BJT Small Signal Analysis

• re transistor model – employs a diode and controlled current source to duplicate the behavior of a transistor in the region of interest.
  
• The re and hybrid models will be used to analyze small-signal AC analysis of standard transistor network configurations.
  

Ex: Common-base, common-emitter and common-collector configurations.

• The network analyzed represent the majority of those appearing in practice today.
  

AC equivalent of a network is obtained by:

1. Setting all DC sources to zero
2. Replacing all capacitors by s/c equiv.
3. Redraw the network in more convenient and logical form

Common-Emitter (CE) Fixed-Bias Configuration

The input (Vi) is applied to the base and the output (Vo) is from the collector.
The Common-Emitter is characterized as having high input impedance and low output impedance with a high voltage and current gain.

Removing DC effects of VCC and Capacitors

re Model

Phase Relationship

The phase relationship between input and output is 180 degrees.
The negative sign used in the voltage gain formulas indicates the inversion.

CE – Voltage-Divider Bias Configuration

re Model

Phase Relationship

A CE amplifier configuration will always have a phase relationship between input and output is 180 degrees. This is independent of the DC bias.

Common-Base (CB) Configuration

The input (Vi) is applied to the emitter and the output (Vo) is from the collector.
The Common-Base is characterized as having low input impedance and high output impedance with a current gain less than 1 and a very high voltage gain.

re Model

Collector DC Feedback Configuration

The network has a dc feedback resistor for increased stability, yet the capacitor C3 will shift portions of the feedback resistance to the input and output sections of the network in the ac domain. The portion of RF shifted to the input or output side will be determined by the desired ac input and output resistance levels.

re Model

Approximate Hybrid Equivalent Circuit

The h-parameters can be derived from the re model:

hie = bre hib = re
hfe = b hfb = -a
hoe = 1/ro

The h-parameters are also found in the specification sheet for the transistor.

Approximate Common-Emitter Equivalent Circuit

Hybrid equivalent model re equivalent model

Approximate Common-Base Equivalent Circuit

Hybrid equivalent model re equivalent model