Diode Models?

Question

I know that the exponential model is the most precise, and the constant-voltage model is used the most, but what about the Small Signal model? Unless I'm told to use it, I'm not sure when to use the Small Signal model. So...when should I use it? I'm looking at some problems with 3 diodes in series and it seems that it was used..but I don't know why. Thanks for any responses.

Answer 1

If you know that the diode is already biased on ( possibly by design) and the voltage excursion will be small. Then use the small signal model which would just be a resistor with a value dependent on the bias point.

Answer 2

Small signal modeling is a common analysis method used in electrical engineering to describe nonlinear devices in terms of linear equations. The basic idea is to first calculate (possibly by an iterative process if the circuit is complex) the levels that will be present when no signal is applied, then form linear approximations for the deviations from that base state.

Electronic circuits generally involve small time-varying signals carried over a constant bias. This suggests using a method akin to approximation by differentials to analyze relatively small perturbations about the bias point.

Any nonlinear device which can be described quantitatively using a formula can then be 'linearized' about a bias point by taking partial derivatives of the formula with respect to all governing variables. These partial derivatives can be associated with physical quantities (such as capacitance, resistance and inductance), and a circuit diagram relating them can be formulated. Small signal models exist for diodes, field effect transistors and bipolar transistors.

Large signal DC quantities are denoted by uppercase letters with uppercase subscripts. For example, the DC input bias voltage of a transistor would be denoted VIN.
Small signal quantities are denoted using lowercase letters with lowercase subscripts. For example, the input signal of a transistor would be denoted as vin.
Total quantities, combining both small signal and large signal quantities, are denoted using lower case letters and uppercase subscripts. For example, the total input voltage to the aforementioned transistor would be vIN(t) = VIN(t) + vin(t).

For more information visit http://en.wikipedia.org/wiki/Diode_modelling

http://en.wikipedia.org/wiki/Small_signal_model