Chapter 7: BJT Transistor Modeling

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Presentation transcript:

Chapter 7: BJT Transistor Modeling Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Transistor Modeling Slide 1 A model is an equivalent circuit that represents the AC characteristics of the transistor. It uses circuit elements that approximate the behavior of the transistor. There are 2 models commonly used in small signal AC analysis of a transistor: • re model • hybrid equivalent model Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Important Parameters Slide 2 Zi, Zo, Av, Ai are important parameters for the analysis of the Ac characteristics of a transistor circuit. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Input Impedance, Zi Slide 3 [Formula 7.1] To determine Ii: insert a “sensing resistor” then calculate Ii: [Formula 7.2] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Output Impedance, Zo Slide 4 [Formula 7.5] To determine Io: insert a “sensing resistor” then calculate Io: [Formula 7.4] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Voltage Gain, Av Slide 5 [Formula 7.6] For an amplifier with no load: Note: the no-load voltage gain (AVNL) is always greater than the loaded voltage gain (AV). Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Current Gain, Ai Slide 6 [Formula 7.9] The current gain (Ai) also be calculated using the voltage gain (Av): [Formula 7.10] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Phase Relationship Slide 7 The phase relationship between input and output depends on the amplifier configuration circuit. Common – Emitter ~ 180 degrees Common - Base ~ 0 degrees Common – Collector ~ 0 degrees Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

re Transistor Model Slide 8 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. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common – Base Configuration Slide 9 Using a PNP transistor the Common-Base configuration and the re model: The emitter is the input and the collector is the output. This model indicates: Ic = Ie Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

re Slide 10 [Formula 7.11] where IE is the DC current. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common-Base re Model Slide 11 The diode in the previously shown re model can be replaced by the resistor re. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Impedance in Common-Base Configuration Slide 12 The re model indicates: The input impedance (Zi) is quite small: [Formula 7.12] The output impedance (Zo) is quite large: [Formula 7.13] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Gain calculations for the Common-Base using the re model Slide 13 Voltage Gain: [Formula 7.14] Current Gain: [Formula 7.15] The phase relationship between input and output is 0 degrees. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

NPN Common-Base Configuration Slide 14 The npn transistor will use the same calculation. The only difference is that the voltage polarities and current directions will be the opposite. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common-Emitter Configuration Slide 15 The base current is the input and the collector is the output. This model indicates: [Formula 7.16] [Formula 7.17] [Formula 7.18] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Impedance in Common-Emitter Configuration Slide 16 The input impedance (Zi): [Formula 7.19] The output impedance (Zo): [Formula 7.20] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Gain calculations for the Common-Emitter using the re model Slide 17 Voltage Gain (Av): [Formula 7.21] Current Gain (Ai): [Formula 7.22] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common-Collector Configuration Slide 18 The Common-Emitter Model is used for Common-Collector. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Hybrid Equivalent Model Slide 19 The hybrid parameters: hie, hre, hfe, hoe are developed and used to model the transistor. These parameters can be found in a specification sheet for a transistor. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

General h-Parameters for any Transistor Configuration Slide 20 hi = input resistance hr = reverse transfer voltage ratio (Vi/Vo) hf = forward transfer current ratio (Io/Ii) ho = output conductance Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Simplified General h-Parameter Model Slide 21 The above model can be simplified based on these approximations: hr  0 therefore hrVo = 0 and ho   Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common-Emitter re vs. h-Parameter Model Slide 22 hie = re hfe =  hoe = 1/ro Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common-Emitter h-Parameters Slide 23 [Formula 7.28] [Formula 7.29] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common-Base re vs. h-Parameter Model Slide 24 hib = re hfb = - Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Common-Base h-Parameters Slide 25 [Formula 7.30] [Formula 7.31] Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.