LIC & AC Lab Manual CBCS Scheme 15ECL48RC Phase shift and Wein’s bridge oscillators Aim: To design RC Phase shift and Wein’s bridge oscillators using Op-amp. Components Required: Sl No Components Specification Quantity 1 Op-amp µA741 1 2 AFO and Power - 1 each Supply IT 3 CRO - 1 4 Resistors 1.5 kΩ, 1 MΩ, 15 kΩ 3,1,2 5 Capacitors 0.1 µF 3 M 6 Breadboard - 1 7 CRO probes - 3 8 Connecting Wires - 1 Set Theory: G An oscillator is a circuit, which generates ac output signal without giving any input ac E signal. This circuit is usually applied for audio frequencies only. The basic requirement for an EC oscillator is positive feedback. An oscillator consists of an amplifier and a feedback network. Active device i.e. Op Amp is used as an amplifier. Passive components such as R-C or L-C combinations are used as feed back network. C To start the oscillation with the constant amplitude, positive feedback is not the only sufficient condition. Oscillator circuit must satisfy the following two conditions known as LI Barkhausen conditions: 1. The first condition is that the magnitude of the loop gain (Aβ) = 1 where A = Amplifier gain β = Feedback gain. 2. The second condition is that the phase shift around the loop must be 360° or 0°. Dept ECE, GMIT, Davangere Page 1 LIC & AC Lab Manual CBCS Scheme 15ECL48 RC Phase Shift Oscillator: Phase-shift oscillator is a simple electronic oscillator. It contains an inverting amplifier, and a feedback filter which 'shifts' the phase of the amplifier output by 180 degrees at the oscillation frequency. The filter produces a phase shift that increases with frequency. It must have a maximum phase shift of considerably greater than 180° at high frequencies, so that the phase shift at the desired oscillation frequency is 180°. The most common way of achieving this kind of filter is using three identical cascaded resistor-capacitor filters, which together produce a phase shift of zero at low frequencies, and 270 degrees at high frequencies. At the oscillation frequency each filter produces a phase shift of IT 60 degrees and the whole filter circuit produces a phase shift of 180 degrees. M Circuit Diagram: G E EC C LI Figure 1: RC Phase-shift oscillator Dept ECE, GMIT, Davangere Page 2 LIC & AC Lab Manual CBCS Scheme 15ECL48 Design: Frequency of oscillation is given as, Consider fo = 500 Hz, Voltage gain Av = 29 Voltage gain for inverting amplifier is given as, IT Choose M Consider C = 0.1µF Choose G E EC C Output Waveform: LI Figure 2: RC Phase-shift oscillator Output Dept ECE, GMIT, Davangere Page 3 LIC & AC Lab Manual CBCS Scheme 15ECL48 Tabulation: Theoretical Readings Practical Readings Frequency Time period Frequency Amplitude Time period Hz Sec Hz V Sec Procedure RC Phase Shift Oscillator: For the given frequency f0 values of circuit elements are calculated as per the design. IT Circuit is rigged up as shown in figure 1. Power supply is switched on and the waveform is observed on CRO. M Amplitude and time period of the waveform is tabulated. Trace the waveform on tracing sheet. G E EC C LI Dept ECE, GMIT, Davangere Page 4 LIC & AC Lab Manual CBCS Scheme 15ECL48 Wein Bridge Oscillator: A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies. The frequency of oscillation is given by: f= 1/2πRC. The Wien Bridge Oscillator uses a feedback circuit consisting of a series RC circuit connected with a parallel RC of the same component values producing a phase delay or phase advance circuit depending upon the frequency. At the resonant frequency fr the phase shift is 0°. Circuit Diagram: IT M G E EC C LI Figure 3: Wein Bridge oscillator Dept ECE, GMIT, Davangere Page 5 LIC & AC Lab Manual CBCS Scheme 15ECL48 Design: Frequency of oscillation is given as, Consider fo = 1 KHz, Voltage gain Av = 3 Voltage gain for non-inverting amplifier is given as, IT M Consider C = 0.047 µF Choose G E EC Output Waveform: C LI Figure 4: RC Phase-shift oscillator Output Dept ECE, GMIT, Davangere Page 6 LIC & AC Lab Manual CBCS Scheme 15ECL48 Tabulation: Theoretical Readings Practical Readings Frequency Time period Frequency Amplitude Time period Hz Sec Hz V Sec IT Procedure Wein Bridge Oscillator: For the given frequency f0 values of circuit elements are calculated as per the design. M Circuit is rigged up as shown in figure 3. Power supply is switched on and the waveform is observed on CRO. G Amplitude and time period of the waveform is tabulated. Trace the waveform on tracing sheet. E Result: Thus the RC Phase Shift oscillator and Wein Bridge oscillator are designed, constructed and EC verified. RC phase shift oscillator fo = _____________ Wein Bridge oscillator fo = _______________ C LI Dept ECE, GMIT, Davangere Page 7