Question

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(c) Figure 3 shows the schematic diagram of a typical Wein-bridge oscillator. Sub- Circuit A Ri Vout C2 R2 Rj Sub-circuit B Figure 3 (i) Identify which are the components of the Wein-bridge oscillator that (2 marks) contributed to its postive feedback network. (i) Explain how the sub-circuit B and the sub-circuit A work together to (2 marks) generate an oscillating output. (ii) If the sub-circuit B consists of 3.3 k2 resistors (i.e. R2 R3) and 4.7 nF capacitors (i.e. C2 Ca), what is the oscillating frequency of the oscillator? (3 marks) (iv) If the gain of the sub-circuit B is 1/3, and R1 of sub-circuit A is 2 k2, determine the value of R to generate an oscillating output.(3 marks) (v) Determine the gain of sub-circuit A when the resistor R1 is broken and (2 marks) becomes open circuit. (vi) Sketch the output waveform of the oscillator immediately after the resistor (3 marks) R1 has broken and become open circuit. EAEA (009783) Sample Paper 2

Answer 1

(i) Feedback network contains bridge network which does not provide phase shift at oscillating frequency as one arm consists of lead circuit and other arm consists of lag circuit. The total phase shift around the loop is zero as the bridge network and the noninverting amplifier does not give any phase shift. So the components R2,C2,R3, and C3 contribute to the positive feedback network.

When the resistor R, is broken and becomes open circuit, the gain of the non - inverting amplifier will be become 1 as below Gain = 1 + R_f/R_1 = 1 + R_f/infinity rightarrow (due to open circuit) = 1 + 0 = 1 Given feedback network gain = 1/3 when R, is broken the gain of the non - inverting amplifier = 1 so the overall loop gain. A beta = 1 1/3 = 1/3 i.e. A beta = 1/3 < 1 so the oscillator output will be a decaying signal as it cannot maintain sustained oscillation