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Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering – Stage 2
(NFQ – Level 8)
Summer 2007
Analogue Electronics
(Time: 3 Hours)
Answer five questions [each 20 marks]
Maximum available marks: 100
Examiners: Dr. M. Cranitch Prof. G. Hurley Dr. S. Foley
Q1. (a) Explain the terms ‘virtual earth’ and ‘output offset voltage’, as applied to an operational amplifier. [4 marks]
(b) Draw the circuit diagram and derive the equation for the output voltage of (i) an op-amp integrator, and (ii) an op-amp differentiator. Sketch time-related input and output waveforms in each case when a square wave is applied. [10 marks]
(c) If a ‘practical’ integrator has Ri = 10kΩ, Rf = 20kΩ and C = 0.1μF, determine the amplitude of the output if the input is a 1kHz square wave of peak amplitude 2V. Draw sketches of the input and output waveforms on dimensioned axes. [6 marks]
Q2. (a)^ With^ reference^ to^ an^ operational^ amplifier,^ explain^ the^ terms^ ‘transition frequency’ and ‘slew rate’. Quote figures typical of the 741 op-amp. (^) [6 marks]
(b) Show that the highest frequency which can be amplified by an op-amp, without distortion due to slew rate, is given by f (^) max = K
S
, where K is the peak output voltage and S is the slew rate. [6 marks]
(c) An op-amp with a slew rate of 1.2V/μs is used in an inverting amplifier which has Ri = 20kΩ and Rf = 140kΩ. Determine whether or not the output will be distorted due to slew rate limitation for each of the following sinusoidal input signals: (i) 0.7Vrms at 30kHz, (ii) 1.0Vrms at 15kHz, and (iii) 0.1Vrms at 40kHz. [8 marks]
Q3. (a) Draw the small-signal model of the bipolar junction transistor, and hence derive equations for voltage gain and input impedance of (i) common emitter amplifier, and (ii) emitter follower. [10 marks]
(b) Sketch the overall amplitude-frequency response curve of an RC-coupled common-emitter amplifier. Explain the shape of this curve, with particular reference to the low frequency and high frequency responses, respectively. [5 marks]
(c) Explain the effect of the Miller capacitance on the bandwidth of a common- emitter amplifier, and indicate how this can be demonstrated using PSPICE computer-based circuit simulation. [5 marks]
Q4. (a) Explain the terms ‘negative feedback’ and ‘positive feedback’. Outline the difference between them, and give a circuit example in which each is applied. [6 marks]
(b) Derive the relevant equation to show that negative feedback in an amplifier circuit leads to a reduction in voltage gain, and list the effects of negative feedback on the other parameters of an amplifier. [8 marks]
(c) An amplifier without feedback provides an output signal of 12V, with 8% second-harmonic distortion, when the input signal is 12mV. The input and output impedances are both 2kΩ. If 1.5% voltage negative feedback is applied in series with the input, determine the output voltage, the percentage distortion, input and output impedances. [6 marks]
Q5. (a) Draw the circuit diagrams of (i) Wien bridge oscillator, (ii) Phase-shift oscillator, (iii) LC tuned collector oscillator. [6 marks]
(b) State the criteria required in a circuit for oscillations to occur and be maintained. What are these conditions usually called? [6 marks]
(c) For the circuit in part (a)(i), derive the equation for frequency of operation, and indicate how the amplitude of the output can be stabilised to ensure that saturation does not occur. [8 marks]
