1. Small Signal Model
Section 4.4

2. BJT in the active region
Electrons cross the forward biased BE junction
and are swept reverse biased BC junction.

3. Large Signal Model of a BJT
Called “large” signal model
because this model is
applicable even if VBE
changes from 300 mV to 800 mV

4. Experiments

5. Problem
IS,Q1=5 x A
VBE=800 mV
β=100
What is the voltage at VX?

6. Problem
What value of RC will take Q1 out of the active region?

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

8. Illustration of Transconductance

9. But there is something else….
A in VBE creates a change in base current!

10. Example 4.10 Small Signal Equivalent Circuit VBE=800 mV β=100
Signal Generated
By a microphone
VBE=800 mV
β=100
IS,Q1=3 x A
Small Signal Equivalent Circuit
Question: If a microphone generates a 1 mV signal,
how much change is observed in the collector and
base current?

11. A Simple Amplifier
Determine the output signal level if the microphone produces a 1 mV signal.

12. AC Ground The voltage produced by a voltage source is constant.
The small signal model is concerned only with changes in quantities.
Therefore, a DC voltage source must be replaced with a ground in small signal analysis.

13. Example

14. 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.

15. Early Effect

16. James M. Early

17. Modeling of Early Effect

18. What Doesn’t Change with Early Effect?

19. Modification of the Small Signal Model

20. Summary