CMOS Receivers for Optical Wireless Communications
Graduate Research Student: Scott Harris
Summer Intern: Anuj Bhatia
Funding: School of Engineering, Tufts University

In line-of-sight (LOS) wireless optical applications, a large photodiode area is needed at the input of the receiver in order to allow enough of the incoming light to impinge upon it. The large photodiode area means that there will be a large capacitance at the input to the preamplifier stage. The transimpedance amplifier designed in this case must account for this large capacitance which will directly affect the bandwidth and gain of the receiver. Another issue when it comes to LOS applications is the varying optical power that will impinge upon the photodetector, which depends on the distance between the transmitter and receiver. In order to account for this, a method of automatically controlling the gain can be used. This automatic gain control (AGC) will decrease the gain as the impinging power is increased in order to ensure that the amplifier will not amplify the signal such that it is too large for the amplifier to handle. This can be done using a low-pass filter to extract the DC signal at the output and using this voltage to adjust a voltage divider. This voltage can then be used to vary the feedback resistance and thus the transimpedance gain. A generalized block diagram of this configuration is shown with the feedback resistor implemented as an NMOS transistor.

Keywords: Transimpedance amplifier, automatic gain control (AGC), optical wireless links

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