1    a)      Consider the circuit of  Fig Q1a).  The transistor can be considered to be ideal and VBE is a fixed value of about 0.6 V.  (Assume the transistor always operates in the active regime.)

i)          If the resistor RC  is doubled in value, do you expect IC  to increase, decrease or stay the same?

ii)         What would happen to IC  ifVCC  is doubled?

Briefly explain the reasons for your answers.

b)     Copy table Q1b) into your answer book. Complete the table with the words ‘infinite’ or ‘zero’, to specify the values of input and output resistances for the four generic types of amplifier.  All amplifiers can be considered ideal.

c)    Figure Q1c) shows an amplifier system. The source has low internal impedance and the load has a small 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.

d)    An ideal transconductance amplifier is connected as shown in Fig. Q1d).  The overall voltage gain of the circuit, vo/vg  is 300 when RL  is 10 kΩ. Calculate the gain of the ideal transconductance amplifier.

e)    Use the given equation for the drain current of the MOSFET in saturation to show that the transconductance, gm  and output conductance, gds  of the MOSFET are given by the following expressions:

f)     What is the function of the section of circuit shown in Figure Q1f).

Calculate the value of Io  ifR2 = 4R1  and RE  = 7.3 kΩ .

g)    An operation amplifier with a slew-rate limit of 1V/µs is required to amplify a sinusoidal 100 kHz signal.  Calculate the maximum amplitude of the output voltage that can be achieved without distortion.

h)    Figure Q1.h) shows a schematic diagram of a feedback amplifier with bias components and coupling capacitors removed.  Identify the feedback topology and the amplifier type. Hence write down an expression for the feedback fraction, β .

Total       40

i)          Draw a schematic circuit of your amplifier, omitting the bias resistors and coupling caacitors.

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

iii)        Work out the gain of the first stage and hence find the required gain of the 2nd stage to meet the specification.

iv)        Estimate the minimum value of load resistance, RL  to satisfy the specification of voltage gain.

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

Total 20

3.            Figure Q3 shows an amplifier where the bias resistors and coupling capacitors are omitted for simplicity.

The amplifier operates with a bias current of IC  = 1 mA and you may assume that   rbb’ = 0 Ω.  Other parameters are βo  = 100, RC  = 2.5 kΩ, Cbc  = 1 pF, RS  = 1 kΩ and  with fT  = 200 MHz,.  (recall that  g   = 40 × IC  ).

a)    Draw the ac equivalent circuit of the amplifier at high frequency.  Apply Miller’s theorem, and hence represent the circuit as two independent first order circuits. Estimate values for the input and output time constants (recall that gm  × rbe   = βo   ).

b)    Hence show that the voltage gain can be given by:

where the bandwidth,fH   is given by  with R = rbe //RS and

C ≈ Cbe  + g m RC Cbc

Calculate a value for the bandwidth.

[Ensure that you show all necessary circuit diagrams in your solutions]

c)    The transistor is now loaded with a capacitor of value 1 nF (attached across the output terminal and ground).  Estimate the new time constant for the output side of the circuit and hence the bandwidth.  Comment on the result.

Total 20

4    a)        Draw the block diagram of a simple negative feedback system. Define the terms: open loop gain, Aoℓ , feedback fraction, β and closed loop gain,  Af. Hence derive the  expression for the closed loop gain:

Total 20