Early transistors were germanium devices. They were noisy, low gain, and with characteristics that depended on the temperature of the day! They were extremely temperature sensitive, so much so that early transistor radios could not be left in the sun without the sound distorting!.
As transistors were improved, their whole structure changed. One of the first things to change was the material from which they were made. It changed from germanium to silicon. With is came higher voltage capability, higher current and temperature of operation, better stability and higher gain values.
Let me qualify this.
The gain of a transistor (in the category in which it is placed), is very high or at least sufficient for the intended application.
For any transistor, there is a connection between gain and current handling capability. In most cases it is not possible to produce a transistor with high gain AND high current handing capability.
hat's why small signal transistors have a high gain and power transistor have a low gain.
when designing a circuit you have to take this into account. the amplification (voltage amplification) must be done in the early stages of the circuit so that the current amplification can be achieved in the final stages.
To overcome the problem of high-power, low-gain; manufacturers have introduced a special type of transistor called a Darlington. It combines two transistors in the one package so that very high amplification can be achieved as well as high current handing.
But Darlington transistors form a very small portion of the overall transistor types.
The most common types are low current, medium current and high current varieties. These are available in both PNP and NPN. Each of these groups can be divided further into low frequency and high frequency.
Let's stop there before we get too involved with groupings.
CURRENT GAIN
The main element of this discussion is the gain of a transistor. there are 4 different current gains. These are:
1 The DC current gain (as measured by the Combo-2 tester).
2 The AC current gain when the transistor is operating as an oscillator in a test circuit.
3 The gain when the transistor is placed in a normal circuit.
4 The current gain at the maximum frequency of operation (for the transistor).
These are different values and we will see why.
1. THE DC CURRENT GAIN
This is determined by measuring the current in the collector circuit and dividing it by the current in the base circuit.
The highest result obtained from a number of determinations is selected and used as the value for the transistor. It is the value supplied in the specification sheets. This value is quite often completely different from what you get when using the transistor in practice as it is generated under ideal conditions.
It is determined under very low current conditions. When a higher current is required to be controlled by the transistor, the gain drops considerably.
For example, a transistor may be capable of handling 500mA, but the best gain factor may be determined at 10mA! The gain at 10mA may be 300 but at 500mA it may be only 70.
This applies to all transistors and that's why the gain values you will be getting on the Combo-2 are the highest the transistor will produce. When the transistor is placed in a normal circuit the gain factor will fall according to the current required by the circuit.
2. THE AC CURRENT GAIN.
The AC Current gain is the gain you get when the transistor is operating as an amplifier in oscillator mode, but again under very favourable conditions. Instead of taking static conditions as in the DC gain above, the SLOPE OF A GRAPH is measured and a value obtained. The result is less than the DC current gain but not very much less.
3. THE GAIN IN A CIRCUIT
When a transistor is placed in a circuit, things change completely. All the components around the transistor have a loading effect that reduces the gain.
The result is called STAGE GAIN and this is the MOST IMPORTANT GAIN, as it is the gain you REALLY get.
For instance, a transistor with a gain of 300 may only produce a gain of 50-70 when fitted into a stage. It's the effect of all the components around the transistor that reduce the gain. Such things as output capacitors, coils, loudspeakers and stages that follow, all require to be driven and/or have losses and when the transistor has to drive these components, its gain suffers.
There are also components called self-biasing components and negative feed-back components that reduce the gain. So you can see why a transistor has to have a high gain in itself to end up providing a gain when fitted to a circuit.
4. THE FREQUENCY
The third factor is frequency. As the frequency of operation of a transistor increases, its gain decreases. This is a characteristic of the transistor itself. It is due to the way the transistor is made and the size of the chip making up the device. It is something that cannot be altered after the transistor is made. It's only by improved technology that transistor frequencies have increased from the first audio transistors to the modern gigahertz devices.
As the frequency of operation of a transistor increases, its gain decreases to unity and this point is called Ft, the cut-off point and determines the maximum frequency at which the transistor will operate. This is usually in the MHz range for small signal transistors and about 1MHz for power transistors. For high frequency transistors Ft will be 500MHz and higher. We generally do not have to worry about the maximum frequency capability unless we are designing high frequency circuits.
This leaves the question "What is the gain of a transistor?" unanswered, but you can see how complicated it is.
THE GAIN is what you get when the transistor is fitted to a circuit. If you are designing a circuit it is always a wise idea to take CRO measurements of the input and output of each stage, to determine the gain as well as view the waveforms and also see if any high frequency noise is present.