Transcript Electronic Devices and Circuit Theory

Electronic Devices and Circuit Theory
Boylestad
DC Biasing - BJTs
Chapter 4
Ch.4 Summary
Biasing
Biasing: Applying DC voltages to a transistor in order
to turn it on so that it can amplify AC signals.
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Ch.4 Summary
Operating Point
The DC input
establishes an
operating or
quiescent point
called the Q-point.
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Ch.4 Summary
The Three Operating Regions
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
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Ch.4 Summary
DC Biasing Circuits
Fixed-bias circuit
Emitter-stabilized bias circuit
Collector-emitter loop
Voltage divider bias circuit
DC bias with voltage feedback
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Ch.4 Summary
Fixed Bias
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Ch.4 Summary
The Base-Emitter Loop
From Kirchhoff’s voltage
law:
+VCC – IBRB – VBE = 0
Solving for base current:
VCC  VBE
IB 
RB
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Ch.4 Summary
Collector-Emitter Loop
Collector current:
IC  IB
From Kirchhoff’s voltage law:
VCE  VCC  IC RC
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Ch.4 Summary
Saturation
When the transistor is operating in saturation, current
through the transistor is at its maximum possible value.
V
ICsat  CC
R
C
VCE  0 V
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Ch.4 Summary
Load Line Analysis
The load line end points 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 controls the values of VCE and IC .
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Ch.4 Summary
The Effect of VCC on the Q-Point
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Ch.4 Summary
The Effect of RC on the Q-Point
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Ch.4 Summary
The Effect of IB on the Q-Point
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Ch.4 Summary
Emitter-Stabilized Bias Circuit
Adding a resistor
(RE) to the emitter
circuit stabilizes
the bias circuit.
Electronic Devices and Circuit Theory
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Ch.4 Summary
Base-Emitter Loop
From Kirchhoff’s voltage law:
 VCC  IE RE  VBE  IE RE  0
Since IE = ( + 1)IB:
VCC  IBRB  (β  1)I BRE  0
Solving for IB:
VCC  VBE
IB 
RB  (β  1)RE
Electronic Devices and Circuit Theory
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Ch.4 Summary
Collector-Emitter Loop
From Kirchhoff’s voltage law:
I E RE  VCE  IC RC  VCC  0
Since IE  IC:
VCE  VCC – I C(RC  RE )
Also:
VE  I E RE
VC  VCE  VE  VCC  IC RC
VB  VCC – I R RB  VBE  VE
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Ch.4 Summary
Improved Biased Stability
Stability refers to a condition in which the currents and
voltages 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
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Ch.4 Summary
Saturation Level
The endpoints can be determined from the load line.
VCEcutoff:
VCE  VCC
IC  0 mA
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ICsat:
VCE  0 V
VCC
IC 
RC  RE
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Ch.4 Summary
Voltage Divider Bias
This is a very stable bias circuit.
The currents and
voltages are nearly
independent of any
variations in .
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Ch.4 Summary
Approximate Analysis
Where IB << I1 and I1  I2 :
VB 
R2VCC
R1  R2
Where RE > 10R2:
IE 
VE
RE
VE  VB  VBE
From Kirchhoff’s voltage law:
VCE  VCC  IC RC  IE RE
IE  IC
VCE  V CC IC (RC  RE )
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Ch.4 Summary
Voltage Divider Bias Analysis
Transistor Saturation Level
I Csat  ICmax
V CC

RC  RE
Load Line Analysis
Cutoff:
VCE  VCC
IC  0 mA
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Saturation:
V
CC
I 
C R R
C
E
VCE  0 V
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Ch.4 Summary
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
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Ch.4 Summary
Base-Emitter Loop
From Kirchhoff’s voltage law:
VCC – IC RC –IBRB –VBE –IE RE  0
I' C  IC  IB  IC
Where IB << IC:
Knowing IC = IB and IE  IC, the
loop equation becomes:
VCC – β–BRC  IBRB  VBE  βIBRE  0
Solving for IB:
IB 
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VCC  VBE
RB  β(RC  RE )
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Ch.4 Summary
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)
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Ch.4 Summary
Base-Emitter Bias Analysis
Transistor Saturation Level
I Csat  ICmax 
V CC
RC  RE
Load Line Analysis
Cutoff
Saturation
VCE  VCC
V
CC
I 
C R R
C
E
IC  0 mA
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VCE  0 V
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Ch.4 Summary
Transistor Switching Networks
Transistors with only the DC source applied can be
used as electronic switches.
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Ch.4 Summary
Switching Circuit Calculations
Saturation current:
ICsat
VCC

RC
To ensure saturation:
IB 
ICsat
βdc
Emitter-collector
resistance at
saturation and cutoff:
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Rsat
VCEsat

ICsat
Rcutoff
VCC

ICEO
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Ch.4 Summary
Switching Time
Transistor switching times:
t on  t r  t d
t off  t s  t f
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Ch.4 Summary
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
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Ch.4 Summary
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
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