Transcript Introduction to Small Signal Model

Introduction to Small Signal
Model
Section 4.4
Implement an amplifier with a
BJT
sinusoidal
voltage source
DC voltage source
Cadence simulation
1. Sinusoidal source (vsin)
2. Use of 2n3904 as the NPN
3. Peak to peak swing at the output
4. Peak to peak swing at the input
5. Concept of voltage gain
Small Signal Analysis
1. Replace each ideal DC voltage source with a
small signal ground.
2. Replace each ideal DC current source with an
open circuit.
3. Replace each transistor by its small signal model
4. Analyze the small signal equivalent circuit.
Small Signal Model
(NPN)
(PNP)
Statements that are always true for both NPN and PNP.
1. rπ is between B and E.
2. the direction of the dependent current source always points
from the collector to emitter.
3. ro is always between B and C.
Example
Computation of Small Signal
Parameters
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?
But there is something else….
Small signal model
A change in VBE creates a change in base current!
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.
Early Effect
Modification of the Small Signal
Model