Transcript Ch04 Lecture NotesRev2

DMT 121/3 : ELECTRONIC I
.…Electronic I.…
..DMT 121/3..
ChapTer FoUr
DC BIASING - BIPOLAR
JUNCTION TRANSISTORS (BJTs)
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Mohd Khairuddin B Md Arshad
DMT 121/3 : ELECTRONIC I
Objectives
 Discuss the concept of dc biasing of a transistor
 Analyze voltage-divider bias, base bias, emitter bias
and collector-feedback bias circuits.
 Basic troubleshooting for transistor bias circuits.
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Introduction
For the transistor to properly operate it must be
biased. There are several methods to establish
the DC operating point.
We will discuss some of the methods used for
biasing transistors as well as troubleshooting
methods used for transistor bias circuits.
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Biasing and 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
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The DC Operating Point
The goal of amplification in most cases is to increase the
amplitude of an ac signal without altering it.
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The DC Operating Point
For a transistor circuit to amplify it must be properly biased
with dc voltages. The dc operating point between saturation
and cutoff is called the Q-point. The goal is to set the Q-point
such that that it does not go into saturation or cutoff when an a
ac signal is applied.
IB   IC and VCE 
IB IC  and VCE 
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The DC Operating Point
Recall that the collector characteristic curves graphically show the
relationship of collector current and VCE for different base
currents. With the dc load line superimposed across the collector
curves for this particular transistor we see that 30 mA (IB = 300 A)
of collector current is best for maximum amplification, giving equal
amount above and below the Q-point. Note that this is three
different scenarios of collector current being viewed
simultaneously.
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The DC Operating Point
With a good Q-point established, let’s look at the effect a
superimposed ac voltage has on the circuit. Note the collector current
swings do not exceed the limits of operation (saturation and
cutoff). However, as you might already know, applying too much ac
voltage to the base would result in driving the collector current into
saturation or cutoff resulting in a distorted or clipped waveform.
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Waveform Distortion
FIGURE 5-6 Graphical load line illustration of a transistor being driven into
saturation and/or cutoff…continues at next page
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Waveform Distortion
FIGURE 5-6 Graphical load line illustration of a transistor being driven into saturation
and/or cutoff………
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Fixed – Bias (Base Bias) Circuit
• Simplest transistor bias configuration.
• Commonly used in relay driver circuits.
• Extremely beta-dependant and very unstable
Fig. 4.2
Fixed-bias circuit.
Fig. 4.3
DC equivalent of Fig. 4.2.
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Fixed Bias (Base Bias) Circuit
Fig. 4.4
Base–emitter loop.
Fig. 4.6
Fig. 4.5
Base – Emitter loop
VCC  IBRB  VBE  0
VCC  VBE
IB 
RB
Collector–emitter loop.
Collector – Emitter loop
VCC – ICRC – VCE = 0
VCC  VCE
VCE = VCC – ICRC; then IC 
RC
= VC – VE since VE = 0
VBE = VB – VE (since VE = 0)
VCE = VC
VBE = VB
Since IC = IB, then
Sensitive to Beta
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Measuring VCE and VC.
 VCC  VBE 
IC   

 RB 12
DMT 121/3 : ELECTRONIC I
Emitter Bias
•
Fig. 4.17
Use both a positive and a negative supply voltage on emitter or it just contain an
emitter resistor to improve stability level over fixed – bias configuration.
BJT bias circuit with emitter resistor.
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FIGURE 5-21 An npn transistor with emitter bias.
Polarities are reversed for a pnp transistor. Single
subscripts indicate voltages with respect to ground.
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Emitter Bias – only RE
Collector – Emitter loop
VCC – ICRC – VCE – IERE = 0
IE  IC
VCC – ICRC – VCE –ICRE = 0
VCC – VCE = IC (RC + RE)
Fig. 4.17
resistor.
BJT bias circuit with emitter
Base – Emitter loop
VCC – IBRB – VBE – IERE = 0
IE = ( + 1) IB
Then, VCC – IBRB – VBE – ( + 1)IBRE = 0.
IB 
VCC  VBE
RB  (   1) RE
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VCC  VCE
IC 
RC RE
Since IC = IB, so IC also equivalent to
 (VCC  VBE)
IC 
RB  (   1) RE
Less sensitivity to beta
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Emitter Bias – RE + DC Voltage Supply
Collector – Emitter loop
+
-
Base – Emitter loop
VEE - IBRB - VBE - IERE = 0
IE = ( + 1) IB
Then, VEE - IBRB - VBE - ( + 1)IBRE = 0.
VEE  VBE
IB 
RB  (   1) RE
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VCC – ICRC – VCE – IERE + VEE = 0
IE  IC
VCC – ICRC – VCE –ICRE + VEE = 0
VCC – VCE + VEE = IC (RC + RE)
VCC  VCE  VEE
IC 
RC RE
Since IC = IB, so IC also equivalent to
 (VEE  VBE )
IC 
RB (   1) RE
Less sensitivity to beta
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Emitter Bias - summary
 (VEE  VBE )
IC 
RB (   1) RE
With DC Voltage supply + Resistor at Emitter
 (VCC  VBE)
IC 
RB  (   1) RE
With only Resistor at Emitter
Previous analysis we use IE = ( + 1) IB; but if use IE  IC  IB, then
from previous slide we can get.
VEE  VBE
IC 
RB
RE
If RE >>> RB/ then we can drop RB/
in equation
VEE  VBE Less sensitivity to beta
I
C


or independent to beta
RE
OR we also can use  ( + 1) If VEE >>> VBE then
to get the same result.
VEE
IC 
RE
Independent to VBE
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Example 4.16 - Boystead
Determine VCEQ and IE for the
network as shown in Fig.
-VEE + IERE + VCE = 0
IE = ( + 1)IB
VEE – IBRB – VBE – IERE = 0
IE = ( + 1)IB
VEE  VBE
IB 
RB  (   1) RE
20V  0.7V
IB 
240k  (91)( 2k )
IB  45.73A
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VCEQ = VEE – (+1)RE
= 20V – (91)(45.73)(2k)
= 11.68 V
IE = 4.16 mA
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Emitter Bias - summary
Adding RE to the emitter improves the stability of a
transistor.
Stability refers to a bias circuit in which the
currents and voltages will remain fairly
constant for a wide range of temperatures and
transistor Beta () values.
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Voltage Divider Bias
• The most widely used type of bias circuit. Only one power supply is
needed and voltage-divider bias is more stable ( independent) than other
bias types.
• Two methods of analysis, exact and approximate analysis
Fig. 4.25 Voltage-divider bias configuration.
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Voltage Divider Bias – Exact Analysis
(BOYLESTAD)
Fig. 4.27 Redrawing the input side of
the network of Fig. 4.25.
Fig. 4.28
Determining RTH.
To determine RTH  The voltage source is replaced by a shortcircuit equivalent, resulting……..
RTH = R1 ǁ R2
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Voltage Divider – Exact Analysis (BOYLESTAD)
To determine ETH  The voltage
source VCC remained on the network
and the open circuit Thevenin voltage
can be determined.
Fig. 4.29
Determining ETH.
ETH
VCCR 2
 VR 2 
R1  R 2
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Voltage Divider – Exact Analysis (BOYLESTAD)
The Thevenin network
is then redrawn as
shown in the Fig. 4.30,
and IBQ can be
determined by applying
Kirchoff’s voltage law.
Fig. 4.30
Inserting the Thévenin equivalent circuit.
ETH – IBRTH – VBE – IERE = 0,
….substitute IE = ( + 1) IB….. then
ETH  VBE
IB 
RTH  (   1) RE
Almost similar with emitter bias
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Voltage differences over resistance.
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Voltage Divider – Exact Analysis (BOYLESTAD)
ETH  VBE
IB 
RTH  (   1) RE
IC = IB ; IE = ( + 1) IB  IB
Substituting between these OR
equation in previous slide (from
derivation), resulting :
Fig. 4.30
Inserting the Thévenin equivalent circuit.
ETH  VBE
IE 
RE  RTH / 
If RE >>> RTH/, then…
Independent to Beta
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ETH  VBE
IE 
RE
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Voltage Divider – Exact Analysis (BOYLESTAD)
Once IB is known, the rest
of the parameters can be
determined.
VCE = VCC – IC (RC + RE)
Fig. 4.25 Voltage-divider bias configuration.
The remaining equations VE, VC
and VB are also similar as
obtained in emitter bias
configuration.
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Voltage Divider – Approximate Analysis
(BOYLESTAD)
R 2VCC
VB 
R1  R 2
and
Ri = ( + 1)RE  RE
with condition
Fig. 4.32 Partial-bias circuit for calculating the
approximate base voltage VB.
Ri = equivalent
transistor between
base and ground for
transistor with an
emitter resistor RE
RE  10R2
If beta times the value RE is at
least 10x the value R2, the
approximate approach can be
applied with high accuracy.
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Voltage Divider – Approximate Analysis
(BOYLESTAD)
Once VB is determined, the
level of VE can be calculated.
VE = VB – VBE
And emitter current
VE
IE 
RE
and IC  IE
Fig. 4.32 Partial-bias circuit for calculating the
approximate base voltage VB.
VCE = VCC –ICRC –IERE but
since IE  IC
VCE= VCC – IE (RC + RE)
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Voltage Divider – FLOYD
If the base current is smaller than the current through
R2, the bias circuit can be viewed as a voltage divider
consisting of R1 and R2 – Fig (a).
If IB is not small enough to neglect compared to I2, then
must consider RIN(base) is in parallel with R2 ( Fig. (b)
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Voltage Divider - FLOYD
To develop formula for the dc input
resistance at the base of a transistor, we
will use this diagram. VIN is applied
between base and ground, and IIN is the
current into the base as shown.
RIN (base)
The input current is the base current:
IN = IB
By substitution: RIN ( base)
VIN  DC I B RE


I IN
IB
Cancelling th IB term gives:
RIN(base)  DCRE
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VIN

I IN
VIN  VBE  I E RE
VIN  I E RE
I E  I C   DC I B
VIN   DC I B RE
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Voltage Divider - FLOYD
RIN(base)  DCRE
Total resistance from base to ground is
R2 || RIN(base)  R2 || DCRE
Voltage divider is formed by R1 and the resistance from base to ground
R2|| DCRE
R2 ||  DC RE
VB  (
)VCC
R1  ( R2 ||  DC RE )
If R2|| DCRE >>> R2 (at least 10X greater) then:
R2
VB  (
)VCC
R1  R2 )
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Voltage Divider - FLOYD
Then VE = VB – VBE
IE 
VE
RE
IC  IE  VC = VCC – ICRC
VCE = VC – VE
VCC  VCC – IC (RC + RE)
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Collector Feedback Bias (DC Bias with Voltage
Feedback)
• An improved level of stability can also be obtained by introducing a
feedback path from collector to base.
• If IC tries to increase, it drops more voltage across RC, thereby causing VC
to decrease. When VC decrease, there is a decrease voltage across RB,
which decrease IB. The decrease in IB produce less IC which in turn, drops
less voltage across RC and thus offsets the decrease in VC.
• These feedbacks keep the Q-point stable.
IC  VRC   VC  VRB   IB 
VC
 IC  VRC   offset the
decrease in VC
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Collector Feedback Bias (DC Bias with Voltage
Feedback)
Base – Emitter Loop
VCC – IC'RC – IBRB – VBE – IERE = 0
Actual case IC' = IC + IB
Approximation can be employed :
IC'  IC = IB and IE  IC
VCC – VBE - IB (RC + RE) – IBRB = 0
Solving for IB, yields
VCC  VBE
IB 
RB   ( RC  RE )
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Collector Feedback Bias (DC Bias with Voltage
Feedback)
Collector – Emitter Loop
VCC – IC'RC – VCE – IERE = 0
Approximation can be employed :
IC'  IC and IE  IC
VCC – VCE - IC (RC + RE) = 0
VCE = VCC – IC (RC + RE)
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Troubleshooting
Shown is a typical voltage divider circuit with correct
voltage readings. Knowing these voltages are required
before logical troubleshooting can be applied. We will
discuss some of the faults and symptoms.
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Troubleshooting
R1 Open
With no bias the
transistor is in
cutoff.
Base voltage goes
down to 0V.
Collector voltage
goes up to 10 V
(VCC).
Emitter voltage goes
down to 0V.
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Troubleshooting
Resistor RE Open:
Transistor is in cutoff.
Base reading voltage will
stay approximately the
same.
Collector voltage goes up
to 10V(VCC).
Emitter voltage will be
approximately the base
voltage + 0.7V.
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Troubleshooting
Base Open Internally:
Transistor is in cutoff.
Base voltage stays
approximately the
same.
Collector voltage goes
up to 10V(VCC).
Emitter voltage goes
down to 0V.
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Troubleshooting
Open BE Junction:
Transistor is in cutoff.
Base voltage stays
approximately the
same.
Collector voltage goes
up to 10V(VCC)
Emitter voltage goes
down to 0V.
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Troubleshooting
Open BC Junction:
Base voltage goes down
to 1.11V because of
more base current flow
through emitter.
Collector voltage goes
up to 10V (VCC).
Emitter voltage will drop
to 0.41V because of
small current flow from
forward biased baseemitter junction.
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Troubleshooting
RC Open:
Base voltage goes down to
1.11V because of more
current flow through the
emitter.
Collector voltage will drop
to 0.41V because of current
flow from forward biased
collector-base junction.
Emitter voltage will drop to
0.41V because of small
current flow from forward
biased base-emitter
junction.
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R2 Open:
Troubleshooting
Transistor pushed close to
or into saturation.
Base voltage goes up
slightly to 3.83V because
of increased bias.
Emitter voltage goes up to
3.13V because of
increased current.
Collector voltage goes
down because of
increased conduction of
transistor.
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Summary
 The purpose of biasing is to establish a stable operating
point (Q-point).
 The Q-point is the best point for operation of a transistor
for a given collector current.
 The dc load line helps to establish the Q-point for a
given collector current.
 The linear region of a transistor is the region of
operation within saturation and cutoff.
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Summary
 Voltage-divider bias is most widely used because it is
stable and uses only one voltage supply
 Base bias is very unstable because it is  dependant.
 Emitter bias is stable but require two voltage supplies.
 Collector-back is relatively stable when compared to base
bias, but not as stable as voltage-divider bias.
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