BJT Small Signal Analysis
[youtube]http://www.youtube.com/watch?v=wT8fZJIl3Nk[/youtube]
Q. Why do we study small signal analysis of BJT, MOSFET, etc.What applications do they have?
Ans. We basically use small signal analysis in order to find a linear model for non-linear devices ( e.g. BJT, MOSFETs, Diodes ) . Non-linear means that these devices depend on the current passing through them, unlike resistors for example which are linear, their resistivity is constant regardless of the current passing through.
If we consider BJTs, we usually find the bias currents at DC, these values for DC currents ( emitter, collector and base currents ) can be used to determine the internal resistances of the transistor ( eg. r(pi), g(m) ) for those certain values of currents.
When we determine these resistance values, we can then replace the BJT with a linear model (e.g Hybrid or T model) by using this model we can treat the transistor as any other linear device in our analysis.
We can use small signal anaysis for many reasons such as finding the gain of amplifiers.
If we consider BJTs, we usually find the bias currents at DC, these values for DC currents ( emitter, collector and base currents ) can be used to determine the internal resistances of the transistor ( eg. r(pi), g(m) ) for those certain values of currents.
When we determine these resistance values, we can then replace the BJT with a linear model (e.g Hybrid or T model) by using this model we can treat the transistor as any other linear device in our analysis.
We can use small signal anaysis for many reasons such as finding the gain of amplifiers.
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