CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

3.3 A Low-Voltage Transimpedance Amplifier 61
sistor sizings and resistor values. The responses vary significantly in their resulting
bandwidths and gains, and the trade-offs become even more complex when we also
consider their noise performance. The analysis of this circuit is challenging. As we
shall see, this circuit possesses no less than three feedback loops that are coupled
together, making it difficult to apply the traditional feedback analysis techniques
reviewed in Chapter 2. Nodal analysis is an option, but the resulting transfer func-
tions are too cumbersome to help guide our design. As an example, the transimped-
ance gain of the circuit is given by
where
g m2 v A
vA
g m3 v A
Figure 3.16 Small-signal circuit for determining the preamplifier output
resistance.
vout iin -------- ( s)
=
– gm3 --------------------------------------------------------------------------------------------------------------
Y AY 1Y f
-------------------- +
Y f
--------Y 1( Y A + Y L)
+ Y AY L +
g m1
Y 1
g m1
Y f
gm3Y f -------- + gm2Y f + Y f ( Y L + Y A)
+ ( gm2 + gm3)Yf g m1
(3.19)
(3.20)
CPD is the photodiode capacitance, Cin and Cout are the total input and output
capacitances contributed by the amplifier, is the total capacitance seen on node
A, and and are the shunt feedback resistor and capacitor. Similarly complex
R f
C f
expressions can be derived for the input and output impedance and noise densities
R f
Y 1 = sC ( PD + Cin) Y L =
=
sC ( L + Cout) Y A
Y f
=
sC A
1 ⁄ R f + sC f
C A
i x
+
v x
-