Home Rack The operating principle of an amplifier based on bipolar transistors - theory. The principle of operation of an amplifier using bipolar transistors Single-stage amplifier with a common emitter

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The operating principle of an amplifier based on bipolar transistors - theory. The principle of operation of an amplifier using bipolar transistors Single-stage amplifier with a common emitter

Federal Agency for Education of the Russian Federation

Ufa State Aviation Technical University

Kumertau branch

Department of PA

Course work

In the discipline "Electronics"

Completed by: student of group ATPP-304

Ignatiev I.A.

Checked by: teacher

Zimin N.V.

Kumertau 2010

Introduction

1. Basic concepts

1.1 Amplifier

1.3 h-parameters of bipolar transistors

1.4 Parameters of transistor P14

2. Calculation of parameters and description of the circuit diagram of the device

2.1 Selecting the operating point

2.2 Determination of the gain factors of transistor P 14

2.3 Calculate the input and output resistance of transistor P 14

2.4 Calculation of amplifier elements

2.5 Calculation of capacitor capacities

Conclusion

Bibliography

Introduction

In this course work, various thermal stabilization schemes are analyzed. During the design process, we made an analytical calculation of the amplifier and its design options.

In this work, we calculated the elements of a single-stage amplifier according to a circuit with a common base and calculated the amplification factors for current, voltage and power, input and output resistance.

As a result of the calculation, a low-frequency amplifier with specified requirements and element ratings was developed, which can be used for practical applications.

The data obtained can be used to create real amplification devices.

1. Basic concepts

1.1 Amplifier

When solving many engineering problems, for example, when measuring electrical and non-electrical quantities, receiving radio signals, monitoring and automating technological processes, there is a need to amplify electrical signals. Amplifiers serve this purpose.

An amplifier is a device that increases the energy of the control signal using the energy of an auxiliary source. The input signal is like a template, according to which the flow of energy from the source to the consumer is regulated.

Modern amplifiers, widely used in industrial electronics, usually use bipolar and field-effect transistors, and more recently integrated circuits. Amplifiers on microcircuits are highly reliable and economical, have high operating speed, have extremely small weight and size, and high sensitivity. They allow you to amplify very weak electrical signals.

In a simplified way, an amplifier (amplifier stage) can be represented in the form of a block diagram (Fig. 1.):

This amplifier contains a nonlinear controlled element, usually a bipolar or field-effect transistor, a consumer and a source of electrical energy. The amplifier stage has an input circuit to which the input voltage is supplied (the amplified signal), and an output circuit to produce the output voltage (the amplified signal). The amplified signal has significantly more power than the input signal. The signal power increases due to the source of electrical energy. The amplification process is carried out by changing the resistance of the nonlinear controlled element, and therefore the current in the output circuit, under the influence of the input voltage or current. The output voltage is removed from the controlled or consumer. Thus, amplification is based on the conversion of electrical energy from a constant EMF source into the energy of the output signal by changing the resistance of the controlled element according to the law specified by the input signal.

The main parameters of the amplifier stage are voltage gain Ku= U out / Uin, current gain K I = I out / I input And power gain

Typically, in amplifier stages, all three gains are significantly greater than unity. However, in some amplifier stages, one of the two gains may be less than unity, i.e. TO U <1 или К I <1. Но в любом случае коэффициент усиления по мощности больше единицы.

Depending on what parameter of the input signal (voltage, current or power) needs to be increased using the amplifier stage, voltage, current and power amplifier stages are distinguished. The voltage amplification stage has a gain, usually equal to several tens. In engineering practice, it is often necessary to obtain a significantly higher voltage gain, reaching several thousand and even millions. To solve this problem, multistage amplifiers are used, in which each subsequent stage is connected to the output of the previous one.

Depending on the type of signals to be amplified, amplifiers are divided into:

1. Harmonic signal amplifiers

(sound signals of the form U (t) =U O +∑Ui*cos (ωt+φ);

2. Pulse signal amplifiers.

3. DC and AC amplifiers.

4. Low and high frequency amplifiers (20Hz - 20KHz).

5. High frequency amplifiers.

6. Narrowband and wideband amplifiers.

7. Selective amplifiers.

8. Aperiodic amplifiers.

Connection methods The (connections) of the stages depend on the multi-stage amplifier. Thus, in DC amplifiers, the input of the subsequent stage is connected to the output of the previous stage directly or using resistors. Such amplifiers are called amplifiers with direct or resistive coupling .

amplifier capacitor single-stage thermal stabilization

In alternating voltage amplifiers (UHF, ULF and TYPU), capacitors and resistors are most often used to couple cascades. Such amplifiers are called amplifiers with resistive-capacitive couplings.

In selective amplifiers and power amplifiers, transformers are sometimes used to connect the stages to each other and to connect the amplifier stage to the load device. Such amplifiers are called amplifiers with transformer coupling.

Capacitors and transformers in alternating voltage amplifiers serve to separate the alternating component of the voltage (output) from the direct voltage component on the nonlinear controlled element, which arises from the direct current component created by a source of constant emf.

Based on the method of switching on the amplification element, there are three main types of amplification stages, both bipolar and field-effect transistors.

One of the most common amplifier stages based on bipolar transistors is common emitter cascade(OE cascade).

The circuit of the amplifier stage of an n-p-n type transistor with an OE is shown in Fig. 2.

Uin, which needs to be amplified, is supplied from the oscillation source to the Base-Emitter section. The Base is also supplied with a positive bias from source E1, which is the forward voltage of the emitter junction.

Current flows in the base circuit, therefore, the input resistance of the transistor is small.

To prevent loss of part of the input alternating voltage, the internal resistance of the source E1 is shunted by a capacitor. At low frequencies it should have a resistance many times less than the input resistance of the transistor.

The collector circuit is powered from source E2. The source voltage of modern amplifier stages based on bipolar transistors is usually 10 - 30 V.

To obtain an enhanced output voltage, a load resistance is included in it.

The operation of the amplifier stage occurs as follows. Let's imagine the collector circuit in the form of an equivalent circuit (Fig. 3.).

The source voltage E2 is divided between Rn and the internal resistance of the transistor, which it provides to the constant collector current.

The internal resistance of the transistor is approximately equal to the resistance of the collector junction for direct current:

If a source of oscillation is included in the input circuit, then when it changes

voltage changes the emitter current. This causes a change in r to, which leads to a redistribution of the voltage of the source E2 between R o and r to. In this case, the alternating voltage at the load can be tens of times greater than the input voltage.

The change in collector current is approximately equal to the change in emitter current and many times greater than the change in base current, so in the circuit under consideration a significant current gain and a very large power gain are obtained.

1.2 Bipolar transistor amplifiers

In amplifiers based on bipolar transistors, three transistor connection schemes are used: with a common base (Fig. 4;

7), with a common emitter (Fig. 5;

8), with a common collector (Fig. 6;

Fig.4 Fig.5 Fig.6

Fig.7 Fig.8 Fig.9

Figures 4-6 show circuits for switching on transistors with the input and output circuits powered from separate power sources, and Figures 7 - 8 show the transistor's input and output circuits powered from a single constant voltage source.

Amplifiers in a common-base transistor circuit are characterized by voltage gain, no current gain, low input resistance and high output resistance.

A typical circuit of an amplifier stage based on a transistor with an OE is shown in Fig. 11.5. The input amplified alternating voltage UВХ is supplied to the input of the transistor through the isolation capacitor CP1. Capacitor CP1 prevents the transfer of the constant voltage component of the input signal to the amplifier input, which can cause a violation of the direct current operating mode of the VT transistor. The amplified alternating voltage generated at the collector of the transistor VT is supplied to an external load with resistance RN through an isolation capacitor CP2. This capacitor serves to separate the output collector circuit from the external load by the constant component of the collector current IK0. The values of IK0 and other constant components of current and voltage in the transistor circuits depend on the initial operating mode (initial position of the operating point), set in the absence of a signal.

Fig. 11.5. Bipolar transistor amplifier with OE

The operating point of the transistor is the point of intersection of the dynamic characteristics (load straight line) with one of the static current-voltage characteristics. This position is determined on the characteristics by a set of direct components of currents and voltages in the output IK0, UKE0 and input IB0, UBE0 circuits.

The operation of the amplification stage is illustrated in Fig. 11.6.

Fig. 11.6. Graphic illustration of the operation of an amplifier stage on a transistor with OE

The process of signal amplification can be reflected by the following relationship of electrical quantities:

UВХm→IБm→IКm→IКmRК→(UКЭm= EPIT - IКmRК) = UOUTm.

The figure shows that the input signal voltage with amplitude UВХm =UBЭm changes the value of the base current in phase. These changes in the base current cause proportional changes in the collector current and collector voltage in the collector circuit, and the amplitude of the collector voltage turns out to be significantly greater than the amplitude of the base voltage. The signal voltages at the input and output of the cascade are shifted in phase by 180º, i.e. are in antiphase. When the transistor is operating in active (amplifying) mode, the operating point should be approximately in the middle of the segment AB of the load straight line. The maximum changes in the base input current must be such that the operating point does not go beyond the limits of the segment AB. Figure 11.7 shows the timing diagrams of the operation of the transistor stage with the correct choice of the rest point and the magnitude of the input signal. It is very important to ensure that not only the magnitude of the input signal is correct, but also the quiescent current. With a small initial quiescent current and a minimum signal, the transistor will not open and will be in cutoff mode; with a large bias and a high signal level, it may go into saturation. Rice. 11.8. shows the voltage at the collector of the transistor: a - with insufficient bias current; b - with excess bias current; c - when the input signal is excessive.

Fig. 11.7. Timing diagrams of the operation of a transistor amplifier in a circuit with an OE

The initial position of the operating point is provided by a voltage divider consisting of resistors R1 and R2, the resistance values of which are determined from the ratios: R1 = (EK - UBE0 - URE) / (ID + IB0); R2 = (UBE0 + URE) / ID, where ID = (2…5) IB0 is the current in the divider circuit.

When ensuring the operating mode of the transistor, it is necessary to carry out temperature stabilization of the operating point position (reduce the influence of temperature on the initial position of the operating point). For this purpose, a resistor RE is introduced into the emitter circuit, which creates an OOS voltage for direct current URE.

Fig. 11.8. Timing diagrams of collector voltage under incorrect conditions

The OOS in this circuit operates as follows: when, for example, the temperature of the transistor changes, the collector current increases. This causes a corresponding increase in the emitter current and voltage drop across it. Consequently, the voltage UBE = UB - UE, which is the control voltage for the transistor, decreases, the transistor is switched off, the collector current decreases and returns to the specified mode. The introduction of OOS reduces the gain of the circuit. In order for the feedback to act only on direct current and to eliminate the negative feedback on alternating current, the resistor RE is shunted with a capacitor SE, the resistance of which at the frequency of the amplified signal should be insignificant. When analyzing the circuit, we can assume that there is no environmental protection for alternating current. In this case, the current stage gain

In amplifiers based on bipolar transistors, three transistor connection schemes are used: with a common one, with a common emitter, with a common collector.

In a transistor circuit with a common emitter, the amplifier provides voltage, current, and power amplification. Such an amplifier has average values of input and output resistance compared to switching circuits with a common base and a common collector.

Transistor parameters largely depend on temperature. A change in ambient temperature leads to a change in the operating mode of the transistor in a simple amplifier circuit when the transistor with a common emitter is turned on.

To stabilize the operating mode of the transistor when the temperature changes, emitter stabilization circuits are used to stabilize the operating mode of the transistor.

Figures 5.14 and 5.15 show circuits of single-stage amplifiers based on n-p-n and p-n-p bipolar transistors with emitter temperature stabilization of the transistor operating mode.

Let's trace the circuits through which direct currents flow in the amplifier according to the diagram in Figure 5.14. The direct current of the voltage divider flows through the circuit: plus the power supply, resistors R1, R2, minus the power supply. The direct current of the base of transistor VT1 flows through the circuit: plus of the power supply, resistor R1, base-emitter junction of transistor VT1, resistor Re, minus of the power supply. The direct collector current of transistor VT1 flows through the circuit: plus of the power supply, resistor RK, collector-emitter terminals of the transistor, resistor Re, minus of the power supply. The bipolar transistor as part of the amplifier operates in a mode where the base-emitter junction is biased in the forward direction, and the base-collector junction is biased in the reverse direction. Therefore, the constant voltage across resistor R2 will be equal to the sum of the voltage at the base-emitter junction of transistor VT1 and the voltage across resistor Re:UR2=Ube+URe. It follows that the constant voltage at the base-emitter junction will be equal to Ube = UR2 - URe.

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In rest mode, i.e. in the absence of an input signal (U input = 0), the direct current I BO under the influence of E K passes through the circuit + E K – E- B- R B - -E K. The magnitude of this current by selecting the values of R B is set such that the transistor is half open, i.e. the voltage across it would be approximately half E K. In turn, with a large base current, the transistor opens completely, i.e. its resistance between the emitter and collector is very small, the voltage U EC is almost zero, and at I B = 0 the transistor is completely closed, i.e. The resistance is high and it practically does not allow current I K to pass through.

Capacitor C p1 serves to connect a source of variable input EMF E in, with internal resistance R in, to the base circuit. The coupling capacitor C p2 serves to isolate the alternating component of the collector voltage at the load Rn.

18. Determination of the initial conditions that ensure the specified operating mode of the amplifier with OE

Let's consider an RC amplifier in which the transistor is connected to a circuit with a common emitter and emitter stabilization of the initial operating mode is used.

Currents in the circuit are found using the formulas:

Suppose that i B = i B2, then:

Let us assume that the supply voltage Ek is given and it is required to ensure the initial operating mode at a given initial current I K N.

Considering that i E » i K:

The current i division of the voltage divider on resistors R 1 and R 2 is selected, flowing when the transistor base is disconnected from the divider.