CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

a)
b)
6.1 A 1V Optical Receiver Front-End 150
Figure 6.16 Eye diagram from preamplifier stage at 75 Mb/s with 3µA input
signal: a) 250MHz bandwidth, b) 100MHz bandwidth.
Numerous measures were required to determine the cause of the high-frequency
signal observed at the preamplifier. We first eliminated any external sources of elec-
tromagnetic interference by moving our testing into an EMCchamber. Doing so
effectively removed some spectral peaks around 100MHz that were caused by FM
radio transmissions. These peaks were present in some of our earlier measurements
that are not included here. We then remeasured the noise spectrum and frequency
responses of the signal path, this time from 1MHz up to 500MHz. The output noise
spectrum did not show any peaks at high frequencies, thereby eliminating the possi-
bility that the circuit was oscillating. The frequency responses at the three stages are
shown in Figure 6.17. The figure clearly indicates a resonant peak near 200MHz.
We resimulated the circuit using the extracted layout including parasitics, and found