Bipolar transistors: switching circuits. The scheme of switching on a bipolar transistor with a common emitter

One type of three - electrode semiconductor devices are bipolar transistors. The switching schemes depend on the conductivity (hole or electronic) and the functions performed.

Classification

Transistors are divided into groups:

1. On materials: gallium arsenide and silicon are most often used.
2. According to the signal frequency: low (up to 3 MHz), medium (up to 30 MHz), high (up to 300 MHz), ultra-high (above 300 MHz).
3. According to the maximum power dissipation: up to 0.3 W, to 3 W, more than 3 W.
4. According to the type of device: three connected layers of a semiconductor with alternating variation of direct and inverse methods of impurity conduction.

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 in conductivity types, but the degree of doping with impurities in the latter is much lower. This provides an increase in the permissible output voltage.

The base, which is the middle layer, has a great resistance, since it is made of a semiconductor with weak doping. It has a large contact area with the collector, which improves the heat dissipation 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 places of extreme layers with the same conductivity, it is impossible to obtain analogous parameters of the semiconductor device.

Bipolar transistor switching circuits can support it in two states: it can be open or closed. In the active mode, when the transistor is open, the emitter junction shift is made in the forward direction. In order to visually see this, for example, on a semiconductor triode of the npn type, it should be powered by sources, as shown in the figure below.

The boundary at the second collector junction is closed, and through it the current should not flow. But in practice, the opposite occurs because of the close location of the transitions to each other and their mutual influence. Since a "minus" of the battery is connected to the emitter, the open transition allows electrons to enter the base zone, where partial recombination occurs with holes-the main carriers. A base current I _{b is formed} . The stronger it is, the more proportional the output current is. On this principle, amplifiers operate on bipolar transistors.

Through the base there is an exclusively diffusion movement of electrons, since there is no action of the electric field. Due to the insignificant thickness of the layer (microns) and the large value of the concentration gradient of the 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 contributes to their active transfer. The collector and emitter currents are practically equal to each other if we neglect the insignificant loss of charges caused by recombination in the base: I _e = I _b + I _k .

Parameters of transistors

1. Voltage amplification factors U _eq / U _b and current: β = I _k / I _b (actual values). Usually the coefficient β does not exceed 300, but it can reach 800 and higher.
2. Input resistance.
3. Frequency response - the operation of the transistor to a specified frequency, when exceeding it transients in it do not keep up with the changes in the signal being supplied.

Bipolar transistor: switching circuits, operation modes

The operating modes differ depending on how the circuit is assembled. The signal should be fed and photographed at two points for each case, and only three outputs are available. Hence it follows that one electrode must simultaneously belong to the input and output. This includes any bipolar transistors. Schemes of inclusion: OB, MA and OK.

1. Scheme with OK

The scheme of switching on a bipolar transistor with a common collector: the signal is applied to the resistor R _L , which also enters the collector circuit. This connection is called a common collector circuit.

This option creates only current gain. The advantage of the emitter follower is to create a large input resistance (10-500 kOhm), which makes it convenient to coordinate the cascades.

2. The scheme with OB

The scheme of switching on a bipolar transistor with a common base: the incoming signal is fed through C ₁ , and after amplification it is removed in the output of the collector circuit, where the base electrode is common. In this case, a voltage gain is created analogous to working with the MA.

The disadvantage is the small input resistance (30-100 Ohm), and the circuit with the OB is used as a oscillator.

3. Scheme with OE

In many cases, when bipolar transistors are used, the switching circuits are preferably made with a common emitter. The supply voltage is supplied through the load resistor R _L , and the negative pole of the external supply is connected to the emitter.

The variable signal from the input goes to the electrodes of the emitter and the base (V _in ), and in the collector circuit it becomes already larger (V _CE ). The main elements of the circuit: transistor, resistor R _L and amplifier output circuit with external power. Auxiliary: capacitor C ₁ , which prevents the DC current from flowing into the circuit of the input signal, and the resistor R ₁ , through which the transistor opens.

In the collector circuit, the voltages at the output of the transistor and on the resistor R _L are equal to the emf value: V _CC = I _C R _L + V _CE .

Thus, a small signal V _in at the input sets the law of the change of the DC supply voltage into an AC output of the controlled transistor converter. The circuit provides an increase in the input current of 20-100 times, and voltage - 10-200 times. Accordingly, the power is also increased.

Disadvantage of the circuit: a small input resistance (500-1000 ohms). For this reason, problems arise in the formation of cascades of amplification. The output resistance is 2-20 kΩ.

The above diagrams show how a bipolar transistor works. If you do not take additional measures, their performance will be strongly influenced by external influences, such as overheating and signal frequency. Also, the emitter ground creates non-linear distortion at the output. To increase the reliability of the work, feedback circuits, filters, etc. are connected in the circuit. At the same time, the gain factor decreases, but the device becomes more efficient.

Modes of operation

The function of the transistor is affected by the value of the connected voltage. All modes of operation can be shown if the previously introduced circuit for switching on a bipolar transistor with a common emitter is used.

1. Cut-off mode

This mode is created when the value of the voltage V _BE is reduced to 0.7 V. In this case, the emitter junction is closed 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 voltage sufficient to open the transistor is applied to the base, a small input current appears and is increased at the 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 is fully opened 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. Saturation mode is characterized by the fact that the transistor is extremely open, and here it serves as a switch in the on state. Schemes for switching bipolar transistors when combining the cut-off and saturation modes allow creating electronic keys with their help.

All modes of operation depend on the nature of the output characteristics shown on the graph.

They can be visually demonstrated if a circuit for connecting a bipolar transistor with an OE is assembled.

If we plot on the axes of the ordinates and abscissas segments corresponding to the maximum possible collector current and the magnitude of the supply voltage V _CC , and then connect their ends to each other, we obtain a load line (red color). It is described by the expression: I _C = (V _CC - V _CE ) / R _C. It follows from the figure that the operating point, which determines the collector current I _C and the voltage V _CE , will shift along the loading line from the bottom up, with the base current I _B increasing.

The zone between the axis V _CE and the first output characteristic (shaded), where I _B = 0, characterizes the cut-off 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 the collector current no longer changes as I increases further. The saturation zone in the graph is the shaded area between the I _C axis and the steepest characteristic.

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

Mostly the transistor serves as an amplifier. A variable input signal results in a change in its output current. Here you can apply schemes with OK or with the OE. In the output circuit, a signal is required for the signal. Usually a resistor installed in the output of the collector circuit is used. If it is selected correctly, the output voltage will be much higher than the input voltage.

The operation of the amplifier is clearly visible on the time diagrams.

When pulse signals are converted, the mode remains the same as for sinusoidal signals. The quality of conversion of their harmonic components is determined by the frequency characteristics of the transistors.

Operation in the switching mode

Transistor keys are designed for non-contact switching of connections in electrical circuits. The principle is a step change in the resistance of the transistor. Bipolar type is quite suitable for the requirements of the key device.

Conclusion

Semiconductor elements are used in electrical signal conversion circuits. Universal capabilities and a large classification make it possible to widely use bipolar transistors. The switching circuits determine their functions and operating modes. Much also depends on the characteristics.

The main circuits for switching on bipolar transistors amplify, generate and convert input signals, and also switch electrical circuits.