## Aim of the experiment

To construct a Wein bridge oscillator using transistor and study its frequency in oscilloscope.

#### Components Required

Components required for Wien bridge oscillator are –

1. Wein bridge oscillator trainer
2. Cathode Ray Oscilloscope
3. Trace paper
4. Patching wires

### Theory ( Wein bridge oscillator lab manual )

The Wein bridge oscillator uses RC ( resistor Capacitor ) network. However, RC network is a part of Wein bridge network that produces both regenerative and degenerative feedback.

The method used for getting positive feedback in Wein bridge oscillator is to use two staged RC coupled amplifier. The first stage of an RC coupled amplifier introduces a phase shift of 180° and the second stage increases it to 360°. At the frequency of oscillation of the positive feedback network shown in figure 1 makes the input V1 and output V0 in the same phase.

The frequency of oscillation is given as

In addition to positive feedback, the circuit also produces negative feedback to make the oscillation stable. This type of feedback is introduced because emitter resistor is not bypassed. The result is a pure sine wave oscillator that can be used to generate a frequency ranging from 5 Hz to 1 MHz in a circuit known as lead-lag network.

A lead lag network is shown in figure 2. It is a band pass filter comprised of a series RC network ( R1 C1 ) and a parallel RC network ( R2 C2 ). Its is called a lead lag network because the phase angle leads for some frequencies and lag for others. However, at the resonant frequency, the phase is exactly equal to zero degrees. This important characteristic allow the lead lag network to determine the oscillating frequency of the bridge oscillator.

At low frequencies, the series capacitor C1 has such high impedance that it act as an open, and prevents the output. At very high frequencies, the parallel capacitor C2 shunts the output to ground and again, there is no output. However, at the resonant frequency, the output voltage is maximum. This is illustrated by the voltage output versus frequency curve at the output of the circuit. Output is maximum at f0, therefore the RC network is frequency selective. On both sides of f0 the output decreases significantly.

At low frequencies, the phase angle is positive and the circuit acts as a lead network. At high frequencies, the output phase angle is negative and the circuit acts as a lag network. At the circuits resonant frequency, the phase shift of series and parallel circuit cancel. Since the phase shifts are equal, but of opposite polarity, the resultant output is in-phase with the input.

Apply this lead lag network to a Wien bridge oscillator. Figure 3 illustrates the Wien bridge oscillator using a transistor as an active device. The circuit is a two phase common emitter amplifier. The Wien ridge is connected across the base and emitter of transistor Q1. The lead lag network comprised of R1 & C1 and R2 & C2, makes up one side of the bridge. A voltage divider R3 & R4 is the remaining leg of the bridge.

### Step by Step Tutorial

1. Analyse the circuit printed on the front panel of the trainer.

It consists of two sections.

a. Wien Bridge network

b. Transistor amplifier

Note down the values of resistor and capacitor in the circuit.

1. Now, make connections in the circuit as shown in circuit diagram. Observe the amplifier output feedback to the bridge network ( i.e positive feedback ).
2. Switch ON the trainer board Check +15V DC supply by digital multi meter.
3. Observe the sine wave output on the CRO at the output terminal. Adjust 10K potentiometer for getting stable output.

Oscilloscope settings:

Time / Division: 0.2 mS

1. Measure the time period of the signal on the CRO. Calculate the frequency.

Frequency (f) = Hz. ( Approx. 1040 Hz ).

1. Compare the theoretical and practical values.
2. Calculate the oscillation frequency

Here,

R1 = 6.8 KΩ

R2 = 6.8 KΩ

C1 = 0.022 µF

C2 = 0.022 µF

Ignore the 5K potentiometer value as it is for the frequency stability.

#### Calculation ( Practical )

Time / Division = 0.2 mS / division

1 division = 0.2 mS

We know,

1 division = 5 small division,

So, 5 small division = 0.2 mS

1 small division = 0.2 / 5 mS

Oscilloscope reads one complete cycle in 22 small division.

So, 22 small division =( 0.2 / 5 ) x 22 mS = 0.88 mS

Time period = 0.88 mS

Author

Akash Sharma

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