for
synchronous receivers etc.
Here we are discussing wein bridge oscillator
using 741 op amp IC. It is a low frequency oscillator. The op-amp used in
this oscillator circuit is working as non-inverting amplifier mode. Here the feedback network need not provide
any phase shift. The circuit can be viewed as a wein bridge with a series RC
network in one arm and parallel RC network in the adjoining arm. Resistors Ri
and Rf are connected in the remaining two arms.
Circuit Diagram:
Components
Required:-
·
Resistors (1KΩ, 1.5KΩ x2)
·
Potentiometer(4.7KΩ)
·
Capacitor(0.1µF x2)
·
741 Op amp
Working
of Wein bridge Oscillator:-
- The feedback
signal in this oscillator circuit is connected to the non-inverting input
terminal so that the op-amp works as a non-inverting amplifier.
- The
condition of zero phase shift around the circuit is achieved by balancing
the bridge, zero phase shift is essential for sustained oscillations.
- The
frequency of oscillation is the resonant frequency of the balanced bridge
and is given by the expression fo = 1/2Πrc.
- At resonant frequency
( ƒo), the inverting and non-inverting input voltages will be equal and
“in-phase” so that the negative feedback signal will be cancelled out by
the positive feedback causing the circuit to oscillate.
- From the
analysis of the circuit, it can be seen that the feedback factor β= 1/3 at
the frequency of oscillation. Therefore for sustained oscillation, the
amplifier must have a gain of 3 so that the loop gain becomes unity.
- For an
inverting amplifier the gain is set by the feedback resistor network Rf
and Ri and is given as the ratio -Rf/Ri.
Design:-
The required frequency
of oscillation fo=1kHz
we
have,
Take C=0.01µF,
then R=1.6kΩ (Use 1.5kΩ standard)
Gain
of the amplifier section is given by,
Take Ri=1kΩ,
then Rf=2.2kΩ (Use 4.7kΩ Potentio meter for
fine corrections).
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