Sunday, May 20, 2012

Small Signal Diode Models

       The small signal diode models are required to obtain the a.c. response of a circuit consisting diodes, when circuit is excited by a.c. inputs.
       Due to the frequency consideration, diffusion capacitance is dominant. Similarly the diode is replaced by its incremental resistance rd = 1/gd , where
       Thus the small signal diode model forward biased condition is incremental resistance rd in parallel with diffusion capacitance as shown in the Fig. 1.
Fig. 1  Small signal model for forward biased diode

       The total admittance of the circuit is,
       For a reverse biased diode, the incremental resistance is rr while the dominant capacitance is a transition or junction capacitance CT. Hence the corresponding small signal model is rr in parallel with CT, as shown in the Fig. 2.
Fig.2   Small signal model for reverse biased diode

       Such small signal diode models are useful to develop small signal models of transistors and useful in analysing transistor amplifier and other circuits.

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