CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

4.5.2 Transistor Circuit Examples
EXAMPLE 4.4 A SOURCE FOLLOWER AMPLIFIER
4.5 Analyzing Transistor Circuits 100
We want to determine the gain of the source follower amplifier shown in Figure
4.24. The amplifier is biased with a simple current mirror, and it can be easily shown
that the impedance looking into the output of the current mirror is simply . As
such, the SFG for the amplifier is essentially the SFG of transistor M 1 as shown in
Figure 4.25. Since the drain of M 1 is connected to ground in the small-signal sense,
much of the SFG for the MOSFET can be eliminated, as illustrated by the dashed
lines. The gain of the amplifier can be seen to be
v in
(4.16)
which is the expected voltage gain of a source follower when the body effect is
taken into account.
vout r || ds1 rds2 -------- = gm1 × ---------------------------------------------------------------------------
1 + ( r ||
ds1 rds2) × ( gm1+ gs1) g m1
= ------------------------------------------------------------------------gm1
+ gs1 + 1 ⁄ rds1 + 1 ⁄ rds2 ≈
gm1 -----------------------gm1
+ gs1 I bias
V DD
v in
Note that the final gain expression in Equation (4.16) can be obtained directly by
applying the Source Absorption Theorem. 4 From a DPI/SFG perspective, the trans-
M 1
M 3 M2
v out
Figure 4.24 Source follower amplifier.
r ds2