Nodal analysis

1
Lecture 16 Bipolar transistors
Reading transistors Bipolar chapter 6 MOS
chapter 14
2
Topics

• Today
• Bipolar transistors
• IV curve
• Making an amplifier

3
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

Collector
Base
Emitter
4
Beta (ß) and alpha (a)

• 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 ICaIE
  Alpha can be close to one, 0.99 is not uncommon.
• Since IBICIE? IB(1-a)IE
• we define ß(1-a)

5

• 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.

6
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
Emitter
7
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.

8
Currents and voltages

• The currents are labeled by the letter for the
  terminal they come into

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
IC
IB
IE
9
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 IBICIE
• And of course if you know any two of the voltages
  you can calculate the third. We generally use
  VBE and VCE

10
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

11
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 Example, the voltage difference between
the collector and emitter leads is called VCE
The voltage between the base and the emitter is
called VBE
IC
IB
Vout -
Vin -
IE
12
IV curve for common emitter

• To show the IV curve for 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)

Ic
Breakdown
Forward Active

• Cutoff

Vce
Saturation
13
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 ICßIB, so we
  amplify the input current by the factor ß

14
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

RC
IC
IB
Vout -
Vin -
RB
IE
15
Voltage amplification

• The current into the base IB is
• And the current into the collector is
• And if we have a 5 volt supply rail, the output
  voltage is

16
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?

17
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.

18
The bipolar transistor in a logic device

• Bipolar devices have also been used to make logic
  circuits an example of a NOR gate

Output
A
B
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