CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

3.2 A Feedback Topology for Ambient Light Rejection 52
by the differential structure. In practice, however, some mismatch can be expected,
and additional measures such as adding an on-chip tuning capacitor [Yoon,1997]
may be required to improve matching. Another solution is to use the differential
topology presented in [Zand,1999] that altogether eliminates the need for the
dummy capacitor.
The characteristics of the ambient photocurrent rejection circuit can be studied
by breaking the loop at point A in Figure 3.7. The loop gain is given by
where ATIA( s)
and Aerr( s)
are the frequency responses of the transimpedance
(3.7)
amplifier (TIA) and error amplifier respectively, and is the transconductance
of . Since the error amplifier acts as an integrator, in the ideal case its fre-
quency response is given by
where ωTerr is the unity-gain frequency of the error amplifier. The resulting closedloop
response of the feedback structure is
where
v out+
v out-
average+
average-
a) b)
average
Figure 3.8 Differential output waveforms a) without ambient photocurrent
rejection, b) with rejection.
M ctl
Ls ( ) = ATIA( s)
× Aerr( s)
× g
mctl
Aerr( s)
= ωTerr ⁄ s
g mctl
vout ATIA( s)
s
-------- ( s)
= --------------------- ≈ ATIA( s)
× --------------------------is
1 + Ls ( )
1 + s ⁄ ωHP ωHP =
ATIA( 0)
× ωTerr × gmctl (3.8)
(3.9)