Mock Paper

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Mock Paper
3. In the circuit of figure 3, the transistors
are identical and have the parameters b 150,
CBE 10 pF, CBC 4 pF.

• What name is given to the amplifier configuration
  shown in figure 3?
• Cascode amplifier
• What is the key advantage of this circuit
  compared with the common-emitter amplifier?
• Higher upper cut-off frequency

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(c) Calculate the quiescent base, emitter and
collector voltages for the two transistors and
the collector current of Q1.
Quiescent assumptions IB 0, VBE 0.5 V 24V
dropped across three 100kW resistors, 8V across
each

• 150, CBE 10 pF, CBC 4 pF.

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(d) Calculate the mid-band gain, the input
impedance and the output impedance of the circuit.

• 150, CBE 10 pF, CBC 4 pF.

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(e) Calculate the lower and upper cut-off
frequencies of the amplifier for a source
impedance of 1 kW.
Lower cut-off

• 150, CBE 10 pF, CBC 4 pF.

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(e) Calculate the lower and upper cut-off
frequencies of the amplifier for a source
impedance of 1 kW.
Upper cut-off

• 150, CBE 10 pF, CBC 4 pF.

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In the circuit shown in figure 4, assume that
the forward biased diode voltage is 0.5
V. (f) Calculate the thermal noise voltage that
appears across the resistor in terms of V/ÖHz
T 300 K k 1.3810-23 J/K q 1.610-19 C
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(g) Calculate the additional noise voltage that
appears across the resistor due to shot noise in
the diode current (again in V/ÖHz). Hence
calculate the total noise voltage across the
resistor.
T 300 K k 1.3810-23 J/K q 1.610-19 C
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4 The total maximum power dissipation of a
class-B power amplifier is calculated to be 15 W.
Given output transistors with a specification of
TJmax 150 C, qJA 40 C/W, qJC 1.5
C/W i. Calculate the power dissipated by each
output transistor.
ii. Calculate the minimum specifications for
heatsinks that could be used for each transistor.
iii. Calculate the minimum specification for a
single heatsink that could be used by both
transistors.
Single heatsink must be twice the specification
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• (b) In a common-emitter amplifier
• Explain why the base-collector capacitance of the
  transistor usually has the most influence over
  the upper cut-off frequency of the amplifier.
• In a common-emitter amplifier, the
  base-collector capacitance is multiplied by the
  amplifier gain (plus one) due to the Miller
  effect. Consequently, it would usually appear to
  be an order of magnitude larger than the
  base-emitter capacitance
• Suggest three ways in which the upper cut-off
  frequency of an amplifier can be increased.
• Decrease the gain (reduces Miller effect)
• Use a transistor with lower junction
  capacitances
• Adopt a cascode configuration

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(c) Explain, using supporting diagrams, how
generalised impedance converter(s) can be used to
simulate i. A grounded inductance
i.e. the circuit has the same input impedance as
an inductance of CR2 Henries
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(c) Explain, using supporting diagrams, how
generalised impedance converter(s) can be used to
simulate ii. A floating inductance
Again, the circuit has the same impedance as an
inductance of CR2 Henries
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(c) Explain, using supporting diagrams, how
generalised impedance converter(s) can be used to
simulate iii. A frequency dependent negative
resistance
i.e. a frequency dependant negative resistance