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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

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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


Mock paper

(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.


Mock paper

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

  • = 150, CBE = 10 pF, CBC = 4 pF.


Mock paper

(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.


Mock paper

(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.


Mock paper

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


Mock paper

(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


Mock paper

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:


Mock paper

  • (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


Mock paper

(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


Mock paper

(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


Mock paper

(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


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