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(ELEC304)Midterm_2003Fall.pdf
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The Hong Kong University of Science and Technology
Department of Electrical and Electronic Engineering

ELEC304 Midterm Exam 17th Oct. 2003
Name: Solution Student Number:
Time: 16:00-17:30 (1.5-hours closed-book examination) Do ALL problems in the answer book provided
Q1. An amplifier with shunt-shunt feedback is shown in Figure 1. It is noted that VDD is a DC voltage source.

Figure 1 Find
vo
(a) The closed-loop gain . (10 marks)
i
in
(b)
The closed-loop input resistance seen by the current source. (10 marks)

(c)
The closed-loop output resistance. (10 marks)


Note: In parts (a), (b) and (c), you have to include ro of the MOS transistor and you have to show all evaluation steps. No mark is given without steps.
Q2. Consider a system with negative active feedback shown in Figure 2.1. The forward amplifier is M5 only, while the feedback network is composed of R1, R2 and a differential-input single-ended amplifier (a circuit made of active elements - i.e. transistors) with voltage gain A, infinite input resistance and output resistance Roa. One of the terminal of R1 senses the output voltage vO and the amplifier provides a feedback voltage vFB at the gate of M5. Signal subtraction is achieved at the source-gate voltage of M5 (i.e. vSG5 = vIN C vFB). Therefore, the feedback type is series-shunt feedback. Note that VB is a DC voltage source (i.e. NO AC signal is from VB).

Figure 2.1
(a)
Based on Figure 2.1, find the open-loop voltage gain with loading effect. (10 marks)

(b)
Based on Figure 2.1, find the feedback factor. (15 marks)

(c)
Based on Figure 2.1, find the closed-loop voltage gain o . (5 marks)


v
v
in
(d)
Based on Figure 2.1, find the closed-loop input resistance Rincl. (5 marks)

(e)
Based on Figure 2.1, find the closed-loop output resistance Rocl. (5 marks)


The system shown in Figure 2.1 can be implemented using transistors as shown in Figure 2.2, where VDD, VB, IB are DC voltage sources and DC current source, respectively.

Figure 2.2
vo
(f) Based on Figure 2.2 and the results from (a), (b), (c) (d) and (e), find , Rincl and Rocl.
v
in

(5 marks)
Note: In parts (a), (b), (c), (d), (e) and (f), you have to include all ro of all transistors and show all evaluation steps. No mark is given without steps.
Q3. Figure 3.1 and Figure 3.2 show two differential-input single-ended-output amplifiers.

Figure 3.1 Figure 3.2
(a) What is the definition of differential gain? (1 marks)
Given that in both amplifiers, their VDD, IB and transistor sizes are all identical. You have to answer parts (b), (c) and (d) without using calculation but just by intuitive explanation.
(b) Which amplifier has faster small-signal speed? Why? (8 marks)
(c) Which amplifier has a lower offset voltage? Why? (8 marks)
(d) Which amplifier can accept a larger amplitude of small signal? Why? (8 marks)
- END OF MIDTERM EXAMINATION -

Q1.


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