3i.5 Understand that a transistor is a three terminal device (emitter,
base, collector) in which a small base current will control a larger
collector current and this enables the transistor to be used as an
amplifier.
Understand that the ratio of the collector current to the base current (IC/IB) is the current gain β of the transistor.
Note: the student is not required to recall transistor configurations.
Circuits shown will be an npn transistor connected in common emitter
mode.
How a transistor works
A transistor has three pins:
Emitter
Base
Collector
Different types of transistor may have these coming out in a different order. Always check how they are arranged.
The type of transistor shown in the diagram is called an npn. The arrow showing current flow points to the emitter output.
A small current flowing flowing from the base to the emitter (e.g. 2mA)
causes a bigger current (e.g. 100mA) to flow from the collector to the
emitter.
The β (beta) of a transistor is the current gain and is calculated from Collector Current / Base Current:
β=IC/IB
In our example β=100/2 = 50
NB you will not have to calculate this for the Intermediate Examination.
3i.7 Recall that a transistor must be provided with the correct DC
voltages and currents to allow it to function and that this is termed
correct biasing.
Note that calculations are not required.
To make a transistor work as an amplifier it needs some resistors and capacitors as shown in the circuit opposite.
The components shown on the circuit diagram opposite have the following functions:
C1 allows an AC signal to reach the base, but blocks DC
R1 and R2 establish a DC
voltage at the base. This is called the bias and is very important for
the proper working of the transistor. The AC voltage superimposed on the
bias voltage cause a small current to flow from base to emitter. This
in turn causes a larger current to flow from the collector to the
emitter. R3 and R4 limit this current. The voltage drop across R3
produces an alternating voltage at the collector which is output via C2.
This is called a common emitter
circuit because C3 effectively connects the emitter to ground at AC. So
the input and output are, at AC, connected to the emitter.
3i.6 Understand that if the variation in the base current is large
enough the collector current can be turned on and off and the
transistor behaves as a switch. Transistors as switches
There are times when we want to switch a large current from a device only capable of delivering a small current.
In the circuit opposite the +5V
could be the output pin of an IC. If we wanted this to turn on a
power amplifier drawing 5Amps we could use a transistor as a switch
driving a relay.
When the +5v is applied to the
base it causes a current to flow from the base to the emitter. This
causes a larger current, lets say 100mA, to flow through the relay coil
the collector and emitter which turns the relay switch on. This switch
would be capable of carrying
the 5 Amps of current to the power amplifier.
3i.8
Recall that a transistor can be used to generate audio and radio
frequencies by maintaining the oscillations in a tuned or frequency
selective circuit.
Diagrams will show the Colpitts oscillator with the transistor in emitter follower mode.
Students are not expected to recognise other types of oscillator. Transistors as oscillators
By feeding some of the output
back to the base, a transistor can be used as an oscillator to produce
a sine wave at audio and radio frequencies.
This is similar to the
amplifier circuit shown above, but the input has been replaced by a
tuned circuit and some of the collector output is fed back to the
resonant circuit
When the power is turned on, C1 and C2
charge up and then discharge through the coil L1.
The oscillations across the capacitors are
applied to the base-emitter junction and appear, amplified, at
the
collector output. C3 is used to allow the oscillator output to be
applied to the next stage. The choke provides a high impedance to the
RF current, thus preventing it entering the 12V line.
Distinguish between a crystal oscillator and a variable frequency oscillator (VFO) based on a tuned circuit. Crystals as oscillators
Crystals are much more stable than coils and capacitors. They are
useful when an oscillator is required to work on a single frequency
with high stability e.g. as the frequency standard in a frequency
counter or in a frequency synthesiser. Their stability can be further improved by placing them in a
thermostatically controlled oven.
The Colpitts circuit can also be used with a crystal, by making C1 and C2 fixed and replacing L1 with a crystal.
