Semiconductor Amplifiers
Semiconductor Amplifiers
This chapter deals with the BIPOLAR transistor, which is considered to be the "normal" transistor. Besides it there are other types of transistors, like the field effect transistor or the unijunction transistor, which will be dealt with later.
The bipolar transistor received its name from the fact, that the current has to flow through P and N doped layers.
As for diodes, either GERMANIUM or SILICON may be used as semiconducting material for transistors. The first transistors were made of germanium, as the technology for silicon transistors was not yet available. When silicon transistors were introduced, they very quickly replaced the germanium types, as in general they give better performance. Nowadays germanium transistors are only used for a few special applications, but they are still found in older equipment.
Transistors consist of three semiconductor layers, doped in a N-P-N or a P-N-P order. This gives another distinction for two different types of transistors. PNP and NPN transistors have the same behaviour, only that the directions of all voltages and currents are inverse. We will mainly deal with the NPN type in this chapter, as it is the type more commonly used.
Depending on the physical construction we distinguish between germanium and silicon and between NPN and PNP transistors.
This section can only give a rough introduction to the physical principle of the transistor, as the understanding of all of the details would require a large physical background.
The transistor is constructed of three layers of different doping, either in a NPN or a PNP order.
The three layers form two junctions. Each junction on its own behaves as a normal diode. Because of this, the transistor could be represented by two diodes. But it should be pointed out, that the two junctions of the transistor act together and produce a completely new characteristic. This characteristic can not be achieved by two diodes.
The three terminals of the transistor are connected to the three layers. They are called COLLECTOR, BASE and EMITTER. These names have purely historical reasons.
The base is an extremely thin layer of opposite doping as the collector and the emitter. (P for NPN type transistors and N for PNP type transistors)
The collector and the emitter have equal type of doping, still they may not be interchanged, as their grade of doping is different.
To describe what happens within a transistor we will consider the case of a NPN transistor:
First we will consider the transistor to be connected to a circuit in such a way, that the positive voltage is connected to the collector and the negative to the emitter. The base is left unconnected.
A forward voltage will appear across the base-emitter junction, making it conducting. But the base-collector junction will face a reverse voltage, thus this junction is blocked.
All of the holes of the base (which, we should keep in mind, is very thin!) are filled up with electrons from the emitter, making the base practically nonconducting. In the collector the barrier zone is vacated from electrons, making also this area nonconducting.
Applying a voltage to the base which is negative in respect to the emitter, this situation would even increase.
Now we will apply a positive voltage to the base. As soon as this voltage exceeds the barrier voltage of the base-emitter diode (
0.5V for silicon), a current will start to flow. Electrons are now moving from the emitter to the base. As the emitter is much stronger doped than the base, more electrons than there are holes in the base "flood" in from the emitter.
The electrons, which find no place in the crystal structure of the base, can very easily be taken away from the base. These electrons come under the influence of the barrier zone of the collector, which is still bare of electrons. This barrier zone strongly attracts electrons in order to fill up its crystal grid structure and thus pulls the electrons from the base into the collector. This will even happen, if the base is more positive than the collector.
As soon as the electrons enter the collector, they are attracted by the positive voltage of the collector and flow to the positive pole. The number of these electrons, moving to the collector, is much bigger than the number of those, leaving the base via the "ordinary" path, the base terminal.
Thus a small current flowing into the base is able to control a current up to 500 times bigger, flowing into the collector.
To use the transistor it must be connected together with other circuit element in a circuit.
The basic application of a transistor is, that it controls the current through some other circuit element. This circuit element is called the "LOAD" of the transistor. As general case we will assume this load to be a resistor
As the current to be controlled flows from the collector to the emitter, the load has to be connected to the collector.
Then a supply voltage
The load current or collector current
When a base current
The base current
We will now vary the base current to control the current to the load. If we measure the collector current
When the current
Next we will try to vary the collector current by varying the collector-emitter voltage
We will see that the voltage
Only when the collector emitter voltage get to low, e.g. below 1V, the collector current will start to depend strongly on the collector emitter voltage.
If the transistor is operated in such a way, that the collector current is mainly controlled by the base current and depends only little on the collector-emitter voltage, we say it is operated in the ACTIVE REGION.
In the active region, the collector current depends mainly on the base current and only little on the collector-emitter voltage.
In the active region, a transistor behaves as a current controlled current source.
When a transistor is used as amplifier, it is normally operated in the active region.
In the previous section it was seen already, that when the collector-emitter voltage gets to low, the collector current will not be entirely dependent on the base current any more. We will now take a closer look at this case.
We will again control the current through a load resistor, by varying the base current. If we increase the base current, the load current increases, the voltage across
When the base current has been increased that far, that the collector-emitter voltage gets lower than 1V, we will find that the collector current will not further increase. The base current does not control the collector current any more.
The current that flows through the load resistor is always:
If the collector-emitter voltage gets very low, as it is the case here, the collector current becomes:
This means in fact, that now the collector current depends on the supply voltage and on the load resistor only. This is logical, as the transistor can REDUCE the current that flows through the load resistor, but it can not make MORE current flow, than is possible according to Ohm's law for the load resistor.
The base current provides the carriers to the base, which are required to make the base-collector junction conducting. The more carriers are provided, the more collector current can flow. If the collector current is now limited by the external circuit, e.g. by the load resistor, the base current provides more carriers than required. We say the base or the transistor is SATURATED.
In the voltage-current diagram, the saturation of the transistor can be represented by the region, where the characteristic is not flat anymore, but is dropping. This is called the SATURATION REGION.
From the diagram it can be seen, that the transistor changes gradually from active to saturated. When the transistor will be considered to be definitely saturated, will depend on the circuit.
As general rule, the limit between active region and saturation region is where the voltage between collector and base,
| Saturation region: | |
| Active region: |
A transistor is saturated, if the base current is so high, that the collector-emitter voltage has dropped below the base-emitter voltage.
We say a saturated transistor is fully conducting.
Saturation of a transistor is required for switching applications.
Transistors are made of the same materials (germanium or silicon) and by the same processes as diodes. Because of this, they show the same temperature behaviour as these.
The maximum temperatures for transistors are also the same as for diodes: Germanium transistors can stand up to 100°C, silicon transistors up to 200°C. Exceeding these temperatures will destroy the transistors. It should always be considered, that these temperatures refer to the semiconductor material of the transistor, where the temperature is normally higher than at the case-surface.
With rising temperature the transistor conducts more. This means the base and the collector current will increase. As such changes are normally not desired, in the circuit special measures must be taken, to compensate for the temperature behaviour of the transistor.
In the previous chapter, some of the diagrams, describing the characteristics of the transistor, have been introduced.
In this chapter we will take a closer look at them.
This is the most important characteristic for the transistor. It shows, how the collector current varies, if the collector-emitter voltage is changed.
The base current is the parameter. It is kept constant for each curve. The output characteristic normally shows a set of curves, each curve for a different base current.
The base current has to be maintained constant during the plotting of one curve. This has to be done manually or by means of a constant current source.
The output characteristics for different individual transistors of even the same type may vary considerably. The characteristics, given in the data sheets, only show the behaviour of the average transistor of this type.
The output characteristic is normally limited by operational limits for the transistor. These values may not be exceeded, in order not to destroy the transistors. For the output characteristic these values are:
Some important information about the transistor can be found from the output characteristic:
The current gain is one of the most important characteristics for the transistor. It tells the relationship between the collector current
The current that flows through the load resistor is always:
The current gain depends on the collector-emitter voltage. It can be found in the output characteristic, by going vertically at the required collector-emitter voltage and looking for the value of
Practical values of the current gain of transistors lie in the range of 50 to 500.
This resistance is the internal resistance of the output of the transistor. It is represented by the SLOPE of the output characteristic. As this slope is not constant but different for any value of
To find the collector-emitter resistance at the required collector-emitter voltage and collector current, the VARIATIONS of both of them have to be determined. Then
It can be seen, that in the active region the slope of the output characteristic is relatively flat. That means in fact, that the collector-emitter resistance is relatively high.
Practical values for the collector-emitter resistance lie in the range of 10k
to 100k.
Another output characteristic for the transistor can be found, if the BASE EMITTER VOLTAGE
The output characteristic with the base-emitter voltage as parameter has less importance in practice, as the transistor is normally controlled by the base current.
The base is considered to be the input of the transistor. As the emitter is the common terminal, the input signal, which is controlling the output, will be applied between base and emitter.
The input characteristic is thus showing the relationships between the base-emitter voltage
The base-emitter path behaves mainly as a diode. Thus the input characteristic has just the same shape as a diode characteristic. Depending, whether the transistor is made of germanium of silicon, these characteristics will have a threshold voltage of approximately 0.3V or 0.7V.
The base of the transistor is very thin and the collector has an influence on the situation at the base. The collector-emitter voltage will help to pull the electrons from the emitter into the base. So, if the collector-emitter voltage is increased, it will cause an increase in base current, even if the base-emitter voltage remains unchanged.
As a result, we get different characteristics for different collector-emitter voltages.
The input characteristic is normally only given for forward directions of the base-emitter diode, as this the normal operation condition of the transistor.
From the input characteristic the BASE EMITTER RESISTANCE
The slope of the input characteristic represents the base-emitter resistance. To determine it, the difference in
In the active region of the transistor, to 10k.
This characteristic shows, how the input (base) controls the output (collector) of the transistor. Normally the transistor is considered as current controlled current source, thus the input and the output signals are both currents.
When the current control characteristic is plotted, it is found, that it depends slightly on the collector-emitter voltage. Thus
As these characteristics are a straight line over a wide range, base and collector current can be considered almost proportional.
From the current control characteristic the current gain B can again be determined. It is simply the ratio given by the values of
When the transistor is used in an amplifier, the current gain at the very working conditions is of importance. This is the relationship of the collector current VARIATIONS as a result of the base current VARIATIONS at a certain collector current.
This is represented by the SLOPE of the current control characteristic. It is called the differential current gain. Different symbols are used for this quantity, either 
or
From the characteristics above it can be seen, that there is little difference between the static and the differential current gain. Thus for the ACTIVE REGION we can simplify:
= hfeThis is not true for the SATURATION REGION! There the differential current gain is considerably less than the static current gain.
(Refer to the output characteristic!)
Sometimes also the VOLTAGE CONTROL CHARACTERISTIC of the transistor may be given. It shows, how the collector current
As the base current is almost proportional to the collector current, this characteristic looks like an input characteristic, where the base current axis is multiplied by the current gain B.
Also this characteristic has little importance in practice, as the transistor is normally considered to be controlled by current.
A number of different characteristics and values are used to describe the behaviour of the transistor.
(Much more than introduced in this chapter!)
But ONLY THREE CHARACTERISTICS are the most important ones:
From the OUTOUT CHARACTERISTIC we can find:
|
10k ... 100k |
|
50 ... 500 |
From the INPUT CHARACTERISTIC we can find
|
1k ... 10k |
From the CURRENT CONTROL CHARACTERISTIC we can find
or  |
Semiconductor Amplifiers