In the exam you are expected to know about the
transistor as a switch.
At
their simplest, transistors are solid
state electronic switches. Solid
state means that there are no moving parts, and the switching action is
regulated by the arrangement of the semiconductor materials.
The junction or bipolar transistor was at the heart of the revolution in electronics in the 1950’s, and it has formed the heart of circuits simple and complicated ever since. Previously electronic switching was done with thermionic valves, which got hot, were fragile, and wore out. The photograph shows a thermionic valve.
Transistors
can be made very small and very cheaply. The
biggest revolution has been in the manufacture of integrated circuits, which has
allowed the development of ever more powerful (and less expensive) computers.
Question
1 Why
are transistors used in preference to thermionic valves?
ANSWER
Thermionic
valves are still used in up-market hi-fi systems. Audiophiles consider them to be much more ‘musical’ than
their transistor equivalent.
The photograph shows a selection of different kinds of transistor.
Notice how some of the large transistors are mounted on a heatsink,
which conducts the heat away from the component.
Without the heatsink, the component will get too hot.
A
transistor has three terminals:
Collector
– takes current form
the top rail (wire connected to
the positive of the power supply).
Emitter
– delivers current to
the load.
Base
– this is connected to
the sensor part of the circuit. The
increase in voltage at the base turns the transistor on.
Here is a typical transistor circuit:
This
set up is called an emitter-follower.
If we use Ohm’s Law, we can work out that the voltage across the 10 W
resistor is 5.0 V. The base voltage
is at 5.7 V, so there must be a voltage drop of 0.7 V between the base and the
emitter. Not surprisingly, this is
called the base-emitter voltage and
is 0.7 V for most transistors.
Question
2
State Kirchhoff’s First Law. Explain
how the diagram above is consistent with Kirchhoff I. ANSWER
The
voltages are related by:
Vb
= Ve + 0.7
We can measure the currents and voltages going into a transistor and plot graphs called the transfer characteristics. Here is a typical graph:
The
axis labeled VCE is the collector
– emitter voltage. When the
transistor is off, the voltage is high, just like the voltage across an open
switch. The axis labeled VBE
is the base-emitter voltage.
Below about 0.6 volts between the base and the emitter, there is no
current flowing, and the transistor is turned off.
Between 0.6 and 0.7 volts, current starts to flow, and there is a linear
region whereby the voltage across the collector and emitter is proportional
to the current flowing into the base. This
would be the region in which we would be interested if we were going to use the
transistor as an amplifier.
We
won’t be considering the use of a transistor as an amplifier here.
We will look at its use as a switch. In this case, we are looking at the saturated region, in which case the transistor is fully turned on.
In circuits that use the transistor as a switch, the base-emitter voltage
is set to be below 0.6 volts for when the circuit is off, and above 0.7 volts
for when the transistor is turned on. An
example of such a circuit is a light operated switch, such as those found on
lampposts to turn on streetlights.
The
graph shows an ideal transistor. In
reality there is a voltage between the collector and the emitter, since the
transistor has a definite resistance.
Question 3 When carrying a current of 0.5 amps a transistor is found to have a collector emitter voltage of 1.2 V. What is its resistance? What power does it dissipate?
Summary
Transistor
has three terminals, base, collector, emitter. A
Transistor allows a small current to turn on a big current |