chapter 5: bjt ac analysis. copyright ©2009 by pearson education, inc. upper saddle river, new...
TRANSCRIPT
Chapter 5: BJT AC Analysis
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
BJT Transistor ModelingBJT 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
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
The rThe ree Transistor Model Transistor Model
• BJTs are basically current-controlledcurrent-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.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Base ConfigurationCommon-Base Configuration
Input impedance:Input impedance:
Output impedance:Output impedance:
Voltage gain:Voltage gain:
Current gain:Current gain:
ec I I e
e I
mV 26r
ei rZ
oZ
e
L
e
LV r
R
r
RA
1A i
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter ConfigurationCommon-Emitter Configuration
bbe III 1
The diode re model can be replaced by the resistor re.
more…more…
ee I
mV 26r
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter ConfigurationCommon-Emitter Configuration
Input impedance:Input impedance:
Output impedance:Output impedance:
Voltage gain:Voltage gain:
Current gain:Current gain:
ei rZ
oo rZ
e
LV r
RA
oriA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Collector ConfigurationCommon-Collector Configuration
Input impedance:Input impedance:
Output impedance:Output impedance:
Voltage gain:Voltage gain:
Current gain:Current gain:
ei rZ )1(
Eeo RrZ ||
eE
EV rR
RA
1iA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
The Hybrid Equivalent ModelThe Hybrid Equivalent ModelThe following hybrid parameters are developed and used for modeling the transistor. These parameters can be found on the specification sheet for a transistor.
• hi = input resistance• hr = reverse transfer voltage ratio (Vi/Vo)
0 • hf = forward transfer current ratio (Io/Ii)• ho = output conductance
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Simplified General h-Parameter ModelSimplified General h-Parameter Model
• hi = input resistance• hf = forward transfer current ratio (Io/Ii)
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
rree vs. h-Parameter Model vs. h-Parameter Model
acfe
eie
h
rh
Common-EmitterCommon-Emitter
Common-BaseCommon-Base
1h
rh
fb
eib
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
The Hybrid The Hybrid Model Model
The hybrid model is most useful for analysis of high-frequency transistor applications.
At lower frequencies the hybrid model closely approximate the re parameters, and can be replaced by them.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter Fixed-Bias ConfigurationCommon-Emitter Fixed-Bias Configuration
• The input is applied to the base• The output is from the collector• High input impedance• Low output impedance• High voltage and current gain• Phase shift between input and
output is 180
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter Fixed-Bias ConfigurationCommon-Emitter Fixed-Bias Configuration
AC equivalent
re model
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter Fixed-Bias CalculationsCommon-Emitter Fixed-Bias Calculations
Co 10Rre
Cv
e
oC
i
ov
r
RA
r
)r||(R
V
VA
eBCo r10R ,10Rri
eBCo
oB
i
oi
A
)r)(RR(r
rR
I
IA
C
ivi R
ZAA
Current gain from voltage gain:
Input impedance:
Output impedance:
Voltage gain: Current gain:
eE r10Rei
eBi
rZ
r||RZ
Co
O
R10rCo
Co
RZ
r||RZ
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter Voltage-Divider BiasCommon-Emitter Voltage-Divider Bias
re model requires you to determine , re, and ro.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter Voltage-Divider Bias Common-Emitter Voltage-Divider Bias CalculationsCalculations
Current gain from voltage gain:
Input impedance:
Output impedance:
Voltage gain:
Current gain:
ei
21
r||RZ
R||RR
Co 10RrCo
oCo
RZ
r||RZ
Co 10Rre
C
i
ov
e
oC
i
ov
r
R
V
VA
r
r||R
V
VA
eCo
Co
r10R ,10Rri
oi
10Rrei
oi
eCo
o
i
oi
I
IA
rR
R
I
IA
)rR)(R(r
rR
I
IA
C
ivi R
ZAA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter Emitter-Bias Common-Emitter Emitter-Bias ConfigurationConfiguration
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Impedance CalculationsImpedance Calculations
Eb
Eeb
Eeb
bBi
RZ
)R(rZ
1)R(rZ
Z||RZ
Input impedance:
Output impedance:Co RZ
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Gain CalculationsGain Calculations
Current gain from voltage gain:
Voltage gain:
Current gain:
Eb
Eeb
RZE
C
i
ov
)R(rZEe
C
i
ov
b
C
i
ov
R
R
V
VA
Rr
R
V
VA
Z
R
V
VA
bB
B
i
oi ZR
R
I
IA
C
ivi R
ZAA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Emitter-Follower ConfigurationEmitter-Follower Configuration
• This is also known as the common-collector configuration.• The input is applied to the base and the output is taken from the
emitter.• There is no phase shift between input and output.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Impedance CalculationsImpedance Calculations
Input impedance:
Output impedance:
eE rReo
eEo
rZ
r||RZ
Eb
Eeb
Eeb
bBi
RZ
)R(rZ
1)R(rZ
Z||RZ
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Gain CalculationsGain Calculations
Current gain from voltage gain:
Voltage gain:
Current gain:
EeEeE RrR ,rRi
ov
eE
E
i
ov
1V
VA
rR
R
V
VA
bB
Bi ZR
RA
E
ivi R
ZAA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Base ConfigurationCommon-Base Configuration
• The input is applied to the emitter.
• The output is taken from the collector.
• Low input impedance.• High output impedance.• Current gain less than unity.• Very high voltage gain.• No phase shift between input
and output.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
CalculationsCalculations
eEi r||RZ
Co RZ
e
C
e
C
i
ov r
R
r
R
V
VA
1I
IA
i
oi
Input impedance:
Output impedance:
Voltage gain:
Current gain:
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Emitter Collector Feedback Common-Emitter Collector Feedback ConfigurationConfiguration
• This is a variation of the common-emitter fixed-bias configuration• Input is applied to the base• Output is taken from the collector• There is a 180 phase shift between input and output
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
CalculationsCalculations
F
C
ei
R
R1
rZ
FCo R||RZ
e
C
i
ov r
R
V
VA
C
F
i
oi
CF
F
i
oi
R
R
I
IA
RR
R
I
IA
Input impedance:
Output impedance:
Voltage gain:
Current gain:
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Collector DC Feedback ConfigurationCollector DC Feedback Configuration
• This is a variation of the common-emitter, fixed-bias configuration
• The input is applied to the base• The output is taken from the
collector• There is a 180 phase shift
between input and output
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
CalculationsCalculations
F
C
ei
R
R1
rZ
FCo R||RZ
e
C
i
ov r
R
V
VA
C
F
i
oi
CF
F
i
oi
R
R
I
IA
RR
R
I
IA
Input impedance:
Output impedance:
Voltage gain: Current gain:
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Two-Port Systems ApproachTwo-Port Systems ApproachThis 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
ooTh RZZ
ivNLTh VAE
With Vi set to 0 V:
The voltage across the open terminals is:
where AvNL is the no-load voltage gain.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Effect of Load Impedance on GainEffect of Load Impedance on Gain
L
ivi R
ZAA
This model can be applied to any current- or voltage-controlled amplifier.
Adding a load reduces the gain of the amplifier:
vNLoL
L
i
ov A
RR
R
V
VA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Effect of Source Impedance on GainEffect of Source Impedance on Gain
si
sii RR
VRV
The fraction of applied signal that reaches the input of the amplifier is:
vNLsi
i
s
ovs A
RR
R
V
VA
The internal resistance of the signal source reduces the overall gain:
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Combined Effects of RCombined Effects of RSS and R and RLL on Voltage on Voltage
GainGainEffects of RL:
Effects of RL and RS:
L
ivi
oL
vNLL
i
ov
R
RAA
RR
AR
V
VA
L
isvsis
vNLoL
L
si
i
s
ovs
R
RRAA
ARR
R
RR
R
V
VA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Cascaded SystemsCascaded 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
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
R-C Coupled BJT AmplifiersR-C Coupled BJT Amplifiers
Co RZ
Input impedance, first stage:
Output impedance, second stage:
Voltage gain:
e21i r||R||RZ
2v1vv
e
C2V
e
e21C1v
AAA
r
RA
r
r||R||R||RA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Cascode ConnectionCascode 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.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Darlington ConnectionDarlington Connection
The Darlington circuit provides a very high current gain—the product of the individual current gains:
D = 12
The practical significance is that the circuit provides a very high input impedance.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
DC Bias of Darlington CircuitsDC Bias of Darlington Circuits
BDBDE II)1(I
EEE RIV
EDB
BECCB RR
VVI
Base current:
Emitter current:
Emitter voltage:
Base voltage:
BEEB VVV
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Feedback PairFeedback 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.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Current Mirror CircuitsCurrent Mirror Circuits
Current mirror circuits provide constant current in integrated circuits.
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Current Source CircuitsCurrent Source CircuitsConstant-current sources can be built using FETs, BJTs, and combinations of these devices.
IE IC
E
BEZE R
VVII
more…more…
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Current Source CircuitsCurrent Source Circuits
VGS = 0VID = IDSS = 10 mA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Fixed-Bias ConfigurationFixed-Bias Configuration
ieBi h||RZ
ieBi h||RZ
oeCo h/1||RZ
ie
oCfe
i
ov h
eh/1||Rh
V
VA
fei
oi h
I
IA
Input impedance:
Output impedance:
Voltage gain:
Current gain:
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Voltage-Divider ConfigurationVoltage-Divider Configuration
ie
fei hR
RhA
iei h||RZ
Co RZ
ie
oeCfev h
1/h||RhA
Input impedance:
Output impedance:
Voltage gainVoltage gain:
Current gain:
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Emitter-Follower ConfigurationEmitter-Follower Configuration
boi
Efeb
Z||RZ
RhZ
boi
Efeb
Z||RZ
RhZ
Input impedance:
Output impedance:
Voltage gain:
Current gain:
fe
ieEo h
h||RZ
feieE
E
i
ov h/hR
R
V
VA
E
ivi
bB
Bfei
R
ZAA
ZR
RhA
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Common-Base ConfigurationCommon-Base Configuration
ibEi h||RZ
Co RZ
ib
Cfb
i
ov h
Rh
V
VA
1hI
IA fb
i
oi
Input impedance:
Output impedance:
Voltage gain:
Current gain:
Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
TroubleshootingTroubleshooting
Check the DC bias voltagesCheck the DC bias voltages
If not correct, check power supply, resistors, transistor. Also check the coupling capacitor between amplifier stages.
Check the AC voltagesCheck the AC voltages
If not correct check transistor, capacitors and the loading effect of the next stage.