Mumbai University > EXTC > Sem 3 > Analog Electronics I

Marks: 10 M

Year: May 2014

• Wien bridge oscillator is RC phase-shift oscillator

  Diagram

Fig1 Wien bridge oscillator circuit diagram

Construction

• Wien bridge oscillator employs two transistors and R-C bridge circuit.
• Wien bridge oscillator consists of two stage amplifier with common emitter voltage divider configuration.
• R-C bridge circuit is lead-lag network. In bridge circuit $R_1$ in series with $C_1$, $R_3$, $R_4$ and $R_2$ in parallel with $C_2$ forms the four arms. The phase shift across this bridge lags with increase in frequency and leads with decrease in frequency In between, there is a resonant frequency fr at which the phase angle equals 0°
• This bridge circuit can be used as feedback circuit for oscillator provided that the phase shift through the amplifier is zero. This condition is achieved by employing two transistor each producing phase of 180º thus total phase shift 360º or 0º
• Circuit has two transistors $Q_1$ and $Q_2$. Transistor $Q_1$ acts as oscillator circuit and $Q_2$ acts as inverter providing additional 180º phase shift. The output of second transistor is supplied to the R-C bridge circuit and the voltage across $C_2R_2$ parallel combination is given to transistor Q1
• The circuit uses positive and negative feedback. The positive feedback is through $R_1 C_1 R_2, C_2$ to tran¬sistor $Q_1$ and negative feedback is through emitter resistor R4 which is temperature sensitive lamp.

Working:

• The circuit is set in oscillation by random change in base current of $Q_1$ transistor that may be due to change in dc power supply or due to noise.
• The transistor $Q_1$ amplifies this signal with 180º phase shift this signal is input to second transistor Q2 through capacitor $C_4$ which further amplifies the signal with 180º phase shift since the signal has two phase-shift of 180º the o/p signal at the second transistor is in- phase with the input signal.
• A part of output signal of second transistor is supplied to bridge circuit at input point (point A-C).
• A part of feedback signal is applied across emitter resistor $R_4$ which provides negative feedback or degenerative effect and a part of the feedback signal is applied across the base-bias resistor $R_2$ where it produces regenerative effect (or positive feedback). At frequency of oscillation effect of regeneration is made slightly more than that of degeneration so as to obtain sustained oscillations.
• The continuous frequency variation in this oscillator can be had by varying the two capacitors $C_1$ and $C_2$ simultaneously and switching the value of resistors $R_1$ and $R_2$ we can change frequency range of oscillator
• Negative feedback working: negative feedback is provided by resistor $R_4$ which is a temperature sensitive lamp, whose resistance is proportional to current. Increase in amplitude of oscillation increases current through $R_4$ as a result the resistance is increased thus decreasing current through $R_4$ in other words large negative feedback is applied this reduces the loop gain hence signal of amplitude gets reduced and constant amplitude level is maintained. Reverse mechanism occurs for decrease in amplitude.