ELEC2133: Analogue Electronics FINAL EXAMINATION Term 2, 2019
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit
ELEC2133: Analogue Electronics
FINAL EXAMINATION
Term 2, 2019
QUESTION 1 [20 Marks]
Two amplifier circuits with CE and CC-CE configurations are shown in fig. 1(a) and (b), respectively. The parameters for the transistors in the circuits are provided in the information sheet on pages 2 and 3. Note that the bias details are not shown.
0.1mA
Vo
CL=1pF
Vs
(a)
0.1mA
Vo
CL=1pF
0.1mA
(b)
Fig. 1: (a) CE amplifier (b) CC-CE cascade amplifier
(a) Calculate the input resistance, Rin, for CE and CC-CE amplifiers.
(b) Determine the midband gains for both amplifiers.
[6 marks] [8 marks]
(c) How do you compare the two amplifiers in terms of gain-bandwidth product? Note that the bandwidth for each amplifier is the upper 3-dB frequency, fH . If you do not wish to calculate the upper 3-dB frequencies for the amplifiers, then you will need to justify your response with short explanation.
[6 marks]
QUESTION 2 [40 Marks]
(a) Compare the stability behaviours of a single pole, double pole, and three pole systems with negative feedback. You may use root locus or Bode plot to support your explanation.
[6 marks]
(b) With the aid of a diagram, explain base-width modulation characteristic of a BJT and its implication in the small signal model of the transistor. What is responsible for similar behaviour in MOSFET?
[6 marks]
(c) Explain briefly the following terms used to describe the performance of analogue-to- digital converters.
i. Differential non-linearity error [2 marks]
ii. Missing code [2 marks]
(d) A monostable multivibrator circuit is shown in fig. 2(a). The circuit can be triggered by applying a narrow positive input pulse of height greater than Vref. For normal operation, C1R1<< CR. The saturation voltages of the op-amp are VL and –VL .
VB
Trigger
C1
Vo
VA
C
R
Fig. 2(a): Monostable multivibrator
i. In stable state, what are the values of Vo, VA, and VB? [3 marks]
ii. Show the resulting waveforms of Vo and VA after triggered.
[5 marks]
iii. Show that the pulse generated at the output will have a width T given by
T = CRln () [8 marks]
(e) The ladder type of Digital-to-Analogue converter (DAC) is shown in fig. 2(b). The blocks labelled Si represents a switch that connects to Vref when the bit ai=1 and ground when the bit ai=0. Derive an explicit expression for the output voltage Vo in terms of ai
of the binary word. [8 marks]
Fig. 2(b): Ladder type DAC
QUESTION 3 [40 Marks]
Fig. 3 is a three stage amplifier with feedback utilizing MOSFETs. All the three MOSFETs are biased and sized to operate at gm = 4mA/V. You may use the small signal model parameters provided in the information sheet on page 2-3. Furthermore, you may assume the source Vs has no dc signal component.
Fig. 3: Three stage amplifier with feedback
(a) Identify the type of feedback topology employed in the amplifier of fig. 3 assuming the amplifier is a trans-conductance amplifier with output current Io .
[5 marks] (b) Find the value of RF that results in a closed loop trans-conductance (Io/Vs) of
approximately 100mA/V. {you may assume the open loop gain to be large}
[8 marks]
(c) Draw the open loop amplifier circuit taking into account the loading effect of the feedback network.
[6 marks]
(d) Hence, calculate the mid-band open loop gain of the amplifier.
[8 marks]
(e) Determine the closed loop trans-conductance gain (Io/Vs) of the amplifier.
[4 marks] (f) Calculate the output resistance, Rout1, of the closed loop amplifier at mid-band.
[4 marks] (g) Now assume the output is Vo, what will be the type of feedback topology employed in
the amplifier? Identify the associated feedback network. [5 marks]
2022-08-17