CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

Transimpedance (dBΩ)
90
80
60
40
20
5nA
50nA
500nA
5µA
3.2 A Feedback Topology for Ambient Light Rejection 55
0
100Hz 10kHz 1MHz
100MHz 10 GHz
Figure 3.11 Frequency response of preamplifier with dc photocurrent rejection for
different current levels.
ambient light as the level increases. When the level is low, the feedback loop — or
more precisely, transistor M ctl — is nearly off. Since M ctl is located right at the
input, any thermal noise it generates adds directly to the signal current. Having Mctl turn off maximizes the preamplifier’s sensitivity in low ambient light. When the sig-
nal is much stronger than the ambient light, the saturation of the preamplifier can be
prevented by simply adjusting the preamplifier gain. Thus, the ambient photocur-
rent rejection circuit is not critical here, and can be disabled or limited in order to
eliminate this dependency on .
In situations involving large dc photocurrents, provisions are required to limit
both and the highpass cut-off frequency in order to prevent the preamplifier
g mctl
I dc
increasing
I dc
frequency
from filtering the signal. For instance, from Figure 3.11 we see that at 5µA, the cut-
off frequency exceeds 1 MHz, and the outer feedback loop is beginning to affect the
passband response of the transimpedance amplifier. Ideally, we would like some
method of limiting the current that passes through transistor M ctl . This limit must be
greater than the ambient photocurrent generated by the photodiode under the bright-
est conditions (e.g. direct sunlight). A potential solution would be to redirect the
excess photocurrent into another current source that is static, but which can be
switched on when needed. In this way, the new current source does not change the