Introduction to Microelectronics
Chapter 5
S
INGLE
S
TAGE
A
MPLIFIERS
WITH
C
URRENT
M
IRROR
B
IASING
5.1
LargeSignal Biasing with Resistive and Capacitive Elements
We will first start the Chapter with resistortransistor circuits, because the concept is a bit
closer to what you have learned in the basic circuit course.
Do remember that passive elements
of resistors and inductors are very expensive in IC implementation in terms of the layout area,
and their values are not very accurate.
Therefore, our end goal is still to do the biasing and loads
by transistors whenever possible.
Conventionally this is called the “active” loads, in comparison
with the “passive” loads if resistors are used.
For many circuits, active loads are actually much
better and more flexible than the resistive loads, as you will soon find out.
For the resistortransistor circuits in Fig. 1a (this circuit can serve as an amplifier or an
inverter), we can find the circuit solution by:
(
29
(
29
DD
D
D
th
GS
n
DS
GS
D
V
V
RI
V
V
k
L
W
V
V
f
L
W
I
=
+

=
=
2
'
2
?
,
(5.1)
where the “?=” sign means that
I
D
may be equal to the quadratic equation IF the saturation region
condition of
V
DS
> V
Dsat
= V
OV
can be satisfied.
Because the equation in saturation (mostly
depends on
V
GS
and almost independent of
V
DS
) is much simpler than that in linear (depends
almost equally on
V
GS
and
V
DS
), the convenient way to solve this circuit is:
1.
Assume
NMOS in saturation.
2.
Solve KVL ignoring the Early effect.
Edwin C. Kan
Page
51
10/25/2009
I
D
V
DS
Fixed
V
GS
> V
th
R
+

+

V
DD
V
IN
V
OUT
Fig. 5.1.
(a) Resistive loads for transistor circuits; (b) Loadline solutions.
Q
1
I
D
V
Dsat
=V
GS
– V
th
V
DS
V
DD
1/R
large
R
small
R
r
o
(a)
(b)
1
2
3
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Introduction to Microelectronics
3.
Check
if
V
DS
can make NMOS saturate by
V
DS
> V
Dsat
= V
OV
.
4.
If not, we will need to solve the
V
GS
and
V
DS
together in a coupled 2
×
2 matrix.
We can also investigate the solution from the load line plot in Fig. 1b.
We can see for a
given
V
GS
, different values of the load resistor can cause different behavior in the smallsignal
operations at the
Q
points 1, 2 and 3 as indicated.
The smallsignal model for the circuit is
shown in Fig. 2.
Fig. 5.2.
The small signal model for the NMOSresistor circuits.
At the
Q
point 1 in Fig. 5.1, the transistor is in saturation, so
r
o
is reasonably large, and there is a
sufficient swing of
V
OUT
where the transistor can remain in saturation and
A
v
remains large.
At the
Q
point
2 with a smaller load resistance,
A
v
=
g
m
(r
o
R)
gets smaller due to
R
.
At the operating point 3 with a
larger load resistance, the transistor can be easily kicked out of saturation and enters into the linear region
with very small
r
o
.
Again,
A
v
will become small.
We will look at another transistorresistor circuit in Fig. 5.3 to improve your intuition.
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 Spring '07
 SPENCER
 Amplifier, Microelectronics, Transistor, Input impedance, Voltage source, Current Source, Buffer amplifier, Edwin C. Kan

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