Discrete transimpedance amplifier

A discrete transimpedance amplifier circuit which could be used for amplifying PIN or avalanche photodiode (APD) signals is shown above. It uses a GaAsFET (low noise PHEMT - pseudomorphic high electron mobility transistor) as a source follower, bipolar cascode amplifier and long tailed pair combination for low noise, high gain and low input bias current. This allows the transimpedance (current to voltage ratio) to be very high, around 3.2M ohms or 130dB (3.2mV/nA). Without additional compensation the noise and gain performance are poor - the light green and blue traces shown below respectively

By bootstrapping the photodiode capacitance and GaAsFET gate-source capacitance (not shown in the schematic) the results can be improved considerably - the dark green and red traces above. The result is an improvement in the bandwidth from 284kHz to 21MHz and an increase in the noise corner frequency from 200kHz to 7.4 MHz. The overall effect is a dramatic improvement in the signal to noise ratio. It should be remembered that a tenfold increase in the transimpedance results in only a square root of ten increase in the noise contribution from the feedback resistor. The largest value of feedback resistor is therefore beneficial, provided the amplifier has sufficient bandwidth for the desired application.

A good analysis of transimpedance amplifiers can be found in the Burr Brown (Texas Instruments) Application Bulletin "Noise Analysis of FET Transimpedance Amplifiers" SBOA060.pdf and "Optical Sensing Techniques and Signal Processing" P114-123 by Tudor E. Jenkins, ISBN 0-13-638107-3.

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