One type of three-electrode semiconductor devices are bipolar transistors. Inclusion schemes depend on the conductivity (hole or electronic) they have and the functions they perform.
Classification
Transistors are divided into groups:
- According to the materials: most often used are gallium arsenide and silicon.
- By signal frequency: low (up to 3 MHz), medium (up to 30 MHz), high (up to 300 MHz), ultra-high (above 300 MHz).
- By the maximum power dissipation: up to 0.3 W, up to 3 W, more than 3 watts.
- By device type: three interconnected semiconductor layers with alternating changes in the forward and reverse impurity conductivity methods.
How do transistors work?
The outer and inner layers of the transistor are connected to the supply electrodes, called the emitter, collector and base respectively.
The emitter and the collector do not differ from each othertypes of conductivity, but the degree of doping with impurities in the latter is much lower. This ensures an increase in the permissible output voltage.
The base, which is the middle layer, has a largeresistance because it is made of semiconductor with a weak doping. It has a significant area of contact with the collector, which improves the removal of heat generated by the reverse displacement of the transition, and also facilitates the passage of minority carriers - electrons. Although the transition layers are based on one principle, the transistor is an asymmetric device. When changing places of the extreme layers with the same conductivity, it is impossible to obtain similar parameters of the semiconductor device.
Bipolar transistors enablemaintain it in two states: it can be open or closed. In active mode, when the transistor is open, the emitter shift of the junction is done in the forward direction. In order to visually see this, for example, on an n-p-n semiconductor triode, it should be energized from sources, as shown in the figure below.
The boundary at the second collector junction whileit is closed, and no current should flow through it. But in practice, the opposite occurs because of the proximity of the transitions to each other and their mutual influence. Since the “minus” of the battery is connected to the emitter, the open transition allows electrons to enter the base zone, where they are partially recombined with the holes - the main carriers. Base current I is formedb. The stronger it is, the more proportionally the output current is. This principle employs bipolar transistors.
Through the base only diffusion occursthe movement of electrons, since there is no action of an electric field. Due to the insignificant layer thickness (microns) and a large concentration gradient of negatively charged particles, almost all of them fall into the collector region, although the base resistance is quite large. There they are drawn into the electric field of the transition, which facilitates their active transport. Collector and emitter currents are almost equal to each other, if we neglect the insignificant loss of charges caused by recombination in the database: Ie = Andb + Andto.
Transistor Parameters
- Voltage Gains Uek/ Uba and current: β = Ito/ANDb (actual values). Typically, the coefficient β does not exceed 300, but can reach values of 800 and above.
- Input impedance.
- Frequency response - the operation of the transistor to a given frequency, above which transients in it do not have time for changes in the applied signal.
Bipolar transistor: switching circuits, operating modes
Режимы работы отличаются в зависимости от того, how the circuit is assembled. The signal must be fed and removed at two points for each case, and only three outputs are available. It follows that one electrode must simultaneously belong to the input and output. This includes any bipolar transistors. Schemes of inclusion: OB, OE and OK.
1. Scheme with OK
Wiring diagram of a bipolar transistor with a common collector: the signal goes to resistor RLwhich is also included in the collector circuit. Such a connection is called a common collector circuit.
This option only creates current gain. The advantage of the emitter follower is to create a large input resistance (10-500 kΩ), which allows you to conveniently coordinate the cascades.
2. Scheme with OB
The circuit of the bipolar transistor with a common base: the incoming signal comes through C1, and after amplification is removed in the output collector circuit, where the base electrode is common. In this case, a voltage gain is created in the same way as operating an OE.
The disadvantage is the small resistance of the input (30-100 ohms), and the circuit with ON is used as an oscillator.
3. Scheme with OE
In many cases, when bipolar transistors are used, switching circuits are predominantly made with a common emitter. The supply voltage is supplied through the load resistor RL, and the negative pole of the external power supply is connected to the emitter.
The variable signal from the input goes to the emitter and base electrodes (Vin), and in the collector circuit it becomes larger in size (VEC). Main elements of the circuit: transistor, resistor RL and an external powered amplifier output circuit. Auxiliary: capacitor C1that prevents the passage of DC to the input signal, and the resistor R1through which the transistor opens.
In the collector voltage circuit at the output of the transistor and the resistor RL together equal to the value of emf: VCC = AndFROMRL + BEC.
Thus, a small signal Vin at the entrance is given the law of change of constantsupply voltage to AC at the output of a controlled transistor converter. The circuit provides an increase in input current of 20-100 times, and voltage - 10-200 times. Accordingly, the power also rises.
The disadvantage of the circuit: a small input resistance (500-1000 ohms). For this reason, there are problems in the formation of amplification cascades. Output impedance is 2-20 kΩ.
The diagrams show how it works.bipolar transistor. If no additional measures are taken, external influences, such as overheating and the frequency of the signal, will greatly affect their performance. Also, emitter earthing creates a non-linear distortion at the output. In order to increase the reliability of work, in the scheme, feedbacks, filters, etc. are connected. At the same time, the gain decreases, but the device becomes more efficient.
Modes of operation
The function of the transistor is influenced by the value of the connected voltage. All modes of operation can be shown if the previously introduced circuit of a bipolar transistor with a common emitter is used.
1. Cutoff mode
This mode is created when the value of voltage VBE decreases to 0.7 V. At the same time, the emitter junction closes and there is no collector current, since there are no free electrons in the base. Thus, the transistor is locked.
2. Active mode
If the base voltage is sufficient,in order to open the transistor, a small input current appears and a higher output current, depending on the magnitude of the gain. Then the transistor will work as an amplifier.
3. Saturation mode
The mode differs from the active one in that the transistorfully open, and the collector current reaches the maximum possible value. Its increase can be achieved only by changing the applied EMF or the load in the output circuit. At change of base current the collector does not change. Saturation mode is characterized by the fact that the transistor is extremely open, and here it serves as a switch in the on state. The bipolar transistor turn-on schemes when combining cut-off and saturation modes allow you to create electronic keys with their help.
All modes of operation depend on the nature of the output characteristics shown in the graph.
They can be clearly demonstrated if a switching circuit of a bipolar transistor with OE is assembled.
If you put on the axes of the ordinates and abscissa segments corresponding to the maximum possible collector current and the value of the supply voltage VCC, and then connect their ends together, you get a load line (red). It is described by the expression: IFROM = (InCC - ATEC) / RFROM. It follows from the figure that the operating point determining the collector current IFROM and voltage VEC, will shift along the load line from bottom to top with increasing base current IAT.
Zone between the V axisEC and the first output characteristic (hatched), where IAT = 0, characterizes the cutoff mode. In this case, the reverse current IFROM insignificantly small, and the transistor is closed.
The highest characteristic at point A intersects with the direct load, after which, with a further increase in IAT collector current does not change. The saturation zone on the graph is the shaded area between the axis IFROM and the coolest feature.
How does the transistor behave in different modes?
The transistor works with variable or constant signals entering the input circuit.
Bipolar transistor: switching circuits, amplifier
Большей частью транзистор служит в качестве the amplifier. A variable signal at the input leads to a change in its output current. Here you can apply schemes with OK or with OE. The output circuit requires a signal for the signal. Usually use a resistor installed in the output collector circuit. If you choose it correctly, the output voltage will be significantly higher than the input.
The operation of the amplifier is clearly visible in the timing diagrams.
When pulse signals are converted, the mode remains the same as for sinusoidal ones. The quality of the conversion of their harmonic components is determined by the frequency characteristics of the transistors.
Switching mode
Transistor switches are designed tocontactless switching connections in electrical circuits. The principle is a step change in the resistance of the transistor. Bipolar type is well suited for the requirements of the key device.
Conclusion
Semiconductor elements are used in circuits.electrical signal conversion. Universal capabilities and a large classification make it possible to widely use bipolar transistors. Inclusion schemes define their functions and operating modes. Much also depends on the characteristics.
The basic switching circuits of bipolar transistors amplify, generate and convert input signals, as well as switch electrical circuits.
