Electronics II, Exam-3, Spring 2021
Department of Communication Engineering, National Central University
11th June, 2021, Prof. Dah-Chung Chang (E1-311)
Note: The scientific calculator is allowed in all Electronics II exams.
1. (35%) In the circuit shown in Fig. 1, the active load circuit is replaced by a current source in
which the PNP transistors Q2 and Q3 are matched. The common-emitter current gain of
transistors is denoted as β. Suppose that I REF is obtained from another ideal current source.
(a) Derive the exact relationship between iE 1 and iC 2 , and explain in what condition we
have iE1 iC 2 . (10%)
(b) To find the output resistance of the current source looking into the collector of Q1 , we
may treat the emitter terminal of Q1 as virtual ground in small-signal analysis for
simplicity. Draw the small-signal equivalent circuit and write out the resistance looking
into the base and collector of Q3 from the emitter terminal of Q1 , and explain the
reason. (10%)
(c) Draw the small-signal equivalent circuit of the current source and show that the output
resistance RC1 ro1 / 2 , using the approximation obtained in (a) and (b). (10%)
(d) Assume that β = 250 for all transistors, and that VAN 150V , VAP =120V , and
I REF =1mA . Determine the small-signal voltage gain vO / vI . (5%)
Fig. 1
1
2. (30%) The circuit in Fig. 2 is a two-transistor PMOS current mirror. Based on the reference
current I REF , we want to design the PMOS transistor M 3 and to evaluate the maximum
output current I O for a given load resistance R.
(a) Write out the circuit equations that are required to obtain I REF , and explain the reason
why you construct those simultaneous equations and the processes how to solve for
I REF . (10%)
(b) Assume that the circuit and transistor parameters of M 1 and M 2 are V 3V ,
V 3V , VTP1 VTP 2 1V , K p1 1mA/V 2 , and K p 2 9mA/V 2 . Suppose that
R 12k , determine the maximum value of I O such that M 3 remains biased in the
saturation region. (15%)
(c) Determine the ratio parameters (W / L)3 of M 3 given that k p 0.1mA / V 2 and
I O 0.2mA . (5%)
Fig. 2
2
3. (35%) Consider the multi-transistor current source in Fig. 3. Assume that M 1 , M 2 , and M 3
are identical except for their geometry ratios, and the circuit and transistor parameters are
V 5V , VTN 1V , k n 80 A / V 2 , and 0 .
(a) Let I REF 0.1mA and the geometry ratio of M 3 be (W / L)3 4 . Assume that M 1
and M 2 have the same W / L geometry ratio. Determine the ratio and explain how
to use this assumption to obtain that value. (10%)
(b) Suppose that the transistor parameters of M 4 is the same as M 1 except that
4 0.05V 1 . Determine the geometry ratio of M 4 for at least I O1 0.5mA as M 4
remains biased in the saturation region. (10%)
(c) Assume that the transistor parameters of M 5 is the same as M 1 except that
5 0.01V 1 . What is the maximum value of RS for I O 2 40 A ? (5%)
(d) Let RS 10k . Determine the geometry ratio of M 5 for at least I O 2 40 A as
M 5 remains biased in the saturation region, and determine the output resistance R D 5
looking into the drain terminal of M 5 . (10%)
Fig. 3
