One type of three - electrode semiconductor device is bipolar transistor. Switching schemes depend on their conductivity (hole or electron) and the functions performed.
Classification
Transistors are divided into groups:
- Based on materials: gallium arsenide and silicon are most often used.
- By signal frequency: low (up to 3 MHz), medium (up to 30 MHz), high (up to 300 MHz), ultra-high (above 300 MHz).
- By maximum dissipation power: up to 0.3 W, up to 3 W, more than 3 W.
- By device type: three connected semiconductor layers with alternating changes in the direct and reverse impurity conductivity methods.
How do transistors work?
The outer and inner layers of the transistor are connected to the supply electrodes, called emitter, collector, and base, respectively.
The emitter and collector do not differ from each other by the types 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 great resistance, because it is made of a semiconductor with low doping. It has a significant contact area with the collector, which improves the removal of heat generated due to the reverse bias of the transition, and also facilitates the passage of minority carriers - electrons. Despite the fact that the transition layers are based on one principle, the transistor is an asymmetric device. When changing the locations of the extreme layers with the same conductivity, it is impossible to obtain similar parameters of the semiconductor device.
The bipolar transistor switching circuits are capable of supporting it in two states: it can be open or closed. In active mode, when the transistor is open, the emitter shift bias is done in the forward direction. To visually consider this, for example, on an npn type semiconductor triode, voltage from sources should be applied to it, as shown in the figure below.
At the same time, the boundary at the second collector junction is closed, and no current should flow through it. But in practice, the opposite happens because of the close 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 the electrons to enter the base zone, where they partially recombine with holes, the main carriers. Formed basic current I b . The stronger it is, the proportionally greater is the output current. Amplifiers based on bipolar transistors work on this principle.
Through the base, an exclusively diffusive movement of electrons occurs, since there is no action of an electric field. Due to the insignificant layer thickness (microns) and the 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 in by the electric field of the transition, which promotes their active transfer. The collector and emitter currents are almost equal to each other, if we neglect the insignificant loss of charges caused by recombination in the base: I e = I b + I c .
Transistor Parameters
- Gain on voltage U ek / U be and current: β = I to / I b (actual values). Typically, the coefficient β does not exceed a value of 300, but can reach a value of 800 and above.
- Input impedance.
- Frequency response - the transistor is operable to a predetermined frequency, beyond which transients in it do not keep pace with changes in the supplied signal.
Bipolar transistor: switching circuits, operating modes
Modes of operation differ depending on how the circuit is assembled. The signal must be supplied and taken 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. So turn on any bipolar transistors. Switching schemes: OB, OE and OK.
1. Scheme with OK
Switching circuit of a bipolar transistor with a common collector: the signal is fed to a resistor R L , which also enters the collector circuit. Such a connection is called a common collector circuit.
This option creates only current gain. The advantage of the emitter follower is the creation of a large input resistance (10-500 kOhm), which makes it possible to conveniently coordinate the cascades.
2. Scheme with OB
The circuit for switching on a bipolar transistor with a common base: the input signal enters through C 1 , and after amplification it is removed in the output collector circuit, where the base electrode is common. In this case, voltage gain is created similarly to working with OE.
The disadvantage is the small input resistance (30-100 Ohms), and the circuit with OB is used as an oscillation generator.
3. Scheme with OE
In many cases, when bipolar transistors are used, the switching circuits are mainly made with a common emitter. The supply voltage is supplied through the load resistor R L , and the negative pole of the external power is connected to the emitter.
An alternating signal from the input goes to the electrodes of the emitter and base (V in ), and in the collector circuit it already becomes larger in magnitude (V CE ). The main elements of the circuit: a transistor, a resistor R L and an output circuit of an amplifier with external power. Auxiliary: capacitor C 1 , which prevents the passage of direct current into the circuit of the input signal, and resistor R 1 through which the transistor opens.
In the collector circuit, the voltages at the output of the transistor and at the resistor R L together equal the magnitude of the EMF: V CC = I C R L + V CE .
Thus, a small signal V in at the input sets the law of variation of the DC supply voltage into AC at the output of the controlled transistor converter. The circuit provides an increase in the input current by 20-100 times, and the voltage by 10-200 times. Accordingly, the power also increases.
The disadvantage of the circuit: a small input resistance (500-1000 Ohms). For this reason, problems arise in the formation of amplification cascades. The output impedance is 2-20 kΩ.
The above diagrams demonstrate how a bipolar transistor works. If you do not take additional measures, their impact will be greatly affected by external influences, such as overheating and signal frequency. Also, the grounding of the emitter creates nonlinear distortion at the output. In order to increase the reliability of operation, feedbacks, filters, etc. are connected in the circuit. In this case, the gain is reduced, but the device becomes more efficient.
Operating modes
The transistor functions are affected by the value of the connected voltage. All operating modes can be shown if the previously presented scheme for switching on a bipolar transistor with a common emitter is applied.
1. Cutoff mode
This mode is created when the voltage value V of the BE decreases to 0.7 V. In this case, the emitter junction closes, and the collector current is absent, since there are no free electrons in the base. Thus, the transistor is locked.
2. Active mode
If a sufficient voltage is applied to the base to open the transistor, a small input current appears and increased output, 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 transistor fully opens, and the collector current reaches the maximum possible value. Its increase can be achieved only by changing the applied emf or load in the output circuit. When the base current changes, the collector current does not change. The 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 inclusion of bipolar transistors when combining cut-off and saturation modes allows you to create electronic keys with their help.
All operating modes depend on the nature of the output characteristics shown on the graph.
They can be clearly demonstrated if a circuit for switching on a bipolar transistor with OE is assembled.
If we put off the segments corresponding to the maximum possible collector current and the supply voltage V CC on the ordinates and abscissa, and then connect their ends to each other, we get a load line (red). It is described by the expression: I C = (V CC - V CE ) / R C. From the figure it follows that the operating point that determines the collector current I C and voltage V CE will shift along the load line from bottom to top with increasing base current I V.
The area between the V CE axis and the first output characteristic (shaded), where I B = 0, characterizes the cutoff mode. In this case, the reverse current I C is negligible, and the transistor is closed.
The highest characteristic at point A intersects with the direct load, after which, with a further increase in I B, the collector current does not change. The saturation zone on the graph is the shaded area between the I C axis and the coolest characteristic.
How does a transistor behave in different modes?
The transistor works with variable or constant signals entering the input circuit.
Bipolar Transistor: Switching Circuits, Amplifier
For the most part, the transistor serves as an amplifier. An alternating signal at the input changes its output current. Here you can apply schemes with OK or with OE. In the output circuit, a signal is required for the signal. Usually use a resistor mounted in the output collector circuit. If you choose it correctly, the value of 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. The conversion quality of their harmonic components is determined by the frequency characteristics of the transistors.
Switch Mode Operation
Transistor switches are designed for non-contact switching of connections in electrical circuits. The principle is a stepwise change in the resistance of the transistor. The bipolar type is quite suitable for the requirements of a key device.
Conclusion
Semiconductor elements are used in electrical signal conversion circuits. Universal capabilities and a large classification allow the wide use of bipolar transistors. Switching schemes determine their functions and operating modes. Much also depends on the characteristics.
The main switching circuits of bipolar transistors amplify, generate and transform input signals, as well as switch electrical circuits.