ELE311
EXAMPLES
1.
For the circuit shown below, determine the differential and common mode input
resistances calculate R
id
and R
ic
respectively. (2002 Midterm I)
ANSWER:
I
0
= I
REF
= (V
CC
– V
BE
– V
EE
) / R
1
= (5-0.7+5)/18.6k = 0.5mA
I
E1
= I
E2
= I
0
/ 2 = 0.25mA
h
ie
≅
(h
fe
+ 1) V
T
/ I
EQ
= 101
x
26mV / 0.25mA = 10.8k
Ω
In the differential mode, i.e. v
i2
= -v
i1
.
R
id
= 2h
ie
= 2 (10.4k) = 21.6k
Ω
In the differential mode, i.e. v
i2
= v
i1
.
Let us define
r
o1
= 1/h
oe1
, r
o2
= 1/h
oe2
, r
o4
= 1/h
oe4
R
ic
=
(h
fe
+1)(r
o4
|| r
o1
|| r
o2
) = (100 + 1)(200k || 400k || 400k) = 10.1 M
Ω
.
Q
1
≡
Q
2
h
fe
= h
FE
=100
V
BE
= 0.7V
1/h
oe1
=1/h
oe2
= 400k
Ω
1/h
oe4
= 200k
Ω

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