Chapter 5: BJT AC Analysis Two-Port Systems Approach This approach: • Reduces a circuit to a two-port system • Provides a “Thévenin look” at the output terminals • Makes it easier to determine the effects of a changing load With Vi set to 0 V: ZTh Zo Ro The voltage across the open terminals is: ETh AvNLVi where AvNL is the no-load voltage gain. Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Effect of Load Impedance on Gain This model can be applied to any current- or voltage- controlled amplifier. Adding a load reduces the gain of the amplifier: Vo RL A v A vNL Vi RL Ro Zi Ai A v R L Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Effect of Source Impedance on Gain The fraction of applied signal that reaches the input of the amplifier is: R i Vs Vi R i Rs The internal resistance of the signal source reduces the overall gain: Vo Ri A vs A vNL Vs Ri Rs Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Combined Effects of RS and RL on Voltage Gain Effects of RL: Vo RLA vNL A v Vi RL Ro Ri Ai A v RL Effects of RL and RS: Vo Ri RL A vs A vNL Vs Ri Rs RL Ro Rs Ri Ais A vs RL Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Cascaded Systems • The output of one amplifier is the input to the next amplifier • The overall voltage gain is determined by the product of gains of the individual stages • The DC bias circuits are isolated from each other by the coupling capacitors • The DC calculations are independent of the cascading • The AC calculations for gain and impedance are interdependent Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. R-C Coupled BJT Amplifiers This image cannot currently be displayed. Input impedance, first stage: Zi R1 || R 2 || re Output impedance, second stage: Zo RC Voltage gain: RC || R1 || R 2 || re A v1 re RC AV2 re A v A 1v A v2 Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. R-C Coupled BJT Amplifiers Draw the ac equivalent circuit and calculate the voltage gain, input resistance and output resistance for the cascade BJT amplifier in above Figure. Let the parameters are: VCC 20V , Q1 Q2 200,VBE(ON ) 7.0 V ,r0 R1 R3 15k, R2 R4 7.4 k, RC1 RC 2 2.2 k, RE1 RE 2 1k Solution Dc Analysis I 19.89A I BQ1 19.89A BQ2 I CQ1 .3 979mA I CQ2 .3 979mA 1re1 2re2 .1 307 k Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. R-C Coupled BJT Amplifiers Ac Analysis: The voltage gain at Q1 is: AV 1 .0 153 2.2( k // 957.36) 102.06 The voltage gain at Q2 is AV 2 .0 153 2.2( k) 336 6. The overall gain is then, AV AV1 AV 2 (102.06)(336 )6. 34,353 The input resistance is: Ri R1 // R2 // Br e 957.36 The o/p resistance is: Ro RC 2 2.2 k Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Cascode Connection This example is a CE–CB combination. This arrangement provides high input impedance but a low voltage gain. The low voltage gain of the input stage reduces the Miller input capacitance, making this combination suitable for high- frequency applications. Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Cascode Connection Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Darlington Connection The Darlington circuit provides a very high current gain—the product of the individual current gains: bD = b1b2 The practical significance is that the circuit provides a very high input impedance. Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. DC Bias of Darlington Circuits This image cannot currently be displayed. Base current: VCC VBE IB R B DR E Emitter current: IE (D I)1 B DIB Emitter voltage: VE IER E Base voltage: VB VE VBE Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. AC small signal of Darlington Circuits Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Feedback Pair This is a two-transistor circuit that operates like a Darlington pair, but it is not a Darlington pair. It has similar characteristics: • High current gain • Voltage gain near unity • Low output impedance • High input impedance The difference is that a Darlington uses a pair of like transistors, whereas the feedback-pair configuration uses complementary transistors. Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. DC Analysis of Feedback Pair Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. AC Analysis of Feedback Pair Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Current Mirror Circuits Current mirror circuits provide constant current in integrated circuits. Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Current Mirror Example Find I? Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Current Mirror More Configurations Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Current Source Circuits Constant-current sources can be built using FETs, BJTs, and combinations of these devices. VZ VBE IE IC I I E R E more… Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved. Current Source Circuits VGS = 0V ID = IDSS = 10 mA Electronic Devices and Circuit Theory, 10/e Copyright ©2009 by Pearson Education, Inc. Robert L. Boylestad and Louis Nashelsky Upper Saddle River, New Jersey 07458 • All rights reserved..