CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

3.4 SUMMARY
3.4 Summary 71
In this chapter, we presented circuit techniques to reject ambient light, to
enhance the dynamic range, and to enable the low-voltage operation of transimped-
ance amplifiers. Enhanced dynamic range was achieved through a fully-differential,
variable-gain, transimpedance amplifier. The amplifier uses an internal shunt feed-
back topology, and overcomes many of the stability challenges found in previous
designs. Simulation results were presented for a CMOS implementation that con-
sumed 8mW at 3V, and provided 70 MHz bandwidth over a 77dB dynamic range
with a maximum transimpedance gain of 20kΩ and a gain range of 32dB.
Ambient light rejection was achieved by placing the proposed transimpedance
amplifier within a larger feedback loop. The feedback topology eliminated the need
for large passive devices and improved the regulation of the photodiode bias volt-
age. We analyzed the stability requirements of this structure and its implications on
the component error amplifier. The low-frequency behaviour of the circuit was
found to be dependent on the ambient light level, and we discussed ways to regulate
the feedback loop to control this dependency.
Low-voltage operation was achieved with a novel transimpedance amplifier
structure and with dynamic gate biasing. The transimpedance amplifier was capable
of 1V operation without the use of low-threshold devices, yet provided a wide out-
put swing and maximized the available bias voltage for the photodiode. We
described dynamic gate biasing and the use of charge pumps to bias and tune MOS
resistors. The complex internal feedback mechanisms of the proposed amplifier
makes its analysis difficult, and motivates our investigation into the DPI/SFG
method, a graphical circuit analysis technique that is presented in the next chapter.