Lecture #16 Bipolar transistors
Reading: transistors
Bipolar: chapter 6
MOS: chapter 14
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Topics
Today:
Bipolar transistors
IV curve
Making an amplifier
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Electron flow
• So the forward bias on the
emitter-base junction
induces the electrons to
flow, but most of them
make it across to the
collector instead of stopping
in the base and flowing to
the base terminal
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Collector
Base
Emitter
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Beta (β) and alpha (α)
• When the base-emitter junction is forward
biased, and the base-collector junction is
reverse biased, approximately a fixed portion of
the electrons will make it across to the collector
rather than coming from the base contact. The
ratio current from the electrons that make it
across to the total current is alpha IC=αIE Alpha
can be close to one, 0.99 is not uncommon.
• Since IB+IC=IE IB=(1-α)IE
• we define β=(1-α)
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• Both of these definitions for the bipolar
transistor are only approximately true,
but for most bipolar transistors in the
active mode, they are reasonable
approximations.
IC

IE
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IC

IB
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Device model
• As long as the base-collector junction is
reverse biased, and the Emitter-base
junction is forward biased, a good model
of the NPN transistor is:
Collector
Base
I c  I b
Emitter
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Other modes of operation
Cut-off:
• If the Emitter-base junction (the one controlling the
current) is not forward biased, then the transistor is said
to be in cut-off.
• A small amount of current will still flow, usually negligible
Saturation:
• If the Base-collector junction sees so much current flow
that it is no longer forward biased, then the device will no
longer behave as described.
Breakdown:
• If a high enough voltage is applied, the transistor
junctions will break down, and a high current can flow.
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Currents and voltages
• The currents are labeled by the letter for
the terminal they come into
IC
IB
IE
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The voltages are labeled with a
double subscript, with the
subscripts referring to the two
terminals the voltage difference is
taken between:
Example, the voltage difference
between the collector and emitter
leads is called VCE
The voltage between the base and
the emitter is called VBE
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IV curve
• Since the transistor is a three terminal device is a three
terminal device, you might think that 6 variables would
be important:
• Vbc – the voltage between the base and the collector
• Vbe – the voltage between the base and the emitter.
• Vce- The voltage between the collector and the emitter.
• Ib- the current into the base.
• Ic- the current into the collector.
• Ie- the current out of the emitter.
• But the transistor has no net charge, so IB+IC=IE
• And of course if you know any two of the voltages you
can calculate the third. We generally use VBE and VCE
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Transistor circuit configurations
• Typically we will want to use the transistor as a
device which has an input and an output. Since
one of the terminals must be shared, we call that
a common terminal
• The voltages with respect to the common
terminal are then used to describe the operation
of the transistors
• There are three types of connections:
– Common emitter,
– Common collector,
– Common base
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Common Emitter configuration
R
The voltages are labeled with a
double subscript, with the
subscripts referring to the two
terminals the voltage difference is
taken between:
IC
IB
+
Vin
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IE
+
Vout
-
Example, the voltage difference
between the collector and emitter
leads is called VCE
The voltage between the base and
the emitter is called VBE
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IV curve for common emitter
• To show the IV curve for
Ic
a NPN transistor in a
common emitter
configuration, we plot the
voltage from the collector
to the emitter Vce vs the
current from the emitter Ic
• The base current is
shown by setting several
values and then plotting a
curve for each of them
(called steps)
Saturation
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Breakdown
Forward
Active
• Cutoff
Vce
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The NPN bipolar as a
current amplifier
• The bipolar transistor is naturally a current amplifier,
because the voltage VBE is pretty much clamped to .7
volts in the active mode of operation.
• As VBE moves slightly above 0.7 volts, the current gets
very large
• If VBE is slightly below 0.7 volts, the current goes to zero
• Rather than trying to set VBE to a very precise value, we
can just put in a current IB instead.
• The current from the collector is IC=βIB, so we amplify the
input current by the factor β
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The bipolar transistor as a voltage
amplifier
• We can convert a
voltage into a
current by using a
resistor, and we
can also convert a
current into a
voltage, so we can
make a voltage
amplifier from a
NPN transistor
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RC
IC
IB
+
RB
Vin
-
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IE
+
Vout
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Voltage amplification
The current into the base IB is:
(Vin  0.7 volts)
IB 
RB
And the current into the collector is:
IC   I B
And if we have a 5 volt supply rail, the output voltage is:
Vout
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 (Vin  0.7 volts) 

 5volts  RC  
RB


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Amplifiers
• Notice that when the input voltage goes up, the output
voltage goes down (the voltage gain is negative
• This is a very common feature of single transistor
amplifiers
• The input is referenced to the 0.7 volts of the turn on for
the base-emitter diode, and must be higher than 0.7
volts. (Why?)
• The output is offset from the power supply voltage, and
can not go higher than the power supply voltage. (Why?)
Since the output is larger than the input, where
does the power come from?
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Biasing a transistor
• Setting up a transistor circuit so that it will
amplify a voltage without it needing have a
specific offset voltage, and producing an
output referenced to a desired point
instead of whatever you get in terms of an
offset from the power supply, is called
biasing a transistor. We will study biasing
in chapter 8.
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The bipolar transistor in a logic
device
• Bipolar devices have also been used to
make logic circuits: an example of a NOR
gate:
A
B
Output
If A is below 0.7 volts, and B is also below 0.7
volts, then the output is near 5 volts
if either A or B is high, then the output is pulled
down
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