Wlen Bridge Oscillator

Wlen Bridge Oscillator

• The Wien bridge oscillator is also an RC oscillator which uses Wien bridge circuit as its feedback network.

• The amplifier used in this oscillator is a noninverting amplifier which does not introduce any phase shift.

• The feedback network which is a Wien bridge circuit also does not introduce any phase shift.

• Thus phase shift around a loop in a Wien bridge oscillator is 0°.

• The Fig. 10.6.1 shows the basic circuit of Wien bridge oscillator.

• The output of the amplifier is applied between terminals 1 and 3 while amplifier is supplied from terminals 2 and 4 which is the output of the feedback network.

1. Derivation of Frequency

• The Fig. 10.6.2 shows the feedback network of Wien bridge oscillator.

• The two arms which are Rx, C| m series and R2, C2 in parallel are frequency sensitive arms and hence only those arms are considered in the Fig. 10.6.2.

• Input to feedback network is Vin applied between 1 and 3, which is amplifier output.

• Output of feedback network is Vf taken from 2 and 4.

• This network is also called lead-lag network.

Using ω = 1/ RC in the equation (10.6.1) we get the magnitude of the feedback network as,

• As |Aβ|  ≥ 1 hence |A| ≥ 3 for Wien bridge oscillator.

• Thus the gain of amplifier stage must be at least 3 to ensure sustained oscillations in Wien bridge oscillator.

2. Wien Bridge Oscillator using Transistor

• As the amplitude increases, the resistance R4 decreases and the current through it increases. This increases the negative feedback and reduces the gain. This controls the amplitude of the oscillations and avoids the output waveform distortion.

• The expression for the frequency of oscillations is,

f = (1 / 2πRC) Hz

3. Advantages and Disadvantages

• The advantages of Wien bridge oscillator are,

1. Mounting the two capacitors on common shaft and varying their values, the frequency can be varied as per the requirement.

2. Due to the use of two stage amplifier, the gain is high.

3. The stability is high.

4. It provides stable low distortion sinusoidal output.

5. The frequency range can be selected simply by using decade resistance boxes.

6. The circuit is easy from design point of view and gives constant output.

• The disadvantages of Wien bridge oscillator are,

1. It cannot be used to generate high frequencies.

2. The circuit needs two transistors and a large number of other components.

3. The maximum frequency is limited due to the amplitude and the phase shift characteristics of the amplifiers. 

Ex. 10.6.1 The frequency sensitive arms of the Wien bridge oscillator uses C₁ = C₂ = 0.001 pF and R₂ = 10 kΩ while R₂ is kept variable. The frequency is to be varied from 10 kHz to 50 kHz, by varying R2. Find the minimum and maximum values of R₂

Sol. : The frequency of the oscillator is given by,

So minimum value of R₂ is i.013 kΩ while the maximum value of R₂ is 25.33 kΩ

Ex. 10.6.2 In a Wien bridge oscillator R1 = R2 = 100 k and the ganged variable capacitor has a range from 50 pF to 500 pF. Find the range of the frequency of the oscillations possible.

If the frequency desired is 50 kHz more than the maximum frequency calculated above, find the value of the resistance to be connected in parallel with 100 k Ω.

Sol. : For a Wien bridge oscillator,

The corresponding R = R' with an additional resistance R_x in parallel.

Ex. 10.6.3 Design a Wien bridge oscillator circuit to oscillate at a frequency of 20 kHz.

Sol. : f = 20 kHz

For a Wien bridge oscillator with R₁ = R₂ = R and C₁ = C₂    

• The designed circuit is as shown in the Fig. 10.6.3.

4. Comparison of RC Phase Shift and Wien Bridge Oscillators

1. Explain the principle of operation of a Wien bridge oscillator.

2. What are the advantages and disadvantages of Wien bridge oscillator?

3. Draw the circuit of a Wien bridge oscillator. Derive the transfer function P fro) of the phase lead-lag network used and hence explain how Barkhausen conditions are satisfied in this RC oscillator.