The concept behind small-signal operation is that a time varying signal with small amplitudes rides on a de value that may or may not be large. The analysis of circuit is divided into two parts.

1. DC bias.
2. AC signal of small amplitude and the solutions are added together using superposition Fig1 shows the circuit diagram of a diode where$\nu _a(t)$ some time varying waveform. Here the purpose of Vp in this circuit is set the operational of the diode about a point on the forward bias in the characteristic curue of the diode. This called quiscent point or apoint. The total voltage at any time t is the sum of the DC and AC components.

$\nu_0(t)=V_D+\nu_d(t)$

∴ Diode current with iD(t) is

$i_0(t)=I_Se^{\dfrac{\nu_0(t)}{nV_T}}=\underset{I_D}{\underbrace{I_Se^{\dfrac{V_0}{nV_T}}}}e^{\dfrac{\nu_{d(t)}}{nV_T}}\\ i_0(t)=I_0e^{\dfrac{\nu_{d(t)}}{nV_T}}\\ \Rightarrow i_{D(t)}\approx I_D+\dfrac{I_D}{\eta V_T}\nu_d(t)$

So if $\nu_d(t)$ is small enough we can see sum of two component. The term $\dfrac{\eta V_T}{I_D}$ is small signal of diode is