1. Introduction to Small Signal Model
Section 4.4

2. Implement an amplifier with a BJT
sinusoidal
voltage source
DC voltage source
Cadence simulation
Sinusoidal source (vsin)
Use of 2n3904 as the NPN
Peak to peak swing at the output
Peak to peak swing at the input
Concept of voltage gain

3. Small Signal Analysis
Replace each ideal DC voltage source with a small signal ground.
Replace each ideal DC current source with an open circuit.
Replace each transistor by its small signal model
Analyze the small signal equivalent circuit.

4. Small Signal Model (NPN) (PNP)
Statements that are always true for both NPN and PNP.
rπ is between B and E.
the direction of the dependent current source always points
from the collector to emitter.
3. ro is always between B and C.

5. Example

6. Computation of Small Signal Parameters

7. Derivation of Transconductance
Small signal model
of Q1
If a signal changes the base-emitter voltage by a small amount, how much change is produced in the collector current?

8. But there is something else….
Small signal model
A change in VBE creates a change in base current!

9. Output Resistance Due to Early Effect
A larger reverse bias voltage leads to a larger BC depletion region.
The effective base width (WB) is reduced.
The slope of the electron profile increases.
IC increases as VCE is increased.

10. Early Effect

11. Modification of the Small Signal Model