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Mock Paper. 3. In the circuit of figure 3, the transistors are identical and have the parameters: b = 150, C BE = 10 pF, C BC = 4 pF. What name is given to the amplifier configuration shown in figure 3? Cascode amplifier

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Mock paper
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


(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.


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

  • = 150, CBE = 10 pF, CBC = 4 pF.


(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.


(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.


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 = 1.38×10-23 J/K

q = 1.6×10-19 C


(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 = 1.38×10-23 J/K

q = 1.6×10-19 C


4 across the resistor due to shot noise in the diode current (again in V/

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:


  • (b) In a common-emitter amplifier: across the resistor due to shot noise in the diode current (again in V/

  • 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


(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


(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


(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