MEMS2082
Chapter 3-2 Semiconductor devices
Transistors and Amplifiers-BJT
Department of Mechanical Engineering
Bipolar Transistor Construction
Department of Mechanical Engineering
npn BJT
Department of Mechanical Engineering
npn BJT
I E = IC + I B
VBE = VB − VE
VCE = VC − VE
Emitter is more heavily n-doped than collector
VC > VB > VE
Base-to-emitter pn junction is forward biased
Base-to-collector pn junction is reverse biased
Base is a very thin layer of p type region
Department of Mechanical Engineering
NPN Transistor
Department of Mechanical Engineering
NPN Transistor as Current Amplifier


The larger collector current IC is proportional to the base current IB according to
the relationship IC =βIB , or more precisely it is proportional to the base-emitter
voltage VBE . The smaller base current controls the larger collector current,
achieving current amplification.
The analogy to a valve is sometimes helpful. The smaller current in the base acts
as a "valve", controlling the larger current from collector to emitter. A "signal" in
the form of a variation in the base current is reproduced as a larger variation in the
collector-to-emitter current, achieving an amplification of that signal.
β is about 100
Amplification factor
Department of Mechanical Engineering
NPN Transistor as Current Amplifier

Constraints on Transistor Operation
Department of Mechanical Engineering
pnp BJT Transistor
Very thin
Department of Mechanical Engineering
BJT packages
Department of Mechanical Engineering
Terminal characteristics of
a typical BJT
0.2 V when saturation
Department of Mechanical Engineering
Common emitter transistor circuit

Transistor Characteristic Curves
Controlled by collector circuit
ic is proportional to ib
0.2 V when saturation
Department of Mechanical Engineering
Transistors

Transistor Characteristic Curves
Department of Mechanical Engineering
Bipolar Transistor Switch

Vin=0.7V
Department of Mechanical Engineering
Transistor Switch Example


The base resistor is chosen small
enough so that the base current drives
the transistor into saturation. In this
example the mechanical switch is used
to produce the base current to close the
transistor switch to show the principles.
In practice, any voltage on the base
sufficient to drive the transistor to
saturation will close the switch and light
the bulb.
There is no current to the base, so the
transistor is in the cut off condition with
no collector current. All the voltage
drop is across the transistor.
Department of Mechanical Engineering
Transistor Switch Example

NPN Common Emitter Switch
For switching currents less than an ampere,
the transistor switch can be used.
When the switch is open, no current flows in
the base so the collector current is cut off.
The resistor RB must be small enough to
drive the transistor to saturation so that most
of the voltage Vcc appears across the load
Rc.
In a configuration where the output is taken
below the load resistor, this kind of switch
can function as an inverting buffer in digital
circuits.
Department of Mechanical Engineering
Transistor Switch Example

Op-Amp Switching
If an op-amp is used instead
of a mechanical switch to
operate a transistor switch, a
diode is used for protection of
the base-emitter junction in
case the op-amp swings to its
negative supply voltage.
Department of Mechanical Engineering
LED Switch
O V or 5V
Department of Mechanical Engineering
Darlington Pair
The emitter current of TR1 is the base current of TR2.
A change in base current of TR1 can give a change 100 times larger in its emitter current.
A change in the base current of TR2 has a similar effect on its emitter current.
Therefore there is an overall amplification of 100 x 100 = 10000 times.
This circuit is sometimes called the Super Alpha Pair.
It is often used as a power output stage.
The two transistors can come in the same package.
Department of Mechanical Engineering
Phototransistor and Optoisolator
An electronic device designed to
transfer electrical signals by
utilizing light waves to provide
coupling with electrical isolation
between its input and output
Planar (top) and silicone dome
(bottom) layouts - cross-section
through a standard dual in-line
package
Department of Mechanical Engineering
Guarantee a transistor in saturation
Q:
Department of Mechanical Engineering
Common emitter
npn BJT
Department of Mechanical Engineering
Common emitter
npn BJT current amplifier

The Common Emitter Amplifier Circuit
This type of configuration is the most commonly used
circuit for transistor based amplifiers and which represents
the "normal" method of bipolar transistor connection.
Department of Mechanical Engineering
Common emitter
npn BJT current amplifier
I E = IC + I B
VC > VB > VE


The collector current IC is proportional to the base current IB
according to the relationship IC =βIB
β has a value between 20 and 200 for most general purpose
transistors
Department of Mechanical Engineering
Common emitter
npn BJT current amplifier
Department of Mechanical Engineering
BJT current
amplifier
Department of Mechanical Engineering
Common base
pnp BJT voltage amplifier

Common base
The BASE connection is common to both the input signal AND the output signal with
the input signal being applied between the base and the emitter terminals. The
corresponding output signal is taken from between the base and the collector terminals
with the base terminal grounded. The input current flowing into the emitter is quite
large as its the sum of both the base current and collector current respectively
therefore, the collector current output is less than the emitter current input resulting in
a current gain for this type of circuit of "1" (unity) or less, in other words the common
base configuration "attenuates" the input signal.
Department of Mechanical Engineering
pnp BJT voltage amplifier

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that
the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not
very common due to its unusually high voltage gain characteristics due to its high
ratio of output to input resistance or more importantly "load" resistance (RL) to "input"
resistance (Rin) giving it a value of "Resistance Gain". Then the voltage gain (Av for a
common base configuration
Department of Mechanical Engineering
Emitter Follower Circuit
a large Current gain


This type of configuration is commonly known as a Voltage Follower or
Emitter Follower circuit.
This type of bipolar transistor configuration is a non-inverting circuit in that the
signal voltages of Vin and Vout are in-phase. It has a voltage gain that is always
less than "1" (unity).
Department of Mechanical Engineering
Emitter Follower Switch

NPN Emitter Follower Switch
The emitter follower can also be
used for switching. The resistor
RB must be small enough to drive
the transistor to saturation so that
most of the voltage Vcc appears
across the load.
Department of Mechanical Engineering
Angular Position of a Robotic
Scanner

Photo-interrupter
Department of Mechanical Engineering