COMSATS Institute of Information Technology
Virtual campus
Islamabad
Dr. Nasim Zafar
Electronics 1 EEE 231
Fall Semester – 2012
Transistor as an Amplifier Circuit:
Lecture No:
20
.
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Amplifier Gain:
Ø
•
•
Ø
Ø
Amplifiers are 2port networks:
input port
output port
A is called the amplifier gain.
If the gain is constant, we call this a linear amplifier.
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Transistor Specifications:
Ø
Maximum collector current, IC .
Ø
Maximum power dissipated, PD
Ø
PD = IC * VCE
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Transistor Specifications:
Ø
Minimum CE voltage for breakdown, V(BR)CEO
Ø
Carefully examine absolute max ratings.
Ø
DC current gain
–
variable
–
β = hFE in specs.
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Amplifier Gain in Decibels:
Ø
Amplifier gain is expressed in decibels (dB)
–
Ø
Ø
Originally it was expressed as “Bels” (named after
Alexander Graham Bell), but these proved to be of
insufficient size so we multiply “Bels” by 10
“decibels.”
Decibels are a logbased ratio and are therefore
dimensionless.
Purpose: We want to measure the ratio of some value
relative to another (e.g. sound power in a stereo amplifier).
Derivation of dB…(Cont.)
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Derivation of Decibels (Contd.):
Ø
Ratio of power of interest (call it “p1”) to some other reference
power (say, p2):
p1
p2
Ø
However, these values are generally quite huge and tend to be
logarithmically related; thus, creation of “the Bel:”
p1
Bel = log
p2
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Derivation of Decibels (Contd.):
•
•
However, the Bel is a bit too small, so let’s multiply it by
10 and call it a decibel (10 x Bel = 1 dB).
Which gives us the decibel expression for power:
decibel power
p1
= 10 log
p2
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Exercise: dB for Voltage:
First, let’s relate voltage to power:
p = vi
i = v/r
p=v r
2
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Exercise: dB for Voltage:
Upon substitution:
2
�v r �
�v �
�v1 �
10 log �
�= 10 log � �= 10 log � �
�v r �
�v �
�v2 �
2
1
2
2
2
1
2
2
Which gives us the decibel expression for voltage:
decibelvoltage
�v1 �
= 20 log � �
�v2 �
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Some Physical Conclusions:
Ø
If dB is positive, then v1 > v2, the signal is amplified.
Ø
If dB is negative, then v1 < v2, the signal is attenuated.
Ø
If dB is 0, then v1 = v2.
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BJT Transistor Amplifiers:
CommonEmitter Amplifiers:
Ø
Ø
The commonemitter amplifier exhibits high voltage and
current gain.
The output signal is 180º out of phase with the input.
CommonEmitter Amplifiers:
Transistor Biasing as an Amplifier Circuit:
Ø
For this discussion, we consider DC behaviour and
assume that we are working in the normal linear
amplifier region with the:
BE junction forward biased and
CB junction reverse biased.
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CommonEmitter Characteristics:
Ø
Treating the transistor as a current node:
Also:
IE
IC IB
IC α IE Ico
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CommonEmitter Characteristics:
Hence:
IC
α ΙC IB) ICO
which after some rearrangement gives:
IC
IB
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ICO
1 α
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CommonEmitter Characteristics:
Define a common emitter currenttransfer ratio :
Such that:
β
α
1 α
IC βIB
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ICO
1 α
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CommonEmitter Characteristics:
Ø
Ø
Since reverse saturation current is negligible the second term
on the right hand side of this equation can usually be neglected
(even though (1 α) is small)
Thus
IC βIB
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Gain FactorsSummary:
IC
IE
Usually given for common base amplifier
IC
IB
Usually given for common emitter amplifier
IE
IB
Usually given for common collector amplifier
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The CommonEmitter Amplifiers:
Transistor Biasing as an Amplifier Circuit:
Ø BE junction forward biased.
VBE ≈ 0.7 V for Si
Ø CB junction reverse biased.
Ø
KCL: IE = IC + IB
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Transistor Biasing as an Amplifier Circuit:
Ø
The purpose of dc biasing is to establish the Qpoint for operation.
Ø
Ø
The collector curves and load lines help us to relate the Qpoint and its
proximity to cutoff and saturation.
The Qpoint is best established where the signal variations do not cause
the transistor to go into saturation or cutoff.
Ø
What we are most interested in is, the ac signal itself. Since the dc part
of the overall signal is filtered out in most cases, we can view a
transistor circuit in terms of just its ac component.
Characteristic Curves with DC Load Line:
Ø
Drawn on the output characteristic curves.
Ø
Component values in a bias circuit.
Ø
–
Determine quiescent point, Q
–
Q is between saturation and cutoff
Best Q for a linear amplifier.
–
Midway between saturation and cutoff
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Characteristic Curves with DC Load Line:
Ø
Active Region:
Ø
Qpoint, and current gain.
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Common Emitter CharacteristicsSummary:
Ø
βdc not constant
Ø
βdc dependent on dc operating point
Ø
Quiescent point = operating point
Ø
Active region limited by
–
Maximum forward current, IC(MAX)
–
Maximum power dissipation, PD
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Transistor Amplifier Basics:
Ø
We will use a capital (upper case) letter for a DC quantity
(e.g. I, V).
Ø
We will use a lower case letter for a time varying (a.c.)
quantity (e.g. i, v)
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