The basic knowledge of amplifier circuit

I. Basic principles of amplifier circuits

Amplifier circuits are one of the most widely used electronic circuits and the basic unit circuits that constitute other electronic circuits. It can increase the output power of the signal. The amplifier circuit obtains the energy source through the power supply to control the waveform of the output signal to be consistent with the input signal, but with a larger amplitude. According to this, it can also be regarded as an adjustable output power supply, which is used to obtain an output signal stronger than the input signal.

The essence of the amplifier circuit is the control and conversion of energy. The actual amplifier circuit usually consists of a signal source, an amplifier composed of a transistor and a load. amplifier meaning

II. The basic form of the amplifier circuit

According to the different common ends of the input circuit and the output circuit, there are three basic forms of the amplifier circuit: common emitter amplifier circuit, common collector amplifier circuit and common base amplifier circuit. amplifier price

1. Common emitter amplifier circuit

The common emitter amplifier circuit is the most widely used triode connection method in the amplifier circuit. The signal is input from the base and emitter of the triode, and output from the collector and emitter. Because the emitter is very common to the ground, it is named the common emitter amplifier circuit. The structure of the common emitter amplifier circuit is simple, it has a large voltage amplification factor and current amplification factor, and the input and output resistances are moderate.

Features:

(1) The input signal and the output signal are inverted;

(2) Larger current and voltage gain;

(3) Generally used as the intermediate stage of the amplifier circuit;

(4) Between the collector of the common emitter amplifier and the zero potential point is the output terminal, which is connected to the load resistor.

2. Common collector amplifier circuit

In the common collector amplifier circuit, the input signal is input from the base and emitter terminals of the triode. Its output signal is obtained from both ends of the emitter of the triode. The load of the circuit is connected to the emitter, and the collector is directly connected to the power supply UCC. For AC signals, the collector is equivalent to ground and becomes the common terminal of input and output, so this circuit is called a common collector amplifier circuit.

Analytical method:

The three basic amplifying circuit analysis are divided into static analysis and dynamic analysis, also known as DC analysis and AC analysis. When there is no signal input, each current and voltage in the amplifying circuit is a constant DC quantity, which is called static. The analysis of static is called static analysis. When there is an AC signal input, the input AC signal will be superimposed on the DC quantity to be amplified and transmitted in the amplifying circuit. At this time, the voltage and current in the circuit are in a state of change, which is called dynamic. The analysis of dynamics is called dynamic analysis. amplifier speaker

(1) Static analysis: Draw the DC path of the amplifying circuit, and then find the static operating point by analytical method or graphical method, that is, find IBQ, ICQ, UCEQ.

(2) Dynamic analysis: It is necessary to draw the AC path and micro-variation equivalent circuit of the amplifying circuit, and then find the voltage magnification Au, input resistance ri and output resistance ro in the micro-variation equivalent circuit of the amplifying circuit.

Features:

(1) It has the function of power amplification;

(2) The input signal is in phase with the output signal;

(3) Suitable for power amplification and impedance matching circuits;

(4) It is often used as a buffer stage and output stage in multi-stage amplifiers;

(5) The current gain is high, the current iB in the input circuit << the current iE and iC in the output circuit;

(6) The output resistance of the emitter output device is lower than that of the common emitter amplifier circuit, generally ranging from several ohms to tens of ohms;

(7) The input resistance of the common-collector amplifier circuit is much larger than that of the common-emitter amplifier circuit, which can reach tens of thousands of ohms to hundreds of thousands of ohms. The larger the input resistance of the amplifying circuit, the smaller the current drawn by the circuit from the signal source, the lighter the burden on the signal source, and the greater the voltage obtained by the circuit from the signal source. 

3. Common base amplifier circuit

In a common base amplifier circuit, the input signal is input from the emitter and base terminals of the transistor, and then the output signal is obtained from the collector and base terminals of the transistor because the base terminals are common ground terminals, so it is called a common base amplifier circuit. 

Features:

(1) The input signal is in phase with the output signal;

(2) High voltage gain; low current gain (≤1);

(3) The bandwidth is large, usually used as a broadband or high frequency amplifier;

(4) High power gain, suitable for high-frequency circuits, the input impedance of the common base amplifier circuit is very small, which will seriously attenuate the input signal, and is not suitable as a voltage amplifier;

(5) The output current is the emitter current, so the output impedance is low impedance. However, the common base amplifier circuit cannot amplify the current. Because of its good frequency characteristics, it is often used in broadband amplifiers and high-frequency amplifiers;

(6) The common base amplifier circuit can also be used as a "current buffer". The output current of the common base amplifier circuit is less than the input current, and there is no current amplification effect. The input signal is added to the forward-biased emitter junction, and the current is large, so the input impedance is very low. The output signal is a reverse-biased collector junction with high impedance and a large voltage magnification. Common collector amplifier circuit, the input signal is added between the base and collector. Due to the reverse bias voltage applied to the collector junction, the impedance is very high. amplifier for sale

Ⅲ. History and evolution of amplifier circuit

Amplifier circuit has played different roles in the electronic field in different periods:

(1) Amplifier circuits were first used in relay transmission facilities. For example, in old-fashioned telephone lines, it uses a weak current to control the power supply voltage of the outgoing line.

(2) For audio broadcasting. On December 24, 1906, Reginald Fessenden used a carbon grain microphone as an amplifier for the first time in an FM radio transmission device to modulate the sound into a radio frequency source.

(3) In the 1960s, vacuum tubes began to be phased out. At the time, some high-power amplifiers or professional-grade audio applications, such as guitar amplifiers and hi-fi amplifiers, still used vacuum tube amplifier circuits. Many radio transmitters still use vacuum tubes.

(4) Beginning in the 1970s, more and more transistors were connected to a chip to make integrated circuits. A large number of amplifiers in commercial use today are based on integrated circuits.

Ⅳ.  Performance indicators of the amplifier circuit

Voltage magnification, input resistance and output resistance are the three main performance indicators of the amplifier circuit. The most commonly used method to analyze these three indicators is the micro-variation equivalent circuit method. This is a non-linear triode amplifier circuit equivalent to a linear amplifier circuit under the condition of small signal amplification.

(1) Magnification

The magnification, also known as gain, is an index to measure the amplification capability of the amplifying circuit. Depending on the amount of input and output that needs to be processed, the magnifications include voltage, current, mutual resistance, mutual conductance, and power magnification. Among them, voltage magnification is the most widely used.

(2) Input resistance

The input resistance of the amplifier circuit is the equivalent resistance seen from the input terminal to the amplifier circuit, which is equal to the ratio of the input voltage to the input current after the output terminal of the amplifier circuit is connected to the actual load resistance, that is, Ri=Ui/Ii. For a signal source, the input resistance is its equivalent load.

The size of the input resistance reflects the influence of the amplifier circuit on the signal source. The larger the input resistance, the smaller the current drawn by the amplifier circuit from the signal source (that is, the input current); the smaller the voltage drop on the internal resistance of the signal source, and the closer the actual input voltage is to the signal source voltage, often called constant voltage enter. Conversely, when constant current input is required, Ri<<Rs must be made; if maximum power input is required, Ri=Rs is required, which is often called impedance matching.

(3) Output resistance

For the load, the output terminal of the amplifying circuit can be equivalent to a signal source. The smaller the output resistance, the less the output voltage is affected by the load. If Ro=0, the output voltage will not be affected by the size of RL, which is called constant voltage output. Constant current output can be obtained when RL<<Ro. Therefore, the size of the output resistance reflects the size of the load capacity of the amplifier circuit.

Ⅴ. Classification of amplifier circuit

1. According to the input/output signal type:

(1) Voltage Amplifier: used to amplify voltage signals.

(2) Current Amplifier: used to amplify the current signal.

(3) Power amplifier: its task is to output enough power to push the load to work. For example, the sound of the speaker, the action of the relay, the rotation of the motor, etc. Both the power amplifier circuit and the voltage amplifier circuit use the amplification effect of the triode to amplify the signal. The difference is that the power amplifier circuit aims to output sufficient power and works in a large signal state; the purpose of the voltage amplifier circuit is to output a large enough voltage , work in a small signal state.

The power amplifier circuit should meet the following requirements:

The output power is large enough. In order to obtain a large output signal voltage and current, the triode is often required to work in the limit state. In actual application, the limit parameters PCM, ICM and U(BR)CEO of the triode should be taken into account.

Dynamic work analysis let the input signal be a sinusoidal voltage ui, as shown in Figure 6-30a. In the positive half cycle, the emitter junction of the V1 tube is forward-biased, and the emitter junction of the V2 tube is reverse-biased, and the current ic1 provided by +VCC flows to the load through the V1 tube, and the positive half-cycle output voltage uo is obtained on the load RL. Similarly, in the negative half cycle, the emitter junction of V1 is reverse-biased, and the emitter junction of V2 is forward-biased. The current ic2 provided by -VCC flows from the -VCC terminal to the V2 tube through the load, and the negative half-cycle output voltage is obtained on RL. uo. It can be seen that in the entire cycle of ui, the V1 tube and the V2 tube are turned on in turn to complement each other, so that a complete output voltage uo is obtained on RL, so it is called a complementary symmetrical power amplifier circuit. amplifier classes

There are many types of power amplifiers:

① Class A amplifier

When the efficiency requirement is not high, most small-signal linear amplifiers will be designed as class A, that is, the output stage device is always in the conduction region. Class A amplifiers are generally more linear than other types, but it’s very inefficient. These amplifiers are most commonly used in small signal level or low power applications such as driving headphones.

The disadvantage of class A amplifiers is that the output efficiency is very low, and the theoretical value does not exceed 50%. Take driving earphones as an example. Under normal circumstances, the lower the volume, the more power consumption. When there is no signal input, the current flows at the maximum amount. Therefore, when the machine is not listening to music, it consumes the fastest power. Even when listening to music, 50% of the electricity used is dissipated as heat. Therefore, the power consumption of a class A amplifier is no less than that of an air conditioner, and this 50% heat consumption is the reason for the gradual aging of the vacuum tube. At the same time, because of the high heat generation, all parts work under high current and high temperature for a long time, which may easily cause problems in stability and life. If it is a pure class A vacuum tube integrated amplifier, there are also problems such as tube life and future replacement.

② Class B and AB amplifier

In a Class B amplifier, there are two groups of output devices that amplify the positive and negative half-cycles, each alternately conducting at exactly 180 degrees (or half-cycles) of the input signal.

Class B push-pull amplifier

Class AB amplifier is a compromise between Class A and Class B, which improves the linearity of small signal output; the conduction angle is above 180 degrees, and the specific value is determined by the designer. Due to their high efficiency, they are usually used in low frequency amplifiers (such as audio and hi-fi). Or for other designs where linearity and efficiency are important (cell phones, cell towers, TV transmitters).

③ Class C amplifier

Class C amplifiers are often referred to as high power radio frequency (RF) amplifiers. Class C amplifiers are designed to turn on when the input signal is less than 180°. Linearity is bad, but that's not important for a single frequency power amplifier. The signal is restored to an approximate sinusoidal shape by the tuned circuit, and the efficiency is much higher than that of class A, class AB or class B amplifiers.

④ Class D amplifier

Class D amplifiers use fast switching to achieve high power switching. Its principle is similar to that of a switching power supply. The amplified analog signal is output by turning on or off each output device, so the energy loss is minimized; since the original signal is first converted into a series of switching (1 and 0) instructions, it is also called digital Amplifier. Simple methods like pulse-width modulation are sometimes used; however, high-performance switching amplifiers use digital techniques, such as sigma-delta modulation, to achieve higher performance.

They were originally used only in subwoofer amplifiers due to their limited bandwidth and considerable distortion (poor sound quality). Advances in semiconductor devices have made it possible to develop high-fidelity, full-sound-band Class-D amplifiers, so that their signal-to-noise (S/N) and distortion levels have narrowed the gap with other linear devices.

Since the PDM digital signal also belongs to the PWM signal, the original digital music signal can be converted into a PDM signal that is also a digital signal through mathematical calculations, and the PDM signal directly controls the switching device, which is called digital direct output (also known as Power DAC and Sony’s S-master). One of the criticisms of Class D amplifiers is the immaturity and unpopularity of digital direct input. Most audio systems use digital-to-analog converters to convert digital music into analog audio. Then the Class D amplifier converts the analog audio back to a digital signal in the PDM format, and the two digital-to-analog conversions will cause greater distortion. 

2. According to the operating frequency range of the amplifier:

(1) Audio Frequency Amplifier (A.F. Amplifier): for the audio frequency range (20 Hz to 20 kHz).

(2) Radio Frequency Amplifier ( R.F. Amplifier): suitable for radio frequency range (usually from hundreds of kHz to several GHz).

3. According to the working mode of the amplifier:

(1) Classified amplifiers (Class A, B, AB, C, D, E, etc.): classified according to the conduction angle and output waveform characteristics of the amplifier.

(2) Differential Amplifier: it uses differential input to amplify the two input terminals of the input signal, and is often used for differential signal processing and noise suppression.

4. According to the purpose of the amplifier:

(1) Operational Amplifier (Op-Amp): it mainly used in various signal processing and amplification applications.

(2) Instrumentation Amplifier: it used to improve the amplification of sensor signals and suppress noise.

(3) Feedback Amplifier: the performance and stability of the amplifier is adjusted through the feedback network.

5. According to the amplifier technology:

(1) Discrete Amplifier: an amplifier built using discrete components such as transistors, diodes, etc.

(2) Integrated Circuit Amplifier (IC Amplifier): integrates the amplifier on a single chip, providing higher integration and convenience.