CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

3.1 A Differential Transimpedance Amplifier with Wide Dynamic Range 44
(3.1)
which is typically in the range of ten to a few hundred. Coincidentally, the same
expression is obtained for the traditional design assuming the output source fol-
lower has unity gain.
1V
x
B
M 1
3V
1V
i o
The stability of these circuits can be analyzed by breaking their feedback loops
at points B and C. Figure 3.2 shows the main features of the loop gain for both cir-
cuits. The frequency where the loop gain of the transimpedance amplifiers is unity is
approximately
R 1
R f
1V
v out
(3.2)
where is the photodiode capacitance. The stability of the amplifiers is deter-
mined by the relative position of ωt to the non-dominant pole, ω p2 . From Equation
(3.2), we see that ωt increases when R f is reduced. In contrast, we can assume the
non-dominant pole is not significantly affected by . High-frequency gain peak-
ing occurs when ωt comes too close to ω p2
. To prevent this, the unity-gain fre-
i o
⎛ ⎞
A = – ⎜------- × ----- × – 1⎟
⎝ io vin ⎠
≈ gm1 R1 -1
M 2
1V
v out
Figure 3.1 Two transimpedance amplifiers: a) proposed (local shunt feedback),
b) traditional.
C PD
1V
x
C
R 1
1
3V
2V
i o
M 1
a) b)
A gm1 R1 ωt ≈ ----------------- ≈ -----------------
R f CPD R f CPD R f
R f
1V v out