CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

2.3 Transimpedance Amplifier Design Requirements 17
The simplest technique is to limit the output swing as represented by the diode
clamp in Figure 2.7. While limiting does not affect the lower limit of the dynamic
range, it does increase the upper limit by allowing the receiver to accept strong sig-
nals that would have otherwise overloaded the receiver and prevented normal opera-
tion. Limiting can be performed either within the preamplifier [Yamazaki,1997], or
in cases where the dynamic range requirements are more modest, externally by fol-
lowing the preamplifier with a limiter circuit [Nakamura,1999], [Ohhata,1999]. The
advantage of limiting is that it does not require level detection circuitry. However,
the process of limiting destroys the amplitude information of the received signal. As
such, limiting can only be used with binary signalling schemes. In addition, limiting
introduces pulse width distortions that result out of the uneven gain applied to dif-
ferent portions of the pulse. Finally, for applications in which ambient light is an
issue, limiting removes information that helps separate the ambient light from the
information signal.
2
I tail
V ctl
Level
Detection
Figure 2.7 Various methods of increasing dynamic range: 1) output signal limiting,
2) input current steering, and 3) variable transimpedance gain.
The second technique, input current steering, also improves the dynamic range
by increasing the maximum acceptable signal level of the preamplifier. It uses a dif-
R f
r f
1
3