CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

5.2 Developing an Analytic Circuit Model 117
Thus, the pole frequency, ωo , of the complex-conjugate pair is given by
(5.16)
Figure 5.10 shows a plot of ω p1 and ωo as a function of R f and the current mirror
gain. For most of the design space, ωo is significantly higher than ω p1 . However,
along the path marked by the arrow, ωo is closer to ω p1 . By adjusting the Q of the
complex-conjugate pair, it is possible to provide some high-frequency boosting to
help extend the bandwidth of the preamplifier.
Pole frequencies(MHz)
500
450
400
350
300
250
200
150
100
50
0
0
5.2.2 Modeling the Amplifier Noise
The sensitivity of an optical preamplifier is limited by its noise performance.
Figure 5.11 shows the thermal noise sources found within the low-voltage optical
preamplifier. Although MOSFETs also produce flicker noise, flicker noise can be
ignored in our design because the high bandwidth of the preamplifier makes thermal
noise dominant. Figure 5.12 shows the preamplifier’s SFG with the additional noise
sources.
5
R f (kΩ)
2
ωo 10
R f (kohms)
gm3 ------------
R f Ci Cin + R f CL( gm2 ⁄ gm3) ( gm1 + gs1) = × ----------------------------------------------------------------------------------------
C AC L + C f ( C A + CL) 15
1 o f
20
15
10
5
0
−5
−10
Current 20*log (Gain) Gain Kcm (dB)
10
Figure 5.10 Plotting the preamplifier’s pole frequencies vs. feedback resistance and
current mirror gain.
ω o
ω p1
−15
−20
surface —
mesh —
ω p1
ωo