Q1

(a)     Sketch the linear equivalent circuits that represent the behaviour of

(i)      a non-ideal voltage amplifier

(ii)     a non-ideal transconductance amplifier

In both cases, explain the function of any passive components included in the equivalent circuits and state what their ideal values should be.

(4 marks)

(b)     An amplifier operating from ±14V provides an output voltage of 9V rms and an output current

of 14mA peak to a resistive load when a signal of 2.8V peak-to-peak is connected to its input. The current drawn by the input port of the amplifier is 0.1mA rms. A current of 10mA DC is provided by each power supply. Calculate

(i)       the voltage and current gains of the amplifier in dB

(ii)      the power gain of the amplifier in dB

(iii)     the efficiency of the amplifier

(6 marks)

(c)     The amplifier shown in figure Q1, is based on an op-amp which is characterised by a finite open-loop gain, A.

(i)       Derive an expression for the voltage gain of the amplifier vO /vI  as a function of R1, R2 and A

(ii)      Based on the expression derived in (i), state what the relationship between R1, R2 and

A should be in order to minimise the effect of a finite open-loop gain.

(iii)     If A is infinite, show how the gain expression may be simplified.

(10 marks)

Figure Q 1

Q2

(a)     For the circuit in figure Q2, assuming that the op-amp is ideal in all respects,

(i)      Derive an expression for the gain of the circuit vO /vI as a function of frequency

(ii)     Plot the magnitude of the gain in dB versus 业 using a Bode plot. Use the following values

for the  passive  components:  C = 50nF, R = 20kΩ and RF = 2MΩ . Label the  axes carefully so that the slope of any lines may be clearly seen.

(iii)    Derive an expression for the phase of the gain as a function of frequency

(15 marks)

(b)     (i)       Redraw the schematic in figure Q2, using an op-amp model which takes into account its

input bias currents and input offset voltage.

(ii)      Explain what would happen in a practical laboratory environment if the resistor RF was

removed from the circuit in figure Q2

(5 marks)

Figure Q 2

Q3

(a)     The circuit in figure Q3 is an instrumentation amplifier  based on op-amps which may  be

assumed ideal in all respects. The voltage vO  may be expressed as a function of v3, v4, R3 and R4 as shown below

vO  =  (v4  − v3 )

Assuming that R4=R3, derive an expression for vO  as a function of v1, v2, R1 and R2

(10 marks)

(b)     (i)       Describe  the  differences  between  an  instrumentation  amplifier  and  a  difference

amplifier.

(ii)      Show how the voltages v1, v2  shown in figure Q3 may be represented by means of

common mode and differential components

(iii)     Explain the term Common Mode Rejection Ratio of an amplifier and describe briefly the

circumstances where this parameter is important in modelling the behaviour of a circuit.

(10 marks)

Figure Q 3