A bipolar transistor is an electronic semiconductor device, one of the types of transistors designed to amplify, generate and convert electrical signals. The transistor is called bipolar, since two types of charge carriers – electrons and holes – are simultaneously involved in the operation of the device. This is different from the unipolar (field) transistor, in which only one type of charge carrier is involved.
The principle of operation of both types of transistors is similar to the work of a water tap, which regulates the water flow, only through the transistor passes a stream of electrons. In bipolar transistors, two currents pass through the device – the main "big" current and control "little" current. The power of the main current depends on the power of the manager. In field-effect transistors, only one current passes through the device, the power of which depends on the electromagnetic field. In this article we will take a closer look at the work of a bipolar transistor.
The device bipolar transistor.
The bipolar transistor consists of three semiconductor layers and two PN junctions. There are PNP and NPN transistors by the type of alternation of hole and electronic conductivities. This is similar to two diodes connected face to face or vice versa.
The bipolar transistor has three contacts (electrodes). The contact coming out of the center layer is called the base. Extreme electrodes are called collector and emitter (collector and emitter). The base layer is very thin relative to the collector and emitter. In addition to this, the semiconductor regions are asymmetric at the edges of the transistor. The semiconductor layer on the collector side is slightly thicker than on the emitter side. This is necessary for proper operation of the transistor.
The work of a bipolar transistor.
Consider the physical processes occurring during the operation of the bipolar transistor. For example, take the NPN model. The principle of operation of the PNP transistor is similar, only the polarity of the voltage between the collector and the emitter will be opposite.
As already mentioned in the article about the types of conductivity in semiconductors, in a P-type substance there are positively charged ions – holes. N-type substance is saturated with negatively charged electrons. In the transistor, the concentration of electrons in the region N significantly exceeds the concentration of holes in the region P.
Connect the voltage source between the collector and the VCE Emitter (VCE). Under its action, electrons from the upper N part will begin to be attracted to the plus and gather near the collector. However, the current can not go, because the electric field of the voltage source does not reach the emitter. This is prevented by a thick layer of semiconductor collector plus a layer of semiconductor base.
Now we connect the voltage between the base and the emitter VBE, but significantly lower than VCE (for silicon transistors the minimum required VBE is
As a result, the central base layer is enriched with free electrons. Most of them will be directed towards the collector, since the voltage there is much higher. Also, this contributes to a very small thickness of the central layer. Some part of the electrons, though much smaller, will still flow towards the plus base.
As a result, we get two currents: small – from the base to the IBE emitter, and large – from the collector to the ICE emitter.
If you increase the voltage at the base, then in the layer P gather more electrons. As a result, the base current will increase slightly, and the collector current will increase significantly. Thus, with a small change in the base current IB, the collector current of the IC varies greatly. This is how the signal is amplified in a bipolar transistor. The ratio of the collector current of the IC to the base current IB is called the current gain. Denoted by β, hfe or h21e, depending on the specifics of the calculations carried out with the transistor.
The simplest bipolar transistor amplifier
Let us consider in more detail the principle of signal amplification in the electrical plane using the circuit as an example. I will make a reservation in advance that such a scheme is not entirely correct. No one connects a DC voltage source directly to an AC source. But in this case, it will be easier and clearer to understand the amplification mechanism itself using a bipolar transistor. Also, the calculation technique itself in the example below is somewhat simplified.
So, let’s say at our disposal a transistor with a gain of 200 (β = 200). On the collector side, we connect a relatively powerful 20V power source, due to the energy of which amplification will occur. From the base of the transistor, we connect a weak 2V power supply. Connect the AC voltage source in the form of a sine, with the amplitude of oscillations in
2. Calculation of input current base Ib
Now we calculate the base current Ib. Since we are dealing with a variable voltage, it is necessary to calculate two current values - at maximum voltage (Vmax) and minimum (Vmin). We call these current values, respectively, Ibmax and Ibmin.
Also, in order to calculate the base current, it is necessary to know the base-emitter voltage VBE. There is one PN junction between the base and the emitter. It turns out that the base current “meets” in its path a semiconductor diode. The voltage at which the semiconductor diode begins to conduct is about
Calculate Ibmax and Ibmin using Ohm’s law:
2. The calculation of the output current collector IC
Now, knowing the gain (β = 200), one can easily calculate the maximum and minimum values of the collector current (Icmax and Icmin).
3. The calculation of the output voltage Vout
It remains to calculate the voltage at the output of our amplifier Vout. In this circuit is the voltage at the collector VC.
Through the resistor Rc flows the collector current, which we have already calculated. It remains to substitute the values:
4. Analysis of the results
As can be seen from the results, VCmax turned out to be less than VCmin. This is due to the fact that the voltage across the resistor VRc is subtracted from the supply voltage VCC. However, in most cases it does not matter, because we are interested in the variable component of the signal – the amplitude, which has increased c
So, let’s summarize the principle of operation of an amplifier on a bipolar transistor. Through the base current flows Ib, carrying a constant and variable components. The constant component is needed in order for the PN transition between the base and the emitter to start – “opened”. The variable component is, in fact, the signal itself (useful information). The current collector-emitter inside the transistor is the result of multiplying the base current by the gain factor β. In turn, the voltage across the resistor Rc above the collector is the result of multiplying the amplified collector current by the value of the resistor.
Thus, a signal arrives at the Vout pin with an increased vibration amplitude, but with a preserved shape and frequency. It is important to emphasize that the transistor takes the energy to amplify from the VCC power supply. If the supply voltage is not enough, the transistor will not be able to fully operate, and the output signal may be distorted.
Modes of operation of the bipolar transistor
In accordance with the voltage levels on the electrodes of the transistor, there are four modes of its operation:
Cut-off mode (cut off mode).
Active mode (active mode).
Saturation mode (saturation mode).
Inverse mode (reverse mode).
When the base-emitter voltage is lower than
In active mode, the voltage on the base is sufficient for the PN junction between the base and the emitter to open. In this state, the transistor has base and collector currents. The collector current is equal to the base current multiplied by the gain.
Sometimes the base current may be too large. As a result, the power supply is simply not enough to provide such a magnitude of the collector current, which would correspond to the gain of the transistor. In saturation mode, the collector current will be the maximum that the power supply can provide and will not depend on the base current. In this state, the transistor is not able to amplify the signal, since the collector current does not respond to changes in the base current.
In the saturation mode, the conductivity of the transistor is maximum, and it is more suitable for the function of the switch (key) in the “on” state. Similarly, in the cut-off mode, the conductivity of the transistor is minimal, and this corresponds to a switch in the off state.
In this mode, the collector and emitter switch roles: the collector PN junction is biased in the forward direction, and the emitter switch is reversed. As a result, the current flows from the base to the collector. The collector semiconductor region is asymmetric to the emitter, and the gain in the inverse mode is lower than in the normal active mode. The design of the transistor is designed so that it works as effectively as possible in active mode. Therefore, in the inverse mode, the transistor is practically not used.
The main parameters of the bipolar transistor.
Current Gain – The ratio of the collector current IC to the base current IB. Denoted by β, hfe or h21e, depending on the specifics of the calculations carried out with transistors.
β is a constant value for a single transistor, and depends on the physical structure of the device. High gain is calculated in hundreds of units, low – in dozens. For two separate transistors of the same type, even if they were “pipeline neighbors” during production, β may be slightly different. This characteristic of a bipolar transistor is perhaps the most important. If other parameters of the device can often be neglected in the calculations, then the current gain is almost impossible.
Input resistance – resistance in the transistor, which “meets” the base current. Denoted by Rin (Rin). The larger it is, the better for the amplifying characteristics of the device, since from the base side there is usually a weak signal source that needs to consume as little current as possible. The ideal option is when the input impedance equals infinity.
Rвх for an average bipolar transistor is a few hundred kΩ (kilo). Here, the bipolar transistor is very much inferior to the field-effect transistor, where the input impedance reaches hundreds of GΩ (gig).
Output Conductivity – The conductivity of the transistor between the collector and the emitter. The greater the output conductivity, the more current the collector-emitter will be able to pass through the transistor at a lower power.
Also, as the output conductance increases (or the output impedance decreases), the maximum load that the amplifier can withstand with slight losses in overall gain increases. For example, if a transistor with a low output conductance amplifies a signal 100 times without a load, then when connecting a load of 1 KΩ, it will already amplify only 50 times. A transistor with the same gain, but with a higher output conductivity, will have a smaller gain drop. The ideal option is when the output conductivity equals infinity (or the output resistance Rout = 0 (Rout = 0)).
Frequency response – the dependence of the gain of the transistor on the frequency of the incoming signal. With increasing frequency, the ability of the transistor to amplify the signal gradually decreases. The reason for this is the parasitic capacitances formed in PN-junctions. The transistor reacts to changes in the input signal in the base not instantaneously, but with a certain deceleration due to the time spent filling the capacitors with charge. Therefore, at very high frequencies, the transistor simply does not have time to react and fully amplify the signal.