ECE 3150 Lab 5 3/26/2010
A S
IMPLE
MOS D
IFFERENTIAL
A
MPLIFIER
1
Objectives
In this lab, you will examine the small-signal voltage transfer and output characteristics of a
simple MOS differential amplifier comprising an
n
MOS differential pair and a simple
p
MOS current
mirror. You will also investigate the large-signal range where the differential amplifier can work properly.
Although you will only make the large-signal measurements from the instrument, you can derive the
small-signal transconductance and voltage gains, as well as the small-signal output resistance in the quasi-
static operations.
You will then compare them with the intuitive models we develop in the class. A
realistic analog amplifier needs to contain three parts: large-signal biasing, small-signal amplification and
loads.
In the previous labs, you have only seen the individual parts.
The five-transistor differential
amplifier in this lab is one of the simplest but most elegant complete analog amplifiers.
2
Prelab
Unless otherwise stated, you should assume that like transistors are matched and that the Early
effect is negligible.
1. Consider the five-transistor circuit shown in Fig. 1, comprising an
n
MOS differential pair and a
simple
p
MOS current mirror. This circuit shows most of the important characteristics for an ideal
differential amplifier. As with the differential pair,
V
1
and
V
2
are the large-signal inputs to the
circuit, and
V
b
is a constant bias voltage that sets up the current flowing in the circuit. Suppose that
transistors
M
1
and
M
2
are well matched and that transistors
M
3
and
M
4
are also matched. Also,
suppose that the Early effect is negligible. If
V
1
and
V
2
were high enough to keep
M
b
saturated and
we were to hold
V
out
with a voltage source so that
M
2
and
M
4
are also saturated, what would be the
output current,
I
out
in relation to
I
1
and
I
2
?
2. If we were to let go of the output by disconnecting the voltage source, in what direction would
V
out
go if
I
out
were (a) positive, (b) negative, and (c) zero?
3. Which input is the noninverting input (i.e., the one that causes the input to move in the same
direction)?
Which input is the inverting input?
4. First assume the Early effect were negligible. If
V
1
=V
2
=V
CM
and if
V
out
were fixed as explained in
the first question, what would
I
out
be?
If we were to change
V
CM
, what would
I
out
be?
Under these
circumstances, what would be the incremental common-mode voltage gain of the circuit (i.e.,
A
cm
2245
δ
V
out
/
V
cm
)?
If we were to increase
V
dm
by a small amount, what would happen to
I
out
?
If we
were to let go of the output by disconnecting the voltage source, to what value would
V
out
eventually move?
Instead, if we were to decrease
V
dm
by a small amount, what would happen to