Demonstrating the wonderful amplifying action of a transistor

Summary
A simple demonstration is described which shows how a transistor works. By including the audience into the actual circuit and by lighting LED's we show the potential of the simple transistor amplifier. The extension to two transistors - the Darlington Pair - is also described.

Note: this article has been published: Demonstrating the wonderful amplifying action of a transistor
J. P. Hare, IOP press, Journal of Physics Education, March 2004, p.128-131 reproduced here by permission of IOP)

Introduction
In 1956 the Nobel prize for physics was awarded to Shockley, Bardeen and Brattain for the invention of the transistor. In 2000 Alferov, Kroemer and Kilby were awarded the Nobel prize for 'basic work on information and communication technology' and the development of the integrated circuit - all work that would have been impossible without the humble transistor.

The transistor has enabled the modern telecommunications revolution. However, as time goes by the transistor itself has almost become invisible within our high-tech society and this ubiquitous invention is becoming unappreciated and even misunderstood. I believe it is important for people have an idea of what a transistor is, how it works and what it can do and to show that this wonderful device can be understood in terms of relatively simple physics.

The transistor
The transistor is an electronic device that transforms small electrical currents (and voltages) into larger copies of the original - it is what is called an amplifier and is said to have 'gain' (magnification). The transistor has three wire connections called; the emitter (E), the base (B) and the collector (C), see Fig. 1. By wiring the device up with other simple components an amplifier can easily be constructed. A typical transistor has a gain of ca. 100 times.

The physical theory describing the transistor is complex and involves understanding the movement of electrons (and the absence of electrons - holes) in P and / or N doped semiconductor materials. Readable accounts of the theory can be found in various sources (see the reference section below). What follows here is not a detailed account of the theory but a simple set of experiments that demonstrates the transistor working.

How it works
A diode is a two wire electronic component that only conducts electricity when connected the 'correct way round' i.e. with the potentials applied correctly. It is composed of a P and N semiconductor junction. The transistor is a three wire component composed of a sandwich of either PNP or NPN junctions. Electrically it is as if the transistor is composed of two diodes wired back to back, see Fig 1. The common middle region (the base - B) of the transistor is much thinner than the other two regions.

Because the diodes are opposed to each other no current would normally flow when a voltage is applied between the emitter and the collector - EC (although there may be a tiny leakage current). If a voltage is applied across BE (B positive and E negative for an NPN transistor) this junction will be forward biased so a current will flow in this circuit. However, because the base region is very thin (and also because when wired up correctly the collector is at a high potential and so attracts electrons) as much as 99% of this current will actually flow right across the base region to reach the collector (C). So we have actually made the EC circuit of the transistor conduct by applying a current into B (set up by a small voltage across BC).

Now the current flowing from the emitter must be equal to the sum of i) the 99% arriving at the collector and ii) the 1% that is left flowing through the base. So the base current is small, only 1% or so. But as we have seen the collector current can not exist without the little base current and so it is effectively controlling the collector current. This collector current is a larger copy of the base signal and so we find the transistor produces a current gain! Current gains of 100-200 are typical for a transistor. Usually the EC part of the circuit is used as the output and the base is used as the input of the amplifier.

The EB circuit is low voltage and low current while the EC is at a much higher potential and higher current. As power = voltage x current we must therefore have a higher power in EC and so a power-gain is possible with such a simple circuit. Of course the transistor does not amplify this small base signal by 'magic', the extra power is derived from the supply driving the transistor circuitry. The transistor needs a battery, or other supply, to work its 'magic'.

Experiment 1 - a simple series circuit
Connect in series a 9V battery, an LED and a 560 ohm resistor as shown in Fig. 2. The LED should light if connected correctly (reverse LED connections if nothing happens). The LED needs about 3V at 10mA to light and this can be achieved by putting an appropriate sized resistor in the circuit:

R = V / I = (9 - 3)V/0.01A = 600 ohms
(Note: actually we use a 560 ohm resistor in these experiments as it is an easily obtained 'preferred value' that is close enough to work)

Experiment 2 - a simple series circuit that won't work!
Please read Note (1) before going on with this experiment.
Now disconnect the lead to the positive battery terminal. Place a finger of one hand onto the positive 9V battery terminal and a finger of the other hand to make connection with the free lead. Now we have a series circuit as before but with the additional resistance of the body included. The resistance of the body is complex and will depend on the voltage applied and most importantly the contact resistance between the skin and wire connections (see note (2) below). The body can present a wide range of resistance anywhere between 10,000 - 1,000,000 ohms. For the sake of argument let us say it is 50,000 ohms = 50k ohm.

We now get:

I = V / R = (9 - 3) / 50,000 = 0.0001A = about 0.1mA

which we see is about 1/100 the current needed to light the LED (ca. 10mA) and so not surprisingly nothing happens!

Experiment 3 - The transistor amplifier
We have heard that a transistor can amplify about 100 times so we can make use of this property to boost the signal from the small current flowing through the body so that it can light an LED.

Carefully wire up the simple transistor circuit shown in Fig 3. When one finger on one hand is placed on the positive battery terminal and the other finger on the other hand is connected to the base connection of the transistor a tiny current (of a magnitude that we have just calculated - ca. 0.1mA) flows into the BE circuit of the transistor. Because of the gain of the transistor this sets up a CE current (where the LED is connected) of roughly 100 times this:

0.0001 x 100 = 0.01A = 10mA and so the LED lights !!

Experiment 4 - The Darlington Pair
So what will happen if we have two transistors in cascade (one feeding another) ? This is indeed possible and is called the Darlington Pair, see Fig. 4. Quiz the students / pupils as to what they might think the total gain of such a system will be. For example, will it be 2 x 100 = 200 or 100 x 100 = 10,000 times (see note 4). Try getting the whole class to form a human series chain, holding hands, with one hand from the first, and one hand of the last person making the connection between the positive 9V battery connection and the base of the Darlington pair (see Note 4 below).

Summary
I have found this little demonstration to be a most effective way of showing the action of a transistor. Please make sure that the students / pupils understand that for safety reasons the experiments must only be made using batteries (see Note. (1) below). In these very simple experiments we have used the transistor as an amplifying switch. To amplify more subtly changing signals (rather than ones that simply go ON or OFF) such as audio or radio we need to 'bias' the transistor so that a linear (lower distortion) amplification can take place. These next important steps are not dealt with here but details can be found in the suggested reading material in the reference section at the end of this article.

Components and parts
1) 2 x NPN transistor: most NPN transistor will work e.g. BC109C
2) 1 x LED; any light emitting diode LED will work
3) PP3 9V battery
4) 560 ohm resistor (any wattage)
5) 100 k ohm (see Note (4))
6) wire for connections (croc-clips are useful)

Acknowledgements
I would like to thank the many students and pupils who have helped in these demonstrations and in particular I would like to dedicate this article to the memory of Jan Meering who worked at the Angmering School (West Sussex). I would also like to thank NESTA (The National Endowment for Science Technology and the Arts) for support.

References and web sites:
1) For the physics of the transistor see the following good books:

Essential Theory for the Electronics Hobbyist, G T Rubaroe, 1988. ISBN 0 900162 69 4
From Atoms to Amperes, F. A. Wilson, ISBN 0 85934 199 2
Quantum Physics, R Eisberg and R Resnick, 1985. ISBN 0 471 87373 X

2) for information on the 1956 Nobel prize for Physics: 'for their researches on semiconductors and their discovery of the transistor effect' see the web site:
1956 Nobel Prize

for information on the 2000 Nobel Prize for Physics: 'for basic work on information and communication technology', see the web site:
2000 Nobel Prize

3) for details of the CSC Xmas LED W/S see: Xmas LED W/S

Notes
1) As the human body is used in these experiments electrical shock hazards need to be considered. With a 9V PP3 battery these experiments are completely safe. These experiments must only be made with batteries. A mains powered supply or 'battery eliminator' should never be used.

2) The human body is not like a standard resistor. Most important is the contacts resistance to the skin and this will depend on the moisture present (i.e. sweet etc) . If your audience is particularly 'cool' (or perhaps it is not a very humid day) then the contact resistance can be reduced (and these experiment improved) by slightly wetting the fingers (with tap water or saliva).

3) For several years I have run Christmas LED workshops for young children. The idea is to teach them about components, series and parallel circuits and wiring of LED's etc. We wire up 4 - 6 LEDs and the children decorate homemade cards and Xmas trees ! The transistor demo started as a part of this workshop.

4) The demonstrator should be aware that if the positive supply directly touches the base, and there is a short to the positive supply, the current that passes will damage the transistor(s). In the Darlington pair circuit for example a 100k ohm resistor should be placed in series with the base circuit to limit current (the resistor will hardly affect the function of the circuit because the gain is so very high around 100 x 100 = 10,000 !) .

5) The practical uses for such a circuit could include: i) as a simple transistor tester ii) as a damp detector when probing walls in old houses, or iii) as a light switch for an LED torch (which only lights when the torch is held) etc.

Please note that this article was published in Physics Education
see Physics Education webpage