CMOS Optical Preamplifier Design Using Graphical Circuit Analysis

6.2 Variable-Gain Transimpedance Amplifier with Ambient Light Rejection 158
short focal length (i.e., 1.5 mm) of the LED lens. Table 6.2 summarizes the perfor-
mance of the transimpedance amplifier.
Transimpedance gain [dBΩ]
Technology 0.35 µm digital CMOS
Supply voltage 3 V
Bandwidth 70 MHz ± 20 %
Transimpedance 19kΩ−500Ω
Input noise current density 6.7 pA ⁄ Hz
Input capacitance 5 pF
Maximum signal current 400 µA
Preamplifier active area 0.04 mm
Power dissipation 8 mW (preamplifier)
43 mW (output buffer)
2
Table 6.2 Performance summary of the transimpedance amplifier.
90
85
80
75
70
65
60
55
50
45
85 MHz
103 MHz
101 MHz
96 MHz
89 MHz
40
1 10 100 150
frequency (MHz)
Measured
------- Simulated
Figure 6.23 Simulated and measured frequency response of transimpedance
amplifier.
The operation of the ambient light rejection feedback loop is illustrated in the
step response shown in Figure 6.25. Here, a 10 µA dc current is injected into the
preamplifier. The dc rejection loop is initially disabled, and then activated at the