Chapter 4 Islamic University Of Gaza DC Biasing BJTs Dr. Talal Skaik

Chapter 4DC Biasing–BJTsIslamic University of GazaDr. Talal Skaik

BiasingBiasing: The DC voltages applied to a transistor inorder to turn it on so that it can amplify the AC signal.Recall the following basic relationships for a transistor:V BE 0.7 VI E ( 1)I IC I Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 20142Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Operating PointThe DC inputestablishes anoperating orquiescent pointcalled the Q-point.Various operating points within thelimits of operation of a transistor.Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 20143Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

The Three States of Operation Active or Linear Region OperationBase–Emitter junction is forward biasedBase–Collector junction is reverse biased Cutoff Region OperationBase–Emitter junction is reverse biased Saturation Region OperationBase–Emitter junction is forward biasedBase–Collector junction is forward biasedElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 20144Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

DC Biasing Circuits Fixed-bias circuitEmitter-stabilized bias circuitCollector-emitter loopVoltage divider bias circuitDC bias with voltage feedbackElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 20145Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Fixed Bias configurationElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 20146Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Fixed Bias configurationFixed bias circuitElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDC equivalentDr. Talal Skaik 20147Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

The Base-Emitter LoopFrom Kirchhoff’s voltagelaw: VCC – IBRB – VBE 0Solving for base current:VCC VBEIB RBElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 20148Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Collector-Emitter LoopCollector current:I C I BFrom Kirchhoff’s voltage law:VCE VCC I C R CElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 20149Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Example 4.1Find IBQ , ICQ , VCEQ , VB, VC , VBC.Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201410Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Load Line AnalysisV CE V CC I C RCElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201411Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Load Line AnalysisV CE V CC I C RCV CE V CCV CCIC RCI C 0 mAV CE 0VElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201412Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Load Line AnalysisMovement of the Q-point with increasing level of IB.(The level of IB is changed by varying the value of RB)Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201413Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Load Line AnalysisEffect of an increasing level of RC on the load line and the Q-point.(VCC fixed)Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201414Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Load Line AnalysisEffect of lower values of VCC on the load line and the Q-point.Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201415Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Example 4.3Find VCC , RC , RB forthe fixed biasingconfigurationElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201416Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Emitter-Stabilized Bias CircuitAdding a resistor(RE) to the emittercircuit stabilizes thebias circuit.Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201417Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Base-Emitter LoopFrom Kirchhoff’s voltage law: VCC - I E R E - VBE - I E R E 0Since IE ( 1)IB:VCC - I B R B - ( 1)I B R E 0Solving for IB:IB VCC - VBER B ( 1)R EElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201418Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Collector-Emitter LoopFrom Kirchhoff’s voltage law:I R V I R V 0E ECE C CCCSince IE IC:VCE VCC – I C (R C R E )Also:VE I E R EVC VCE VE VCC - I C R CVB VCC – I R R B VBE VEElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201419Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Example 4.4Find IB , IC , VCE , VC , VE , VB , VBC .Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201420Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Improved Biased StabilityStability refers to a circuit condition in which thecurrents and voltages will remain fairly constant overa wide range of temperatures and transistor Beta ( )values.Adding RE to the emitter improves the stability of a transistor.Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201421Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Improved Biased StabilityElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201422Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Load Line AnalysisV CE V CC I C (RC R E )V CE V CCI C 0 mAV CCIC RC R EV CE 0VElectronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis NashelskyDr. Talal Skaik 201423Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

The Three States 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 Dr. Talal Skaik 2014 4