1.    a)     Consider the circuit of Figure Q1a.  The transistor can be considered to be ideal, it is operating in the active regime and VBE  is a fixed value of about 0.6 V.  If the resistor RC   is doubled in value, do you expect IC to increase, decrease or stay the same?  What would happen to IC if VCC is doubled? Explain the reasons for your answers.

b)    Figure Q1b shows a common-emitter amplifier circuit.  Sketch the small signal equivalent circuit for the amplifier and show that the theoretical maximum possible  voltage gain is given by 40VCC. Comment on the ideal values required for RS  and RB for the maximum gain condition.

c)    Figure Q1c shows a common emitter amplifier with emitter degradation.  Calculate the dc collector current required to produce an ac input resistance of 100 kΩ, Take RE = 100 Ω and the ac current gain of the transistor as βo  = 150.

d)     For the level shifter circuit of Figure Q1d, work out a value for the voltage difference (V1 – V2). The transistors are identical, dc current gains are large and VBE  = 0.6 V.

e)     Copy the table shown in Figure Q1e into your answer script and complete it with the words ‘infinite’ or ‘zero’, to specify the values of input and output resistance for the four generic types of amplifier (R = resistance and G = conductance).

f)     The triangular element in the circuit shown in Figure Q1f represents an ideal   current amplifier with a gain of 100. Work out the value of the voltage gain if Rg = 1 kΩ and RL  = 15 kΩ .

g)    Draw a mid-frequency ac equivalent circuit of the amplifier shown in Figure Q1g. (Note that bias and other components are omitted). Conduct an analysis to    find a value for the capacitor, Ci  to give a low-frequency roll-off point (- 3 dB) of 100 Hz. Which circuit capacitor is usually designed to set the low-frequency roll-off point in common-emitter amplifiers and why?

The ac current gain, βO  = 100 and the circuit is biased at IC = 0.1 mA.

h)    An operation amplifier with a slew-rate limit of 1 V/us is required to amplify a sinusoidal 100 kHz signal.  Calculate the maximum amplitude of the output voltage that can be achieved without distortion. What type of output waveform distortion is seen when slew rate occurs and why? (consider a 741 op-amp)

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2.            The figure below shows a Widlar current mirror to generate the dc current, IO.

a)    Neglecting base currents and assuming T1  and T2  are identical transistors, derive the design equation for the current IO, as given by:

3.    Figure Q3 shows an operational amplifier circuit.

4.    a)    List the advantages and disadvantages of the use of negative feedback in electronic systems.

b)    Draw a block diagram of an amplifier with feedback, labelling clearly the open loop gain, Aol and feedback fraction, β. Show that the gain with feedback is given by;

Under what conditions is the closed loop gain insensitive to variations in the open loop gain?

c)    Figure Q4c shows an amplifier with feedback.

i)       Identify the feedback topology and hence the type of amplifier.

ii)      Represent the circuit as a negative feedback system and hence estimate the appropriate gain, assuming that the open-loop gain of the amplifier is large Consider the open-loop gain and use the equations given in the ‘amplifier properties’ page to justify the assumption.

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