Download 20 Bipolar Transistors and Amplifiers

Transistors
"The Transistor was probably the most important invention of the 20th
Century…”
The American Institute of Physics
The Nobel Prize in Physics 1956
William Bradford
Shockley
1/3 of the prize
John Bardeen
1/3 of the prize
Walter Houser Brattain
1/3 of the prize
USA
USA
USA
Semiconductor Laboratory of
Beckman Instruments, Inc.
Mountain View, CA, USA
University of Illinois
Urbana, IL, USA
Bell Telephone Laboratories
Murray Hill, NJ, USA
b. 1910
(in London, United Kingdom)
d. 1989
b. 1908
d. 1991
b. 1902
d. 1987
Transistors
Zhores I. Alferov, Russian, 70, A.F. Ioffe Physico-Technical Institute, St. Petersburg
Herbert Kroemer, German-born American, 72, Univ. of California, Santa Barbara
Jack S. Kilby, American, 76, Texas Instruments, Dallas
The prize is being awarded with one half jointly to:
ZHORES I. ALFEROV, and HERBERT KROEMER for developing
semiconductor heterostructures used in high-speed- and opto-electronics
and one half to:
JACK ST. CLAIR KILBY for his part in the invention of the integrated circuit
Transistors
First
Transistor,
1947
(Schokley,
Bardeen,
Brattain)
First Integrated
Circuit,
1958
(Jack Kilby)
Intel’s 1.7 Billion
Transistor Chip
2004
What are transistors for?
Transistor is a solid-state switch
Mechanical switch
3V
120V
Thermo-switch
Solar triggered switch
(opto-switch)
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What are transistors for?
Transistor is a logical gate – a heart of computer
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What are transistors for?
Transistor is an amplifier (audio, video, microwave)
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Bipolar Junction Transistors (BJT)
A Bipolar Transistor essentially consists of a pair of PN Junction
Diodes that are joined back-to-back.
This forms a sort of a sandwich where one kind of semiconductor
is placed in between two others.
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BJT operation
n
p
Forward polarity is
applied to the left diode.
Positive battery polarity
attracts electrons.
If the battery voltage
exceeds the built-in
voltage, there is a high
current flowing through.
p
n
Reverse polarity is
applied to the right diode
Positive battery polarity
attracts electrons.
There is no electrons on
the p-side of the diode.
The current is zero.
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BJT operation
n
p
Battery 1
Forward polarity is
applied to the left diode.
There is a high current
flowing through it.
The electrons entering
p-region recombine with
holes to form a current
loop.
p
n
Battery 2
Reverse polarity is
applied to the right diode
There is still no
electrons on the p-side
of the diode (they all
move into battery 1.
The current is zero.
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BJT operation
p
n
Battery 1
n
Battery 2
The P-layer is made VERY THIN
• Forward polarity is applied to the left diode.
• Electrons from the left n-region are driven to the positive
battery 1 terminal through the layer p.
• However, as the p-layer is very thin, most of the electrons
miss the holes and shoot through the second barrier, toward
the positive terminal of the battery 2. There is a high current
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flowing through it.
n-p-n bipolar transistor
Device structure
Emitter
n
Circuit symbol
C
Base Collector
p
B
n
E
p-n-p bipolar transistor
Device structure
Emitter
p
Circuit symbol
C
Base Collector
n
p
B
E
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BJT Current-Voltage Characteristics
VCE, V
Very small base current (~5-75 µA)
causes much higher collector current (up to 7.5 mA).
The current gain is ~ 100
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Transistor amplifiers characteristics
Amplifiers
An amplifier makes an
electrical signal bigger and
mirror precisely every small
change of the input level.
GAIN = 10
Vin
Iin
Vout
Iout
Amplifiers are used for analog
signals, such as speech or
music and for digital signals
such as radar transmitters or
communication system
repeaters.
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BJT amplifier circuit analysis: Operating point
RL
VCE, V
Collector current depends on two circuit parameters:
the base current and the collector voltage.
At high collector voltage the collector current depends on the base
current only.
For Ibase = 40 µA, Icoll = 4 mA for any EC-E greater than 1.5 V
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BJT amplifier circuit analysis: Operating point
VCC
RL
VCE
VCE, V
For an arbitrary collector voltage, collector current can be found using the KVL.
The KVL for the collector – emitter circuit, VCC = IRL×RL + VCE;
V −V
I RL = CC CE The RL current depends linearly on the collector voltage VCE
RL
Resistor RL and the C-E circuit of BJT are connected in series, hence IRL = IC
For Ib = 40 µA and VCC = 13V, the collector current IC = 4 mA
For Ib = 75 µA and VCC = 14V, the collector current IC = 4 mA
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BJT amplifier gain analysis: 1
1. Input circuit
The input voltage has two components: the DC bias and the AC signal
Vin
AC signal amplitude
DC bias
Time
DC voltage component biases the base-emitter p-n junction in the forward direction
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AC component is the input signal to be amplified by the BJT.
BJT amplifier gain analysis: 2
VCE, V
2. Output circuit
The collector current has two components too.
IC
AC current amplitude
Base current
DC current
DC and AC collector currents flow through the BJT
in accordance with its I-V characteristics
Time
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BJT amplifier gain analysis: 3
VCE, V
3. Amplifier gain
The resistance of the forward-biased B-E junction is very low.
Hence the base current IB ≅ Vin/R1 = (VinDC+VinAC)/R1
The collector current IC does not depend on the collector voltage if the latter is high enough.
Hence, IC ≅ β IB;
The voltage drop across the load resistance R2: V2= IC R2;
The output voltage Vout = VCC - V2 = VCC - IC R2;
Vout = VCC - IC R2 = VCC - β IB R2 = VCC - β R2(VinDC+VinAC)/R1;
In most amplifiers only AC component of the output voltage is important:
VoutAC = - β R2VinAC/R1;
The amplifier voltage gain: kV = VoutAC/VinAC= - β R2/R1;
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