A transistor, as a semiconductor device with three electrodes (emitter, base, collector), can be turned on in three main ways (Fig. 3.1 - 3.6). As you know, the input signal enters the amplifier via two wires; The output signal is also routed through two wires. Consequently, for a three-electrode amplifying device, when an input signal is supplied and an output signal is received via two wires, one of the electrodes will certainly be common. According to which of the electrodes in the transistor switching circuit will be common, three main switching circuits are distinguished: with a common emitter (CE), a common collector (CC) and a common base (CB).

Rice. 3.1. Common emitter (CE) circuit

Rice. 3.2. Circuit with a common collector (OK)

Practical options for transistor switching circuits structures p-p-p and p-p-p are shown in Fig. 3.1 - 3.6. As follows from a comparison of the figures, these circuits are identical and differ only in the polarity of the supplied voltage.

To determine the input (RBX.) and output (RBbix.) resistance of each of the switching circuits, as well as the amplification factors for current (K,), voltage (Ki) and power (KR = K|ХKi), calculated and experimental values ​​and formulas are given in tables 3.1 and 3.2.

The table with formulas is given for approximate calculations, and for the initial, primary assessment and comparison of the properties of the main circuits for connecting transistors, a second table with numerical estimates is intended.

Rice. 3.3. Common base (CB) scheme

The designations in the table are as follows: RH - load resistance; R3 is the emitter resistance or the ratio of the change in voltage at the emitter junction to the change in the emitter current in the mode of a short circuit in the AC output circuit; RB - base resistance or the ratio of the change in voltage between the emitter and the base to the change in the collector current in the no-load mode of the input circuit in alternating current; a is the current gain for a circuit with a common base; p is the current gain for a circuit with a common emitter.

Rice. 3.4. Common emitter (CE) circuit

Rice. 3.5. Circuit with a common collector (OK)

Rice. 3.6. Common base (CB) scheme

Most often in practical circuits, the switching mode of a transistor with a common emitter is used (as it has the highest power gain).

Emitter followers (common collector circuits) are used to match the high output impedance of the signal source with the low input impedance of the load. To build high-frequency amplifiers (having low input impedance), circuits with a common base are used.

Depending on the presence, polarity and magnitude of potentials on the electrodes of transistors, several modes of its operation are distinguished. Saturation - the transistor is open, the voltage at the K-E junction is minimal, the current through the junctions is maximum. Cutoff - the transistor is closed, the voltage at the K-E junction is maximum, the current through the transitions is minimum. Active - intermediate between saturation and cutoff modes. Inverse - characterized by applying reverse (inverse) operating voltage polarity to the electrodes of the transistor.

In switching circuits that have only two states: on (the resistance of the key element is close to zero) and off (the resistance of the key element tends to infinity), saturation and cutoff modes are used. Active mode is widely used to amplify signals. The inverse mode is used quite rarely, since it is not possible to improve the performance of the circuit when the transistor is turned on in this way.

In order to initially estimate the values ​​of the RC elements included in the circuits without calculations (Fig. 3.1, 3.2, 3.4, 3.5), we can take the resistance value in the collector (emitter) circuit to be equal to several kOhms, and the resistance value in the base circuit to be 30 ...50 times larger. In this case, the voltage at the collector (emitter) should be equal to half the supply voltage. For a circuit with a common base (Fig. 3.3, 3.6), the value of resistance R3 usually does not exceed 0.1... 1 kOhm, the value of resistance R2 is several kOhm.

The values ​​of the reactance of capacitors C1 - SZ for the lowest frequencies that need to be amplified should be approximately an order of magnitude lower than the active resistances R1 - R3 connected to them (Fig. 3.1 - 3.6). In principle, the values ​​of these capacitors could be chosen with a significant margin, but in this case the dimensions of the transition capacitors, their cost, leakage currents, duration of transient processes, etc. increase.

As an example, let's use Table 3.3 to quickly determine the reactance value of capacitors for several frequencies.

Let us recall that the reactance of the capacitor Xc, Ohm, can be calculated using the formula:

For direct current, the reactance of capacitors tends to infinity. Therefore, for DC amplifiers (the lower cut-off frequency of the gain is zero), coupling capacitors are not required, and special measures must be taken to separate the stages. Capacitors in DC circuits are equivalent to an open circuit. Therefore, when constructing DC amplifier circuits, circuits with direct connections between stages are used. Of course, in this case it is necessary to coordinate the levels of interstage voltages.

When amplified alternating current inductive elements are often used in the load circuit of amplifier stages. Note that the reactance of inductances increases with increasing frequency. Accordingly, as the load resistance changes with frequency, the gain of such a cascade also increases.

In addition to bipolar transistors, more modern elements - field-effect transistors (Fig. 3.7 - 3.9) have become widespread.

Rice. 3.7. Common source circuit (CS)

Rice. 3.8. Common drain circuit (OS)

By analogy with switching circuits for bipolar transistors, field-effect transistors are switched on with a common source, a common drain, and a common gate.

Rice. 3.9. Common gate circuit (03)

The main calculated relationships for these circuits for switching field-effect transistors are given in Table 3.4, where S is the slope of the field-effect transistor characteristic, mA/V; R is the internal resistance of the transistor.

The approximate value of R1 (Fig. 3.7 - 3.9) can be from several Ohms to units of MOhm R2 - several kOhms. Note that, as for bipolar transistors, field-effect transistors also allow operation with cutoff and saturation; active and inverse modes.

To increase the current transfer coefficient bipolar transistor they use “composite” transistors connected according to the Darlington circuit (Fig. 3.10 - 3.13). Their overall gain is somewhat different from the product of the gains of each transistor. At the same time, the temperature stability of the circuit deteriorates.

Literature: Shustov M.A. Practical circuit design (Book 1), 2003

There are two main types of transistors - bipolar and field-effect. Bipolar transistors are made of alloyed materials and can be of two types - NPN and PNP. A transistor has three terminals known as emitter (E), base (B) and collector (K). The figure below shows an NPN transistor where, in the main operating modes (active, saturation, cutoff), the collector has a positive potential, the emitter is negative, and the base is used to control the state of the transistor.

An NPN transistor has one P region sandwiched between two N regions:

The junctions between the N and P regions are similar to junctions in diodes, and they can also be forward or reverse biased p-n junction. These devices can operate in different modes depending on the type of displacement:

• Cut-off: work in this mode also occurs when switching. No current flows between the emitter and collector, practically an “open circuit”, that is, “the contact is open”.
• Active mode: The transistor operates in amplifier circuits. In this mode, its characteristic is almost linear. A current flows between the emitter and collector, the magnitude of which depends on the value of the bias (control) voltage between the emitter and the base.
• Saturation: works when switching. There is practically a “short circuit” between the emitter and the collector, that is, “the contact is closed.”
• Inverse active mode: As in active mode, the transistor current is proportional to the base current, but flows in the opposite direction. Very rarely used.

In an NPN transistor, a positive voltage is applied to the collector to create a current from the collector to the emitter. In a PNP transistor, a positive voltage is applied to the emitter to create a current from the emitter to the collector. In NPN, current flows from the collector (K) to the emitter (E):

It is clear that the directions of current and voltage polarity in PNP and NPN are always opposite to each other. NPN transistors require a supply with positive polarity relative to the common terminals, and PNP transistors require a negative supply.

PNP and NPN work almost identically, but their modes are different due to the polarities. For example, to put NPN into saturation mode, U B must be higher than U K and U E. Below is short description operating modes depending on their voltage:

The basic operating principle of any bipolar transistor is to control the base current to regulate the flow of current between the emitter and collector. The operating principle of NPN and PNP transistors is the same. The only difference is the polarity of the voltages applied to their N-P-N and P-N-P junctions, that is, the emitter-base-collector.

There are three main circuits for connecting transistors. In this case, one of the electrodes of the transistor is the common input and output point of the cascade. We must remember that by input (output) we mean the points between which the input (output) alternating voltage operates. The main switching circuits are called circuits with a common emitter (CE), a common base (CB) and a common collector (CC).

Common emitter (CE) circuit. Such a circuit is shown in Figure 1. All the books say that this circuit is the most common, because it gives the greatest power gain.

Rice. 1 - Circuit diagram for connecting a transistor with a common emitter

The amplification properties of a transistor are characterized by one of its main parameters - the static base current transfer coefficient or the static current gain? Since it should characterize only the transistor itself, it is determined in no-load mode (Rk = 0). Numerically it is equal to:

at U k-e = const

This coefficient can be equal to tens or hundreds, but the real coefficient k i is always less than?, because when the load is turned on, the collector current decreases.

The voltage gain of the cascade k u is equal to the ratio of the amplitude or effective values output and input AC voltage. The input is the alternating voltage u b-e, and the output is the alternating voltage across the resistor, or what is the same, the collector-emitter voltage. The base-emitter voltage does not exceed tenths of a volt, and the output reaches units and tens of volts (with sufficient load resistance and source voltage E 2). It follows that the power gain of the cascade is equal to hundreds, thousands, and sometimes tens of thousands.

An important characteristic is the input resistance Rin, which is determined by Ohm’s law:

and usually ranges from hundreds of ohms to units of kilo-ohms. The input resistance of the transistor when switched on according to the OE circuit, as can be seen, turns out to be relatively small, which is a significant drawback. It is also important to note that the cascade according to the OE circuit reverses the voltage phase by 180°

The advantages of the OE circuit include the convenience of powering it from a single source, since supply voltages of the same sign are supplied to the base and collector. The disadvantages include worse frequency and temperature properties (for example, in comparison with the OB circuit). As the frequency increases, the gain in the OE circuit decreases. In addition, the cascade according to the OE circuit introduces significant distortion when amplified.

Scheme with a common base (CB). The OB diagram is shown in Figure 2.

Rice. 2 - Connection diagram for a transistor with a common base

This switching circuit does not provide significant gain, but has good frequency and temperature properties. It is not used as often as the OE scheme.

The current gain of the OB circuit is always slightly less than unity:

because the collector current is always only slightly less than the emitter current.

Is the static current transfer coefficient for the OB circuit indicated? and is defined:

with u k-b = const

This coefficient is always less than 1 and the closer it is to 1, the better the transistor. The voltage gain is the same as in the OE circuit. The input impedance of the OB circuit is tens of times lower than in the OE circuit.

For the OB circuit, there is no phase shift between the input and output voltage, that is, the phase of the voltage does not reverse when amplified. In addition, when amplified, the OB circuit introduces much less distortion than the OE circuit.

Circuit with a common collector (OK). The connection circuit with a common collector is shown in Figure 3. This circuit is more often called an emitter follower.

Rice. 3 - Connection diagram for a transistor with a common collector

The peculiarity of this circuit is that the input voltage is completely transmitted back to the input, i.e. the negative feedback is very strong. The current gain is almost the same as in the OE circuit. The voltage gain approaches unity, but is always less than it. As a result, the power gain is approximately equal to k i, i.e. several tens.

In the OK circuit, there is no phase shift between the input and output voltage. Since the voltage gain is close to unity, the output voltage matches the input voltage in phase and amplitude, i.e., repeats it. That is why such a circuit is called an emitter follower. Emitter - because the output voltage is removed from the emitter relative to the common wire.

The input impedance of the OK circuit is quite high (tens of kilo-ohms), and the output impedance is relatively small. This is an important advantage of the scheme.

The OE circuit has the highest power gain, so it remains the most common solution for high-frequency amplifiers, GPS, GSM, and WiFi systems. Currently, it is usually used in the form of ready-made integrated circuits (MAXIM, VISHAY, RF Micro Devices), but without knowing the basics of its operation, it is almost impossible to obtain the parameters given in the description of the microcircuit. That is why when hiring and searching for employees, the main requirement is is knowledge of the principles of operation of amplifiers with OE.

An amplifier, no matter what it is (audio amplifier, tube amplifier or radio frequency amplifier) ​​is a four-terminal network in which two pins are input and two pins are output. The block diagram of the amplifier is shown in Figure 1.

Figure 1 Block diagram of the amplifier

The main amplifier element, the transistor, has only three terminals, so one of the transistor terminals must be used simultaneously to connect a signal source (as an input terminal) and to connect a load (as an output terminal). A common emitter circuit is an amplifier where the emitter of the transistor is used to connect both the input signal and the load. The functional diagram of an amplifier with a transistor connected according to a common emitter circuit is shown in Figure 2.

Figure 2 Functional diagram of connecting a transistor with a common emitter

In this diagram, the dotted line shows the boundaries of the amplifier shown in Figure 1. It does not show the transistor power circuits. Currently, the common emitter circuit is practically not used in audio amplifiers, however, it finds wide application in television signal amplifier circuits, GSM amplifiers or other high-frequency amplifiers. You can use two power supplies to power a transistor in a common emitter circuit, but this will require two voltage regulators. In battery-powered equipment this can be problematic, so a single power source is usually used. To power an amplifier with a common emitter, any of the circuits we have considered may be suitable:

• emitter-stabilized circuit.

Let's look at an example of an amplifier circuit with a common emitter and emitter stabilization of the transistor operating mode. Figure 3 shows a schematic diagram of a cascade based on a bipolar NPN transistor, designed to amplify audio frequencies.

Figure 3 Schematic diagram amplifier stage with common emitter

Calculation of the elements of this scheme according to DC can be found in the article. Now we will be interested in the parameters of the amplifier stage assembled according to a circuit with a common emitter. The most important characteristics of an amplifier stage are its input and output impedance and power gain. Basically, these characteristics are determined by the parameters of the transistor.

Common emitter input impedance

In a common emitter circuit, the input resistance of the transistor is R The input HOE can be determined by its input characteristic. This characteristic coincides with the current-voltage p-n characteristic transition. An example of the input characteristic of a silicon transistor (voltage dependence U b from the base current I b) is shown in Figure 4.

Figure 4 Input characteristic of a silicon transistor

As can be seen from this figure, the input resistance of the transistor R IOE depends on base current I b0 and is determined by the following formula:

(1)

How to determine Δ U b0 and Δ I b0 in the vicinity of the transistor operating point in a circuit with a common emitter is shown in Figure 5.

Figure 5 Determination of the input resistance of a common-emitter circuit from the input characteristic of a silicon transistor

Determining resistance using formula (1) is the most accurate way to determine input resistance. However, when calculating an amplifier, we do not always have at hand the transistors that we will use, so it would be nice to be able to calculate the input resistance in an analytical way. The current-voltage characteristic of a pn junction is well approximated by an exponential function.

(2)

Where I b - base current at the operating point;
U bе is the base voltage at the operating point;
I s is the reverse current of the emitter-base transition;
— temperature potential;
k— Boltzmann constant;
q— electron charge;
T— temperature expressed in degrees Kelvin.

In this expression, the coefficient normalizing the exponent is the current I s, therefore, the more accurately it is determined, the better the match between the real and approximate input characteristics of the transistor will be. If we neglect unity in expression (2), then the voltage at the base of the transistor can be calculated using the following formula:

(3)

From expression (1) it is clear that the input resistance is the derivative of the voltage at the base of the transistor with respect to the current. Let us differentiate expression (3), then the input resistance of a circuit with a common emitter can be determined by the following formula:

(4)

However, the graph of the actual input characteristic of a transistor connected in a common-emitter circuit differs from the exponential function. This is due to the fact that the ohmic resistance of the semiconductor in the base of the transistor is not zero, therefore, at high base currents of the transistor in a circuit with a common emitter, its input resistance will tend to the ohmic resistance of the base rbb".

The input current of a common-emitter circuit flows not only through the input resistance of the transistor, but also through all resistors of the voltage-forming circuits at the base of the transistor. Therefore, the input resistance of a common emitter circuit is defined as a parallel connection of all these resistances. The input current paths for a common emitter circuit are shown in Figure 6.

Figure 6 Current flow through the input circuits of a circuit with a common emitter

It is much easier to analyze this circuit using the equivalent circuit of the input circuit, where only those circuits through which the input current flows from the signal source are shown. The equivalent input circuit of a common emitter circuit is shown in Figure 7.

Figure 7 Equivalent circuit of the input circuit of a common-emitter circuit

This circuit is built for medium frequencies using a transistor equivalent circuit. At mid frequencies, the transistor's input capacitance has no effect, so we do not display it on the equivalent circuit. The resistance of capacitor C3 at mid frequencies is close to zero, so there are no R4C3 elements in the circuit. Elements R out and h 21× i inputs do not affect the input circuit and are shown in the diagram to display the amplifying properties of the transistor.

Finally, we can write down the formula for the input impedance of a common emitter circuit:

(5)

After manufacturing an amplifier calculated using the above methods, it is necessary to measure the input resistance of the circuit with a common emitter. To measure the input resistance, use the circuit for measuring the input resistance of the amplifier, shown in Figure 8. In this circuit, an alternating voltage measuring generator and two high-frequency AC voltmeters are used to measure the input resistance (you can use one and make two measurements).

Figure 8 Circuit for measuring the input impedance of the amplifier stage

In case the resistance R and will be equal to the input resistance of the amplifier, the voltage that the AC voltmeter V2 will show will be half the voltage V1. If it is not possible to change the resistance R and when measuring the input impedance, the input impedance of the amplifier can be calculated using the following formula:

(6)

Output impedance of a common emitter circuit

The output resistance of the transistor depends on the design features of the transistor, the thickness of its base, and the volume resistance of the collector. The output resistance of a transistor connected in a common-emitter circuit can be determined from the output characteristics of the transistor. An example of the output characteristics of a transistor is shown in Figure 9.

Figure 9 Output characteristics of a silicon transistor

Unfortunately, the output characteristics are usually not given in the characteristics of modern transistors. This is due to the fact that their output impedance is quite high and the output impedance transistor stage with a common emitter is determined by the load resistance. In the circuit shown in Figure 6, this is the resistance of resistor R3.

Literature:

Together with the article "Common-emitter circuit (common-emitter cascade)" read:

http://site/Sxemoteh/ShTrzKask/KollStab/

http://site/Sxemoteh/ShTrzKask/EmitStab/

Bipolartransistor - three-electrode semiconductor device, one of the types transistor. The electrodes are connected to three successive layers of alternating semiconductor impurity conductivity. According to this method of alternation, they distinguish n-p-n And p-n-p transistors ( n (negative) - electronic type of impurity conductivity, p (positive) - hole). In a bipolar transistor, unlike field effect transistor, charges of two types are used simultaneously, the carriers of which are electrons and holes (from the word “bi” - “two”). The schematic structure of the transistor is shown in the second figure.

The electrode connected to the central layer is called base, the electrodes connected to the outer layers are called collector And emitter. On the simplest scheme the differences between collector and emitter are not visible. In reality, the main difference between the collector is its larger area Pn junction. In addition, for the transistor to operate, a thin base thickness is required.

Device and principle of operation

Simplified cross-sectional diagram of an NPN bipolar transistor

The first transistors were made based on Germany. Currently, they are made mainly from silicon And gallium arsenide. Gallium arsenide transistors are used in ultra-high-speed logic circuits and high-frequency amplifier circuits.

A bipolar transistor is composed of three different ways alloyed semiconductor layers: emitter E, bases B and collector C. Depending on the type of conductivity of these zones, they are distinguished NPN(emitter − n-semiconductor, base − p-semiconductor, collector − n-semiconductor) and PNP transistors. Conductive non-rectifying contacts are connected to each of the zones. The base is located between the emitter and collector and is lightly doped, therefore it has a high ohmic resistance. The total base-emitter contact area is significantly smaller than the collector-base contact area (this is done for two reasons - the large area of ​​the collector-base junction increases the likelihood of the capture of minority charge carriers from the base to the collector and, since in the operating mode the collector-base junction is usually switched on with reverse bias, which increases heat generation and promotes heat removal from the collector), therefore a general bipolar transistor is an asymmetrical device (it is impractical to swap the emitter and collector by changing the connection polarity and resulting in a bipolar transistor similar to the original one - inverse switching).

In the active amplification mode of operation, the transistor is turned on so that its emitter junction displaced in the forward direction (open), and the collector junction is shifted in the opposite direction (closed). For definiteness, let's consider npn transistor, all reasoning is repeated absolutely similarly for the case pnp transistor, replacing the word “electrons” with “holes”, and vice versa, as well as replacing all voltages with opposite signs. IN npn In a transistor, electrons, the main current carriers in the emitter, pass through the open emitter-base junction ( are injected) to the base area. Some of these electrons recombines with the main charge carriers in the base (holes). However, because the base is made very thin and relatively lightly doped, most of the electrons injected from the emitter diffuse into the collector region . The strong electric field of a reverse-biased collector junction captures minority carriers from the base (electrons) and transfers them to the collector. The collector current is thus practically equal to the emitter current, with the exception of a small recombination loss in the base, which forms the base current ( I uh =I b +I To). Coefficient α connecting the emitter current and collector current ( I To = α I uh) is called the emitter current transfer coefficient. The numerical value of the coefficient α is 0.9-0.999. The higher the coefficient, the more efficiently the transistor transmits current. This coefficient depends little on the collector-base and base-emitter voltages. Therefore, over a wide range of operating voltages, the collector current is proportional to the base current, the proportionality coefficient is β = α/(1 − α), from 10 to 1000. Thus, a small base current can be controlled by a much larger collector current.