c)

Figure Q2c) shows an amplifier system. The source has low internal impedance and the load has a large resistance. Choose a suitable generic amplifier type to ensure good matching between source and load.  Hence sketch a systems diagram with appropriate equivalent circuits for the source (Thevenin or Norton) and generic amplifier type.

Figure Q2c

5

d

Figure Q2d shows a schematic diagram of a feedback amplifier with bias components and coupling capacitors omitted for clarity.  Identify the feedback topology and the amplifier type. Hence calculate the feedback fraction, β.

5

Figure Q2d

e)

How would you design a near ideal voltage amplifier incorporating negative feedback?  What would be the advantages and disadvantages of your design compared to a voltage amplifier without negative feedback?

5

Total

25

3.

a)

Compare and contrast the use of bipolar and MOSFET transistors in analogue circuits.

5

b)

Figure Q3b shows a two-stage voltage amplifier with bias components, coupling and DC blocking capacitors omitted for clarity.

Figure Q3b

You are required to design the amplifier to achieve an input resistance, R_i = value given on cover sheet, and an overall voltage gain equal to or greater than 250, into a resistor output load, R_L.  The common-emitter stage (T₂) has bias current equal to 1.0 mA.

V_CC = 20 V, β_o = 150, R_S = 10 kΩ and the DC levels of the first and second stage outputs need to be set at half the supply voltage. Allow 10% of V_CC across R_E.

Proceed as follows:

i)

Use the chain rule to show that the system voltage gain is given as:

2

ii)

Work out the bias current (I_C1) and emitter resistor value to meet the input specification. Hence find the value of the collector resistor of the first stage.

6

iii)

iv)

Work out the gain of the first stage and hence find the required gain of the 2nd stage to meet the specification. Find also a value for R_C2.

Find a minimum value of the load to achieve the required gain.

4

4

c)

Comment on the ‘quality’ of your voltage amplifier and suggest how it could be improved.

4

Total

25

4.

a)

Explain what is meant by the term ‘small signal equivalent circuit’. How is it used in electronics design?

5

In the circuit shown in figure Q4a, biasing components and coupling capacitors  have been omitted.  The collector currents in transistor T₁, T₂ are 0.1 mA, 0.5 mA respectively, while the collector current for T₃ is given on the cover sheet. Calculate:-

b)

the transistor transconductance (g_m) and emitter-base resistance (r_be) of each transistor.

3

c)

the voltage gain v_o/v_i ,

11

d)

the input resistance R_i,

2

e)

the output resistance R_o,

1

f)

Explain the role of each stage.

3

R_E1 = 1 kW, R_C2 = 10 kW, R_C3 = 2 kW and R_E3 = 200 W.

Assume r_ce and r_b’c are infinite and r_bb’ = 0. The a.c. current gain is b_o = 100.

Figure Q4b