Download Base-Emitter Loop From Kirchhoff`s voltage law

Chapter 4
DC Biasing–BJTs
Biasing
Biasing: The DC voltages applied to a transistor in
order to turn it on so that it can amplify the AC signal.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Operating Point
The DC input
establishes an
operating or
quiescent point
called the Q-point.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©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
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
DC Biasing Circuits
•
•
•
•
•
Fixed-bias circuit
Emitter-stabilized bias circuit
Collector-emitter loop
Voltage divider bias circuit
DC bias with voltage feedback
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Fixed Bias
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
The Base-Emitter Loop
From Kirchhoff’s voltage
law:
+VCC – IBRB – VBE = 0
Solving for base current:
IB 
VCC  VBE
RB
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Collector-Emitter Loop
Collector current:
I C  I B
From Kirchhoff’s voltage law:
VCE  VCC  I C R C
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 1:
Determine the
following for the
fixed-bias
configuration
(a) IBQ and ICQ.
(b) VCEQ.
(c) VB and VC.
(d) VBC.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Saturation
When the transistor is operating in saturation, current
through the transistor is at its maximum possible value.
VCC
I Csat 
RC
VCE  0 V
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Load Line Analysis
The end points of the load line are:
ICsat
IC = VCC / RC
VCE = 0 V
VCEcutoff
VCE = VCC
IC = 0 mA
The Q-point is the operating point:
• where the value of RB sets the value of
IB
• that sets the values of VCE and IC
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Circuit Values Affect the Q-Point
more …
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Circuit Values Affect the Q-Point
more …
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Circuit Values Affect the Q-Point
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Emitter-Stabilized Bias Circuit
Adding a resistor
(RE) to the emitter
circuit stabilizes
the bias circuit.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Base-Emitter Loop
From Kirchhoff’s voltage law:
VCC - I BR B - VBE - I E R E  0
Since IE = ( + 1)IB:
VCC - I B R B - (  1)I B R E  0
Solving for IB:
IB 
VCC - VBE
R B  (  1)R E
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Collector-Emitter Loop
From Kirchhoff’s voltage law:
I R V
I R V
0
E E
CE C C
CC
Since IE  IC:
VCE  VCC – I C (R C  R E )
Also:
VE  I E R E
VC  VCE  VE  VCC - I C R C
VB  VCC – I R R B  VBE  VE
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 2:
For the emitter bias network,
determine:
(a) IB.
(b) IC.
(c) VCE.
(d) VC.
(e) VE.
(f) VB.
(g) VBC.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Improved Biased Stability
Stability refers to a circuit condition in which the currents and voltages
will remain fairly constant over a 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/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Using the results calculated in Example 1 and then repeating for a value of β=100
yields the following:
The BJT collector current is seen to change by 100% due to the 100% change in the
value of β. IB is the same and VCE decreased by 76%.Using the results calculated
in Example 2 and then repeating for a value of β=100, we have the following:
Now the BJT collector current increases by about 81% due to the 100% increase in
β. Notice that IB decreased, helping maintain the value of IC—or at least reducing
the overall change in IC due to the change in β. The change in VCE has dropped to
about 35%. The network of Example 2 is therefore more stable than that of Example
1 for the same change in β.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Saturation Level
The endpoints can be determined from the load line.
VCEcutoff:
ICsat:
VCE  VCC
I C  0 mA
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
VCE  0 V
IC 
VCC
RC  RE
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Voltage Divider Bias
This is a very stable
bias circuit.
The currents and
voltages are nearly
independent of any
variations in .
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Exact Analysis
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Exact Analysis
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 3:
Determine the dc bias voltage VCE and the current IC for the
voltage-divider configuration.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Approximate Analysis
Where IB << I1 and I1  I2 :
VB 
R 2 VCC
R1  R 2
Where RE > 10R2:
VE
RE
VE  VB  VBE
IE 
From Kirchhoff’s voltage law:
VCE  VCC  I C R C  I E R E
IE  IC
VCE  V CC I C (R C  R E )
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 4:
Repeat the analysis of Example 3 using the approximate
technique, and compare solutions for ICQ and VCEQ.
Örnek 5:
Repeat the exact analysis of Example 3 if β is reduced to
70, and compare solutions for ICQ and VCEQ.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 6:
Determine the levels of ICQ and VCEQ for the voltage-divider configuration
using the exact and approximate techniques and compare solutions. In this
case, the conditions of approximate solution will not be satisfied but the
results will reveal the difference in solution if the criterion of approximate
solution is ignored.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Voltage Divider Bias Analysis
Transistor Saturation Level
I Csat  I Cmax 
V CC
RC  RE
Load Line Analysis
Cutoff:
Saturation:
VCE  VCC
I C  0mA
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
VCC
IC 
RC  RE
VCE  0V
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
DC Bias with Voltage Feedback
Another way to
improve the stability
of a bias circuit is to
add a feedback path
from collector to
base.
In this bias circuit
the Q-point is only
slightly dependent on
the transistor beta, .
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Base-Emitter Loop
From Kirchhoff’s voltage law:
VCC – I C R C – I B R B – VBE – I E R E  0
Where IB << IC:
I'  I  I  I
C C B
C
Knowing IC = IB and IE  IC, the loop
equation becomes:
VCC – I B R C  I B R B  VBE  I B R E  0
Solving for IB:
IB 
VCC  VBE
R B  (R C  R E )
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Collector-Emitter Loop
Applying Kirchoff’s voltage law:
IE + VCE + I’CRC – VCC = 0
Since IC  IC and IC = IB:
IC(RC + RE) + VCE – VCC =0
Solving for VCE:
VCE = VCC – IC(RC + RE)
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 7:
Determine the quiescent levels of ICQ and VCEQ for the
network
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 8:
Determine the dc level of IB and VC for the network
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Base-Emitter Bias Analysis
Transistor Saturation Level
I Csat  I Cmax 
V CC
RC  RE
Load Line Analysis
Cutoff:
VCE  VCC
I C  0 mA
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Saturation:
V
CC
I 
C R R
C
E
VCE  0 V
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 9:
For the network:
(a) Determine ICQ and VCEQ.
(b) Find VB, VC, VE, and VBC.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 10:
Determine VC and VB for the network
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 11:
Determine VCEQ and IE for the network
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 12:
Determine the voltage VCB and the current IB for the
common-base configuration
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 13:
Determine VC and VB for the network
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 14:
Given that ICQ=2 mA and VCEQ=10 V, determine R1 and
RC for the network
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Örnek 15:
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Transistor Switching Networks
Transistors with only the DC source applied can be used
as electronic switches.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Switching Circuit Calculations
Saturation current:
I Csat 
VCC
RC
To ensure saturation:
I
I B  Csat
 dc
Emitter-collector resistance
at saturation and cutoff:
R sat 
VCEsat
I Csat
R cutoff 
VCC
I CEO
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Switching Time
Transistor switching times:
t on  t r  t d
t off  t s  t f
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
Troubleshooting Hints
•
•
•
•
•
Approximate voltages
– VBE  .7 V for silicon transistors
– VCE  25% to 75% of VCC
Test for opens and shorts with an ohmmeter.
Test the solder joints.
Test the transistor with a transistor tester or a curve tracer.
Note that the load or the next stage affects the transistor operation.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.
PNP Transistors
The analysis for pnp transistor biasing circuits is the same
as that for npn transistor circuits. The only difference is that
the currents are flowing in the opposite direction.
Electronic Devices and Circuit Theory, 10/e
Robert L. Boylestad and Louis Nashelsky
Copyright ©2009 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458 • All rights reserved.