BASIC
ELECTRONICS COURSE
Page 21
INDEX
Up to now we have
shown how an NPN transistor can be connected to the power rails
with the emitter near (or on) the negative rail (the 0v rail)
and the collector going to the positive rail via a LOAD.
This is called a COMMON EMITTER arrangement.
The voltage on the base must be 0.7v higher than the emitter and
the amount of current delivered to the base is multiplied by the
GAIN OF THE TRANSISTOR and will be the maximum amount of current
that can flow in the load. These are the basics of how a
transistor works. Using these facts we can create a number of
circuits, simply by connecting the transistor to the power rails
in different ways. A single transistor and its associated
parts is called STAGE. Generally, a stage is
separated by a capacitor at the front (called the input
capacitor) and one at the output (called the output capacitor).
This makes it easy to see the components associated with the
stage as these parts play a part in turning the transistor on
and setting the operating conditions for the stage.
There are 3 ways to
connect a transistor and the circuit produced for each
connection has a special name, they are:
1. The Common
EMITTER stage - the most OFTEN-USED
arrangement
2. The Common COLLECTOR - also called EMITTER FOLLOWER stage
3. The Common BASE stage.
It's easy to
identify a COMMON EMITTER stage. The emitter is connected
to the 0v rail. (It may be connected directly or via a low value
resistor or via a resistor and capacitor - this will be
discussed later).
The COMMON COLLECTOR stage has the collector connected
directly to the positive rail.
The COMMON BASE stage has the base connected directly (or
via a low-value resistor) to one of the rails.
The
potentiometer in the animations above and opposite is a VOLTAGE DIVIDER. When
the wiper it is at 0v, (the centre of the pot is near the 0v
rail) the output from the potentiometer is ZERO. As the
potentiometer is rotated, the output rises. The COMMON EMITTER
and COMMON COLLECTOR (EMITTER FOLLOWER) circuits do not turn
on until the voltage on the base is 0.6v. As the
potentiometer is rotated further, the current delivered to the base of the
Common Emitter stage increases and the transistor amplifies this
current as shown in the animation.
For the Common Collector stage, the voltage on the emitter is
0.6v less than the voltage on the base. As the voltage on the
emitter rises, the current through the load increases. The
voltage on the emitter is always 0.6v less than the base. That's
why it is called an EMITTER FOLLOWER CIRCUIT.
There are reasons
why a particular stage is chosen for an application. The
common-emitter stage is the most often used stage as it has
medium input and output impedance (another name for impedance is
"RESISTANCE") and has both voltage and current
gain.
If you want high input impedance, the EMITTER FOLLOWER stage is
used.
If you want very low input impedance, the COMMON BASE stage is
used.
If you have a device (one that produces a signal) and it has a
low output impedance, it should be matched up with an amplifying
circuit that has a low input impedance. This will allow the
maximum amount of energy to be passed from the signal source to
the stage.
A
STAGE
A stage consists of a transistor and its biasing components. The
diagram below shows the COMMON EMITTER stage with two
base-bias resistors and emitter resistor. This is called the
"BRIDGE BIASING" arrangement and a simpler
arrangement is called the "SELF BIASING"
stage.
The self-biasing
stage is shown below: It provide nearly the same features as the
bridge arrangement but uses less components.
The self-bias stage is the
simplest and most often used. It has all the features necessary
for a transistor to amplify a signal. It has:
1. A base-bias resistor to turn on the transistor
2. A load resistor.
HOW
THE SELF-BIAS STAGE WORKS
The self-bias stage sets its own operating conditions due to the
base-bias resistor between the base and collector. This is a
very clever place to put it as it turns the transistor ON so that
the collector sits at exactly half rail voltage. For a
transistor to provide the maximum amplification, the collector
must sit at half rail. This will allow the signal to be
amplified equally in the positive direction as well as the
negative direction. This needs a lot of explaining, so we will
start at the beginning.
When power is first applied, the
transistor is not turned on and current flows through the load
resistor and base-bias resistor to turn it on very
hard. This happens very quickly and when the transistors
turns on, current flows in the
load resistor and a voltage develops across it. This causes the
collector voltage to drop and the voltage across the base-bias
resistor is reduced. This causes less current to flow into the
base and the rate at which the transistor turns on is reduced.
The transistor keeps turning on but its rate of "turn-on"
slows
down as the collector voltage falls.
As the collector voltage falls,
the current through the load resistor increases and to provide
this increased current, the current into the base must increase.
But the base-bias resistor cannot provide this extra current and
so the transistor turns off a slight amount. This creates a
higher voltage across the base-bias resistor and allows more
current to flow into the transistor to turn it on more.
Eventually an equilibrium point is reached where the voltage
across the base-bias resistor is exactly sufficient to allow
current to flow into the base and turn the transistor on to a
level called the "equilibrium point" or "bias
point" or "set point." The value of
the base bias resistor and load resistor creates this
point.
By careful selection of the value for these two resistors the bias point
can be set to HALF RAIL VOLTAGE.
The transistor will sit with
these voltages on the base and collector and the current through
the load is called the QUIESCENT CURRENT. The
quiescent
current is also called the "idle current" or "set
current" or "half current." The voltage on
the collector is shown graphically in the diagram below:
We can now add the input and
output capacitors. These
capacitors have no effect on the biasing of the stage. We have
already learnt that capacitors do not pass DC and so the biasing
voltages (the base and collector voltages) are not affected. The
input capacitor passes the signal from a previous stage into the
self-bias stage and the output capacitor passes the AMPLIFIED
signal to the next stage.
We can now apply a "rising
and falling" voltage to the input line (this is called a WAVEFORM
or AC voltage) and the transistor will amplify it. When
the voltage is rising, (this is called the positive direction)
the transistor is said to be amplifying the positive
excursion and when the voltage is falling, the transistor is
said to be amplifying in the negative direction. The voltage on
the collector never rises above rail voltage (9v in this
case) or below the negative rail (0v in this case).
In the diagram below, you will
notice the output waveform is "opposite" (INVERTED) to
the input waveform. This is due to the transistor. The stage is
called an INVERTING STAGE. When the input voltage is rising, the
output is falling and vice versa. The size of the waveforms do
not indicate the gain of the stage. The stage-gain may be 20,
100 or even more depending on the gain of the transistor, the
value of the resistors and the voltage of the supply. At the
beginning of the input waveform, the voltage is RISING. This
will turn the transistor on MORE and the voltage on the
collector will FALL. This is shown by the output waveform as a
falling or NEGATIVE-GOING waveform. Only one cycle of the
waveform is needed to show how the output responds.
Question 85: Name the three leads
of a transistor:
Ans: collector,
base, emitter
Question 86: What is another name
for the negative rail?
Ans: 0v rail,
Question 87: What is a STAGE?
Ans: A stage consists of a transistor and
its biasing components.
Question 88: Name the three ways
a transistor can be connected the power rails.
Ans: common emitter, common
collector (emitter follower), common base.
Question 89: For a COMMON EMITTER
stage; when the input is rising, is the output:
(a) rising (b) falling
Ans: falling
Question 90: For a common emitter
stage, name the resistor between the collector and positive
rail.
Ans: LOAD
Question 91: For a common emitter
stage, name the resistor between the base and collector.
Ans: base-bias resistor
Question 92: How many resistors
are needed to bias a transistor in a BRIDGE arrangement?
Ans: Four resistors
Question 93: How many resistors
are needed to bias a common emitter transistor in self-bias
arrangement?
Ans: two resistors
Question 94: Is the common
emitter stage called a NON-INVERTING or INVERTING stage?
Ans: Inverting
Question 95: For a self-biasing
common emitter stage, what is the quiescent voltage on the
collector?
Ans: Half-rail voltage
Question 96: For a self-biasing
transistor in a common-emitter stage, name the resistor that
turns the transistor on.
Ans: Base-bias resistor
Question 97: If the base-bias
resistor is removed, what voltage will appear on the collector?
Ans: The collector voltage will rise to
RAIL VOLTAGE. This is because the transistor will turn off and
you can consider it to be removed from the circuit. The only
component will be the LOAD resistor and if no current is flowing
through this resistor, rail voltage will appear on BOTH
ENDS.
Question 98: Name the two
capacitors (we have described them above) that are associated
with a common-emitter transistor STAGE:
Ans: input capacitor, output
capacitor
Question 99: When a transistor
"turns on," will the collector voltage be: (a) rail
voltage (b) lower than rail voltage.
Ans: lower than rail voltage. The exact
value will depend on the type of transistor and the value of the
biasing resistors.
Question 100: To turn a
transistor "ON," the voltage on the base must be (a)
0v, (b) 0.3v (c) 0.7v (d) 1.7v
Ans: 0.7v The other value
0.3v is not sufficient to turn the transistor ON,
and the voltage can NEVER go above 0.7v, so if the
voltage is 1.7v, the transistor is DAMAGED! This voltage is
measured between the base and emitter and we are assuming the
emitter is connected to the 0v rail for these measurements.
Question
101: When a transistor is FULLY TURNED ON, the voltage on the
collector will be:
(a) rail voltage, (b) half-rail
voltage (c) 0.7v
(d) 0.35v (e) 0v
Ans: 0.35v This is the accepted value for a fully turned
on transistor however it may be higher than this due to a number
of reasons. It can NEVER go as low as 0v and 0.7v is reserved
for identifying the voltage on the base to turn a transistor
ON.
We are now going
to describe a product that turns a transistor ON and OFF
at exactly the right instant to energise a number of coils.
These coils are placed inside a circular magnet and the magnet
is allowed to rotate. The result is a BRUSHLESS MOTOR.
This is a very
interesting
product and comes in the form of a BRUSHLESS
FAN and has been incorporated into almost every computer, to
keep the active components (mainly the microprocessors) from
overheating. Miniature versions are also available and are
placed over the individual microprocessors with its own cooling
fin.
It a very simple circuit but the way the motor works needs a
full page of explaining.
The next page
covers the: BRUSHLESS
MOTOR.
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