bjt transistor modeling

ELECTRONIC CIRCUITS ANALYSIS AND DESIGN

BJT Transistor Modeling


Objectives:

1. To use the superposition theorem to draw the dc and ac equivalent circuits

2. To determine the important parameters of a two-port system for analysis and design of an amplifier

3. To identify the two models commonly used in the small-signal ac analysis of transistor networks


Biasing

• The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal


Operating Point

The DC input establishes an establishes an operating or quiescent pointcalled the Q-point


Three Stages of Operation• Active or Linear Region Operation

Base–Emitter junction is forward biased

Base–Collector junction is reverse biased

• Cutoff Region Operation

Base–Emitter junction is reverse biased

• Saturation Region Operation

Base–Emitter junction is forward biased

Base–Collector junction is forward biased


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:

– re model

– Hybrid equivalent model


When introducing the ac model for a BJT:

1. All dc sources are set to zero and replaced by a short-circuit connection to ground.

2. All capacitors are replaced by a short-circuit equivalent.

3. All elements in parallel with an introduced short-circuit equivalent should be removed from the network.

4. The network should be redrawn as often as possible.


Important Parameters: Zi, Z0, Av, Ai


Input Impedance, Zi

• The input impedance of a BJT transistor amplifier is purely resistive in nature, and depending on the manner in which the transistor is employed, can vary from a few ohms to mega-ohms.


Output Impedance, Zo

• The output impedance is determined at the output terminals looking back into the system with the applied signal set to zero. It is resistive in nature and can vary from a few ohms to a level that can exceed 2 MΩ.


Voltage Gain, Av

• Depending on the configuration, the magnitude of the voltage gain for a loaded single-stage amplifier typically ranges from just less than 1 to a few hundred. A multistage system, however, can have a voltage gain in the thousands.


Current Gain, Ai

• For BJT amplifiers, the current gain typically ranges from a level just less than 1 to a level that may exceed 100.


Phase Relationship

• For the typical transistor amplifier at frequencies that permit ignoring the effects of the reactive elements, the input and output signals are either 1800 out of phase or in phase.


Comparison Between the Three-Transistor Configurations


The re Transistor Model

• BJTs are basically current-controlled devices; therefore the re

model uses a diode and a current source to duplicate the behavior of the transistor.

• One disadvantage to this model is its sensitivity to the DC level. This model is designed for specific circuit conditions.


Common Base Configuration


Common Emitter Configuration


The Hybrid Equivalent Model


• Short-circuit input impedance parameter

• Open-circuit reverse transfer voltage ratio parameter


Hybrid input equivalent circuit


• Short-circuit forward transfer current ratio parameter

• Open-circuit output admittance parameter


Hybrid output equivalent circuit


Complete Hybrid Equivalent Circuit


Typical values of h-parameters


Approximate Hybrid Equivalent Circuit


Hybrid versus re model

Common-base configuration



Common-emitter configuration



Common-collector configuration

QUESTIONS


bjt transistor modeling

Documents