CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

3.3 A Low-Voltage Transimpedance Amplifier 57
impedance looking into the source of M 1 is much lower than the impedance looking
into the drain of M 2 , the input current will be redirected up through M 1 and into
node A. The injected charge will adjust the gate voltage of M 2 such that at steady
state, the input current is redirected down through M 2 . Since M 3 has the same gate-
to-source voltage as M 2 , the input current is duplicated at the output. However,
unlike a normal mirror whose input is biased at , the input voltage here is
( V bias – V GS1)
which is adjustable and can be reduced to V DSsat2 , the saturation
voltage of M2 , before device M2 drops out of the active mode of operation.
A transimpedance amplifier based around a current-gain amplifier is shown in
Figure 3.14. The amplifier uses the traditional transimpedance structure of a gain
stage shunted by a feedback resistor, but the traditional voltage amplifier is replaced
by a current amplifier. The schematic convention used here is adopted from
[Johns,1997] 2 . Here, the arrow marks the input and the direction of current flow, and
A i
is the open-loop current gain. It can be shown that the transimpedance gain of
this structure is
R in
where is the amplifier’s input resistance which is typically low. A direct imple-
mentation of Figure 3.14 using the current mirror in Figure 3.13 would require that
2. Chapter 6, p. 266.
i in
A
M 1
M 2
I B
v bias
V DD
V GS2
M 3
Figure 3.13 Sub 1-V current mirror.
v out
--------
i in
R f Ai – Rin = – --------------------------- ≈ – R f
1 + Ai I B
i out = i in