BJT Transistor Modeling

ELECTRONIC CIRCUITS ANALYSIS AND DESIGN

BJT Transistor Modeling

BJT Transistor Modeling Modeling Objectives: 1. To use the the super superpos positio ition n theore theorem m to dr draw the dc and ac equivalent equivalent circuits 2. To dete determin rmine e the importan importantt parame parameter terss of a two-port system system for analysis and design of an amplifier 3. To ident identify ify the the two two model modelss common commonly ly used used in the small-signal ac analysis a nalysis of transistor networks

BJT Transistor Modeling Modeling Objectives: 1. To use the the super superpos positio ition n theore theorem m to dr draw the dc and ac equivalent equivalent circuits 2. To dete determin rmine e the importan importantt parame parameter terss of a two-port system system for analysis and design of an amplifier 3. To ident identify ify the the two two model modelss common commonly ly used used in the small-signal ac analysis a nalysis of transistor networks

BJT Transistor Modeling 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 point called 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

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Cutoff Region Operation Base–Emitter junction is reverse biased

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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 Important Parameters: Zi, Z0, Av, Ai

BJT Transistor Modeling Input Impedance, Zi

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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 megaohms.

BJT Transistor Modeling Output Impedance, Zo

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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 M .

BJT Transistor Modeling Voltage Gain, Av

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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

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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 180 0 out of phase or in phase.


Comparison Between the Three-Transistor Configurations

BJT Transistor Modeling The re Transistor Model •

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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 Common Emitter Configuration



BJT Transistor Modeling The Hybrid Equivalent Model



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Short-circuit input impedance parameter

Open-circuit reverse transfer voltage ratio parameter

BJT Transistor Modeling Hybrid input equivalent circuit


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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 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

BJT Transistor Modeling

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