Lecture 1
Transistor small signal analysis
The transistor can be employed as an amplifying device, that is, the output ac power
is greater than the input ac power.
The factor that permits an ac power output greater than the input ac power is the
applied DC power.
The amplifier is initially biased for the required DC voltages and currents.
Then the ac to be amplified is given as input to the amplifier.
If the applied ac exceeds the limit set by dc level, clipping of the peak region will
result in the output.
Thus, proper (faithful) amplification design requires that the dc and ac components
be sensitive to each other’s requirements and limitations.
The superposition theorem is applicable for the analysis and design of the dc and ac
components of a BJT network, permitting the separation of the analysis of the dc and
ac responses of the system.
BJT Transistor modeling:
• A model is an equivalent circuit that represents the AC characteristics of the transistor.
• A model uses circuit elements that approximate the behavior of the transistor.
• There are two models commonly used in small signal AC analysis of a transistor:
i. re model
ii. Hybrid equivalent model
