# Small Signal Question

Discussion in 'General Electronics Chat' started by crazyengineer, Jan 10, 2012.

1. ### crazyengineer Thread Starter Member

Dec 29, 2010
156
2
I know that under large signal analysis, a capacitor becomes open, but under small, it becomes a short. My main question is what is an inductor under small signal analysis. Is it a short under large signal, but open under small signal?

2. ### PaulEE Member

Dec 23, 2011
423
32
The impedance of a capacitor or inductor depends upon the value of said components as compared to the frequency of the AC waveform across them.

DC is (ideally) blocked by capacitors and let through an inductor.
AC is (ideally) passed by capacitors and blocked by inductors.

The above is a special case when component values are large and frequencies are moderately high. When component values are moderate and frequencies are moderate, you must be more specific with what you want to know.

Large signal analysis is concerned with the operation of macroscopic models of circuits, whereas small signal analysis is concerned with more subtle characteristics.

As an example, a silicon diode, with a large signal model, drops "about 0.7 volts at room temperature". What about when your signal is 20 mVpp AC? How does the diode act then? Id = Ise^(Vbe@ref current)/(k*t for temp) is for larger signals. Smaller signal analysis is carried out by linearizing the larger signal model's curve; basically, you zoom in to the point where the curvature seems to disappear and a straight line results.

Cool?

Last edited: Jan 11, 2012