We can divide analysis of diode and transistor into Small signal analysis and Large signal analysis.
Large signal analysis
While doing a large signal analysis, a large dc voltage is applied and find out the behavior of diode or transistor towards this signal(find an operating point of these components). The voltage applied are of the order of few volts to few tens of volts. Here we use current and voltage equations such as
to find bias points. This analysis is applicable to any value of current and voltage applied to the devices.
Small signal analysis
Small signal analysis approximate nonlinear behavior of electronic components with linear equations. This approximation is taken across the bias point.
While discussing about the operating point of a BJT amplifier, it is said that preferable position for the operating point is at the center of the load line. Suppose after fixing the operating point(by applying dc bias voltages only), we are applying the input-AC signal. Then the operating point will swing around the initial operating point according to the value of A C signal. During this if it enter into saturation or cut-off region the behavior of the amplifier become nonlinear. So while choosing the input A C signal for amplifiers care must be taken to select signals which are within the specified maximum value. Because of this reason input A C signals for amplifiers are usually small and this fact encourages small signal analysis.
Small signal model is used for active region of transistor and hence is used to analyze amplifier characteristics such as gain,input and output impedance etc.
Small signal parameters for BJT are :
Representation of Large signal and Small signal parameters:
Large signal signal parameters are represented by UPPERCASE LETTERS and UPPERCASE SUBSCRIPTS. eg: VBE
Small signal signal parameters are represented by lowercase letters and lowercase subscripts. eg: vbe
Total parameters (while combining small signal parameters with large signal parameters) are represented by lowercase letters and UPPERCASE SUBSCRIPTS. eg: vBE
Small signal parameters and small signal analysis are on coming posts.