Electronics –II (ELEC 312), Winter 2009 class
Mid-term test (March 6, 2009)
( Time: 45 minutes
Student ID#
Instructor: Dr. R. Raut)
Name:
Q.1: Consider the amplifier in Figure 1, built with MOS transistors. The transistor M3 functions
as an active load for the transistors M1 and M2. M1 is a common source stage while M2 is a
common gate stage.
(a) Draw the small signal ac equivalent circuit for the system. Ignore the body effect for M2.
(b) Set up the nodal admittance matrix for the system with a goal to find the small signal voltage
gain vo/vi. You DO NOT have to find the voltage gain.
VDD
VG 2
VDD
M3
vo
VSS
VG1
M2
vi
VSS
M1
VSS
Figure 1:
Q.2: Figures 2(a)-(b) show the cascode and the Wilson current mirror using NMOS transistors.
The bias currents are set as IREF = Io = 20 micro amp. Given that VA=20 volts, Kn=100 micro
2 K n I BIASW
amp./V2, W/L=10, gm=
, gmb= 0.2gm, compare the output resistances that the two
L
mirrors present to an outside circuit. The theoretical expressions for the output resistances are:
(cascode) Rout= ( g m 4  g mb4 )ro3 ro 4 , (Wilson) Rout=
g m1 g m3
ro1ro3
g m2
Q.3 The BJT differential amplifier is supplied with a differential ac signal vD= v1-v2. The
differential output signal is given by the expression
exp( v D / VT )
exp( v D / VT )
vo1  vo 2  Rc I [

]
1  exp( v D / VT ) 1  exp( v D / VT )
Where VT is the thermal voltage (~ 25 mV). The bias current I is arranged to be 2 mA
Determine the voltage gain (vo1-v02)/vD, when (i) vD=20 mV and (ii) vD= 2 mV. How do these
compare with the theoretical small signal voltage gain value of | gmRC|, where gm is the transconductance of each BJT device.
10V
2 k

v1
vo2
vo1
vD
v
2
I

 10V
Figure 3
re   / gm
BJTs
 
h fe
h fe  1
Diodes
****** Some useful formulae**********
gm  IC /VT r  h fe / gm h fe  ic / ib ro  VA / I C
VT 
kT
 25mV at room temperature
q
I=Is exp(vBE/VT)