The need for measuring the gain of a transistor goes back to the early days when the gain was fairly low and to get a good device for a particular application you had to go through a whole batch and pick the best.
Today's transistors are much better, almost all have more gain than you need.
This means we don't really need to know the gain but it is interesting to find the value for those transistors you have in the junk-box or for a transistor which may have damaged during soldering. For these and other reasons we have designed a tester to give you the answers.
It has been designed to give you the gain for small signal devices, medium power devices and high power devices. we mentioned in the main article that small signal devices have a gain in the range 70 - 450, medium power devices have a gain of 50 - 200, and high power devices a gain of 10 - 110.
These are only approximate values and could get a device with a value totally outside this range. For instance, some medium- to high-power devices available in Darlington versions can have a gain of 1,000 to 30,000 - way outside the range of our Tester.
But if the device is BELOW the ranges specified above, I would suggest the transistor is faulty and damaged in some way.
It can quite often happen due to overheating, if you don't use a heatsink a soldering, or overheating when in use.
But this is getting away from our topic.
Let's get on with the theory.
The gain of a transistor is a very variable thing. Even from a single batch of transistors, the gain can vary from less than 70 to more than 400.
GAIN is one of the factors used to grade transistors and is one of the reasons why we have so many thousands of different types. You may have noticed the suffix 'A', 'B' or 'C' after a transistor type. These letters denote the gain category and although none of the transistors are faulty, the numbering and lettering combines with other factors that mean the transistor can only be used in a certain location such as low voltage, low gain or low frequency application.
Since the gain of a transistor varies according to many things such as manufacturing technique, the voltage of the supply (in which the transistor is placed), the frequency of operation of the circuit and the current passing through it, an infinite number of values can be created from a single type, so it is quite often difficult to know which to use and what to expect.
The only way we can cover this complexity is to discuss the three values commonly quoted in specification sheets.
What happens to a transistor, and how it behaves in a particular circuit is another matter and cannot be predicted in any way so we will content ourself with values that can be determined.
This value is generally about 20% higher than the "AC" or "working" gain (to be discussed later) and that's why it is most commonly used.
But some purists don't think the above method is realistic as a transistor is not a static device but a dynamic amplifier such as when used in an audio situation. They want an "operating" value of gain. In other words they want an AC gain value.
In this case an amplifier is set up and varying signals are applied to the base for varying rail voltages. Once again, the best gain is picked off the graph and used as the AC value or BETA or Hfe The third value of gain is the value that applies at the upper operating frequency of the transistor and does not concern us in this discussion. It is the highest operating frequency for the transistor and occurs when the gain falls to unity.
For most requirements, the difference between Hfe and HFE can be ignored if you are working within the limits of the transistor.
Data sheets generally supply HFE or dc. current gain and this is the value we obtain from the tester.
The current gain can be written as: B = IC/IB
and this is exactly what the tester is doing, but in a reverse mode. It is reading off the base current for a particular collector current.
It does this in a very clever way. It knows that when a certain current flows through the 4k7 collector load resistor in the gain section, a voltage will be produced across this resistor to cause the detecting gate between pins 9 and 8 to change state and turn on the "gain LED."
By turning the "gain pot," the base current is increased until the transistor produces the required collector current and the value of gain is read off the scale around the pot.
In other words all the mathematics has been done by the project designer and any transistor fitted to the test socket will duplicate the values already worked out.
In most discussions, transistors are referred to as "transistors," but in effect the writer is referring to the first type of transistor to be invented (in simple terms), namely the bipolar junction transistor, or BJT.
They should be referred to as BJT's, but since this is such a mouthful, they are simply called "transistors."
Since the introduction of this type of transistor there have been a number of other developments such as the Field Effect transistor (the FET), the Darlington transistor and others that can be lumped into the transistor group.
Unfortunately our tester is not capable of testing these devices and you will get a false reading if you try to test them, so it is advisable to know the device you are testing is actually a "common" or "garden" transistor.
Most of the devices in your junk-box will be common NPN or PNP transistors and the tester will identify the base lead and provide a value of gain as well as an indication that the transistor does not have an obvious short between any of the junctions - so you shouldn't have any problems.