Download Department of Electronics and Communication

Transistor Biasing
• What is meant by biasing the transistor?
– Applying external dc voltages to ensure that transistor
operates in the desired region
• Which is the desired region?
– For amplifier application, transistor should operate in active
region
– For switch application, it should operate in cut-off and sat.
• What is meant by quiescent point (Q-point)?
– The point we get by plotting the dc values of IC , IB and VCE
(when ac input is zero) on the transistor characteristics
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
• Transistor characteristics
showing Q-point
– Q-point is in the middle
of active region.
– This is called “Class-A”
operation. (Other classes
are discussed later)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
• Types of biasing:
– Fixed bias and Self bias
• Fixed bias:
– The value of IB is “fixed” by choosing
proper value for RB
Vcc  I B RB  VBE  0
VCC  VBE
IB 
RB
VCC  I C RC  VCE  0
VCE  VCC  I C RC
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
• Pros and Cons of Fixed bias:
– Pros:
1) Simple circuit
2) Uses very few resistors
– Cons:
Q-point is unstable
1) If temperature increases, then β increases, and
hence ICQ and VCEQ vary (effectively Q-point shifts)
2) If the transistor is replaced with another
transistor having different β value, then also Q-point shifts
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
• Load Line
– We have: VCE  VCC  I C RC
– This is an equation of straight line with points VCC/RC and
VCC lying on y-axis and x-axis respectively
– This line is called “Load line” because it depends on
resistor RC considered as “Load” and VCC
– Intersection of load line and transistor characteristic curve
is the Q-point or operating point
– This point is the common solution for characteristics and
load line equation
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
Variation in load line with
circuit parameters VCC, RC
and RB
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
• Voltage divider bias or Self bias
– Resistor RE connected between
emitter and ground
– Voltage-divider resistors R1 & R2
replace RB
– Circuit can be analyzed in two
methods:
• Exact method (using
Thevenin’s theorem)
• Approximation method
(neglecting base current)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
• Exact method:
– Input side of self-bias (Fig. a) transformed into Thevenin’s
equivalent circuit (Fig. b) where, RTH is the resistance
looking into the terminals A & B (Fig. c) and VTH is given by:
VTH
VCC R2

R1  R2
RTH  R1 || R2 
R1 R2
R1  R2
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
VTH  VBE  I B RTH  I E RE  0
I E  (   1) I B
IB 
VTH  VBE
RTH  (   1) RE
• Since β >> 1 and (β+1)RE >> RTH
VCC  I C RC  I E RE  VCE  0
VCE  VCC  I C RC  I E RE
VTH  VBE
IB 
 RE
IC   I B 
VTH  VBE
RE
• Since IC is almost independent of β,
Q-point is stable
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
•
Any change of transistor parameter that causes IC to increase will
cause IE to increase by almost the same amount. An increase in IE
causes an increase in VE
VE = IE . RE
• But, VB is essentially constant, so VBE reduces with an increase in
VE.
VBE= VB – VE
• The reduction in VBE reduces IB, which then reduces IC, thus
compensating for the parameter change that tried to increase IC.
• Changes in the bias value of IC automatically change the input voltage
in a way that has opposite effect on IC, thus tending to restore IC to its
original value. So, the bias circuit is known as self bias and emitter
resistor (stabilizes bias point) is known as emitter stabilization.
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
• Since equation for IC does not contain β, we say that IC is
independent of temperature variation and transistor
replacement.
Advantages of voltage divider bias
. Q-point is stable against variation in temperature and
replacement of transistor.
Disadvantages of voltage divider bias
• Analysis and design are complex
• More circuit components required
Department of Electronics and Communication
School of Engineering, Manipal University Jaipur
Tutorials
1. For a fixed bias circuit using Si transistor, RB = 500 kΩ, RC = 2 kΩ, VCC =
15 V, ICBO = 20 µA and β = 70. Find the collector current ICQ and VCEQ at
Q-point. Take VBE as 0.7 V.
(Ans: 3.422mA, 8.156V)
2. A Si transistor is biased for a constant base current. If β = 80, VCEQ = 8 V,
RC = 3 kΩ and VCC = 15 V, find ICQ and the value of RB required.
(Ans: 2.33 mA, 493 K)
3. Repeat problem 2 if the transistor is a germanium device. (VBE=0.3V)
(Ans: 2.33 mA, 507 K)
4.
For a self bias circuit using silicon transistor, RE = 300 Ω, RC = 500 Ω,
VCC = 15 V, β = 100 and . If 10R2= βRE, then determine the value of R1
to get VCEQ = VCC / 2.
(Ans: 9.85K)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Tutorials
1. Draw the DC load line and mark the Q- point on fixed bias circuit
Assume Beta DC=100 and neglect Base–Emitter voltage.
(Vcc=30V,RB=1.5 Mohm, RC=5 Kohm).
Ans: VCE,max=30v,VCEQ=20v,ICQ=2mA
2. In a fixed bias cct. Find the base current required to establish
VCE=6v, also find RB & IE, (VBE=0.7v, Beta DC=120,
VCC=12v,RC=2.2.Kohm).
Ans: IB=22.75 uA, RB=497 Kohm
3. Determine the region in which the transistor operates.
(VBE=0.2v,RB=120kohm,RC=1kohm,VCC=15v,Beta DC=120).
Department of Electronics and Communication
School of Engineering, Manipal University Jaipur
Tutorials
• Determine the DC bias voltage VCE and the current IC for the voltage
divider configuration.
(IC =0.85 mA, VCE = 12.22 V)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Tutorials
5.
For a self bias circuit using silicon transistor, RE = 300 Ω, RC = 500 Ω, VCC =
15 V, β = 100 and . If 10R2= βRE, then determine the value of R1 to get VCEQ
= VCC / 2.
(Ans: 9.85K)
6.
For a self bias circuit, the transistor is a Si device, RE = 200 Ω, R1 = 10R2 =
10 kΩ, RC = 2 kΩ, β = 100 and VCC = 15 V. Determine the values of ICQ and
VCEQ.
7.
Suppose if the transistor used in problem 6 failed, and was replaced with a
new transistor with β = 75. Is the new transistor still biased for active region
operation?
8.
A self bias circuit uses Si transistor, RC = 330 Ω, RE = 100 Ω and VCC = 12 V.
Estimate the values of R1 and R2 required to provide a base current of 0.3 mA,
so as to locate the operating point at ICQ = 18 mA and VCEQ = 4.25 V.
9.
For a fixed bias circuit, VCC = 12 V and RC = 4 kΩ. The Ge transistor used is
characterized by β = 50, ICEO = 0 and VCE sat = 0.2 V. Find the value of RB
that just results in saturation.
(Ans: 198.3K)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• Amplifier
– Device which gives larger swing in output voltage
proportional to the input voltage swing
– BJT basically amplifies current: Collector current equals
beta times Base current
– With proper circuit designs, we can get voltage
amplification and power (both voltage and current)
amplification
– For faithful amplification (no distortion), BJT should
operate in Active region throughout the input cycle (Class
A)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
– Without any bias,
transistor is in cut-off
(IC=0, VCE=0)
– Biasing circuit fixes the
operating point in the
middle of active region
required for faithful
amplification
– Figure shows commonemitter amplifier circuit
employing fixed bias
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• With reference to the fig in previous slide, as input voltage vin
varies, iin varies, thus base current iB varies
• This variation in base current is amplified beta times to get
variation in collector current iC
• Output voltage vout is VCC – iC RC
• Note that if vin increases, there is proportional decrease in vout,
but of greater magnitude
• Similarly if vin decreases, vout increases proportionally
• Thus output voltage of CE amplifier is 180o out of phase with
input voltage
• (Note that small letters are used to represent ac quantities)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• Voltage gain or voltage
amplification factor is vout /vin
which is dependent on β, RC
and other physical parameters
of the transistor
• Figure shows input and output
waveforms for the amplifier
circuit shown previously
• Note the dc shift in the output
voltage waveform. i.e., when
vin = 0, vout = VCEQ
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• This
animation
shows the
working of
Common
Emitter
transistor
circuit
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• Gain of the amplifier is usually expressed in decibels
• (AV)dB = 20 log10 | AV |
• Usually a gain of 100 (i.e., 40 dB) can be obtained using single
transistor. For higher gain requirement, two or more amplifier
stages are to be cascaded
• Overall gain is product of individual gains; But when expressed
in dB, overall gain is sum of individual gains (in dB)
AV  AV 1 . AV 2 ....... AVN
( AV ) dB  ( AV 1 ) dB  ( AV 2 ) dB  .......  ( AVN ) dB
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• RC coupling
– Fig shows CE amplifier
employing self bias
– Additional components
are CC and CE
– CC is called coupling
capacitor – used to
prevent dc component
from entering or leaving
amplifier stage
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
– CE is called emitter bypass capacitor – used to bypass the
ac emitter current – preventing it from flowing through RE
– If ac emitter current is allowed to pass through RE, then vBE
reduces and hence output voltage reduces
• Frequency response of amplifier
– It’s important to know the behavior of amplifier at different
frequencies
– Gain is NOT constant at all frequencies – depends on
various factors
– Frequency response is a plot of gain versus frequency
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• Figure shows frequency
response plot
• At lower and higher
frequencies, gain is less
• Gain attains const value
at mid frequencies
• Bandwidth of amplifier is
range of frequencies over
which gain is not less
than 3 dB of maximum
gain
20 log | 0.707 AVO | = 20 log | AVO | – 3
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• Analysis of frequency response curve
– At very low frequencies, reactance of coupling capacitors is
high, hence there is loss of signal voltage across capacitors,
resulting in reduced gain
– Also at low frequencies, emitter bypass capacitor does not
fully bypass the ac emitter current, hence ac voltage drop
develops across RE, resulting in reduced gain
– At very high frequencies, shunt capacitances due to wiring
and inter-layer junction capacitances will be prominent,
hence resulting in signal loss
– At mid frequencies, gain is maximum and constant
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Amplifier
• Classification of amplifiers:
– Based on mode of operation:
• Class A: collector current flows throughout the complete input
cycle (360o); Q-point is in the centre of active region (no distortion)
• Class B: collector current flows during (positive or negative) half
cycle of input; Q-point is at “just cut-off” or “just saturation”
• Class AB: collector current flows for more than half cycle, but less
than full cycle of input waveform; Q-point is “near cut-off” or “near
saturation”
• Class C: collector current flows for less than half cycle of input
waveform; Q-point is in “deep cutoff” or “deep saturation”
• (For classes B, AB and C, output is distorted or clipped)
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Tutorials
1.
2.
3.
A three-stage amplifier circuit has first stage gain of 45 dB,
second stage gain of 50 dB and third stage gain of –5 dB.
What is the overall gain? If input to the first stage is 0.1mV,
what is the output of final stage?
An amplifier has maximum gain of 200 and bandwidth of
500 kHz. If lower cutoff freq is 50 Hz, what is the upper
cutoff freq and gain at this frequency?
Design a self bias circuit, given the following parameters:
IC=1mA, VCC=12V, VCE=VCC/2, VB=VCC/10, beta=100,
VBE=0.6V, R2=2K
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
End of Module 3
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur
Transistor Biasing
• Approximate analysis:
– Carried out only if βRE ≥ 10R2
• IB is negligible compared to I1 and I2
• So,
VB 
VCC R2
R1  R2
IE 
VB  VBE
RE
IC  I E
VCE  VCC  I C RC  I E RE
Department of Electronics and Communication Engineering, School Of Engineering,
Manipal University, jaipur