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.
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| 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.
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| 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.
