CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

Optimizing Sensitivity
5.2 Developing an Analytic Circuit Model 131
Thus far, we have identified a region of the design space that provides maxi-
mum gain for a given bandwidth. We must now consider sensitivity. Using the
expression for the input-referred dc noise current in Equation (5.29), we can plot the
noise performance across the design space as shown in Figures 5.22 and 5.23. The
vertical axis is in dB relative to 1 pA ⁄ Hz.
We see here that within the optimized
region projected from Figure 5.20 and marked in grey, the noise level within this
region is about 16dB or 6 pA ⁄ Hz.
The lowest noise performance is achieved by
maximizing both and . Hence, the optimum design choice is
R f
K cm
R f = 5kΩ,
K cm = 16dB .
Notice, however, that the noise performance is rather insensitive around the opti-
mum point, varying by only about 2dB in the grey region. This affords the designer
greater flexibility to deviate from the optimum point without too much concern
from a sensitivity standpoint.
Average input−referred Noise Current (pA/Hz 1/2 ) dB
50
45
40
35
30
25
20
15
10
0
5
10
R f (kΩ)
15
20
10
0
20*log 10 (K cm )
Figure 5.22 The dc input-referred noise current vs. R f and K cm
in dB.
−10
−20