ELECTRONIC CIRCUITS ANALYSIS AND DESIGN

BJT Transistor Modeling

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

BJT Transistor Modeling

Biasing

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

BJT Transistor Modeling

Operating Point

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

BJT Transistor Modeling

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

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

BJT Transistor Modeling

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.

BJT Transistor Modeling

BJT Transistor Modeling

BJT Transistor Modeling

BJT Transistor Modeling

Important Parameters: Zi, Z0, Av, Ai

BJT Transistor Modeling

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.

BJT Transistor Modeling

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.

BJT Transistor Modeling

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.

BJT Transistor Modeling

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.

BJT Transistor Modeling

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.

BJT Transistor Modeling

Comparison Between the Three-Transistor Configurations

BJT Transistor Modeling

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.

BJT Transistor Modeling

Common Base Configuration

BJT Transistor Modeling

BJT Transistor Modeling

BJT Transistor Modeling

Common Emitter Configuration

BJT Transistor Modeling

BJT Transistor Modeling

BJT Transistor Modeling

The Hybrid Equivalent Model

BJT Transistor Modeling

BJT Transistor Modeling

• Short-circuit input impedance parameter

• Open-circuit reverse transfer voltage ratio parameter

BJT Transistor Modeling

Hybrid input equivalent circuit

BJT Transistor Modeling

• Short-circuit forward transfer current ratio parameter

• Open-circuit output admittance parameter

BJT Transistor Modeling

Hybrid output equivalent circuit

BJT Transistor Modeling

Complete Hybrid Equivalent Circuit

BJT Transistor Modeling

BJT Transistor Modeling

Typical values of h-parameters

BJT Transistor Modeling

Approximate Hybrid Equivalent Circuit

BJT Transistor Modeling

Hybrid versus re model

Common-base configuration

BJT Transistor Modeling

Hybrid versus re model

Common-emitter configuration

BJT Transistor Modeling

Hybrid versus re model

Common-collector configuration

QUESTIONS

BJT Transistor Modeling