How to read amplification circuit diagram

A circuit that can amplify weak signals is called an amplifier circuit or amplifier. For example, a key component in a hearing aid is an amplifier.

The purpose and composition of amplifier circuit

Amplifiers include AC amplifiers and DC amplifiers. AC amplifiers can be divided into low frequency, medium source and high frequency according to frequency; the output signal strength can be divided into voltage amplification, power amplification, etc. In addition, there are amplifiers that use integrated operational amplifiers and special transistors as components. It is the most complex and varied circuit among electronic circuits. However, beginners often encounter only a few typical amplifier circuits.

When reading the amplified circuit diagram, we still follow the principles and steps of "decompose step by step, grasp the key, analyze in detail, and comprehensively synthesize". First, separate the entire amplifier circuit step by step according to input and output, and then grasp the key points step by step to analyze and understand the principle. The amplifier circuit has its own characteristics: first, it has two working states: static and dynamic, so sometimes it is necessary to draw its DC path and AC path for analysis; second, the circuit often has negative feedback, and this feedback is sometimes Within this level, sometimes there is feedback from the subsequent level to the previous level, so when analyzing this level, you must be able to "look forward and backward". After understanding the principles of each level, the entire circuit can be connected together for comprehensive synthesis.

Below we introduce several common amplifier circuits:

low frequency voltage amplifier

Low-frequency voltage amplifier refers to an amplifier whose operating frequency is between 20 Hz and 20 kHz, and whose output requires a certain voltage value but does not require a strong current.

(1) Common emitter amplifier circuit

Figure 1 (a) is a common emitter amplifier circuit. C1 is the input capacitor, C2 is the output capacitor, the transistor VT is the amplifying device, RB is the base bias resistor, and RC is the collector load resistor. Terminals 1 and 3 are inputs, terminals 2 and 3 are outputs. Terminal 3 is the common point, usually grounded, also called the "ground" terminal. The static DC path is shown in Figure 1 (b), and the dynamic AC path is shown in Figure 1 (c). The characteristics of the circuit are that the voltage amplification factor ranges from a dozen to more than a hundred, the phase of the output voltage is opposite to the input voltage, and the performance is not stable enough, so it can be used in general situations.

(2) Voltage-divided bias common-emitter amplifier circuit

Figure 2 uses three more components than Figure 1. The base voltage is obtained by dividing the voltage between RB1 and RB2, so it is called voltage division bias. Add resistor RE and capacitor CE to the emitter. CE is called AC bypass capacitor, which is short-circuited to AC; RE has a DC negative feedback effect. The so-called feedback refers to sending changes in the output to the input end in some way as part of the input. If the return part is subtracted from the original input part, it is negative feedback. The real input voltage of the base in the figure is the difference between the voltage on RB2 and the voltage on RE, so it is negative feedback. Due to the above two measures, the circuit's working stability and performance are improved, and it is the most widely used amplifier circuit.

(3) Emitter output device

Figure 3 (a) is an emitter output device. Its output voltage is output from the emitter. Figure 3 (b) is its AC path diagram. You can see that it is a common collector amplifier circuit.

In this figure, the real input of the transistor is the difference between V i and V o , so this is a circuit with deep AC negative feedback. Due to the deep negative feedback, the characteristics of this circuit are: the voltage amplification factor is less than 1 but close to 1, the output voltage is in phase with the input voltage, the input impedance is high and the output impedance is low, the distortion is small, the frequency is wide, and the operation is stable. It is often used as the input stage, output stage of amplifiers or for impedance matching.

(4) Coupling of low-frequency amplifier

An amplifier usually has several stages, and the connection between stages is called coupling. There are three inter-stage coupling methods of amplifiers: ①RC coupling, see Figure 4 (a). The advantages are simplicity and low cost. But the performance is not optimal. ② Transformer coupling, see Figure 4 (b). The advantages are good impedance matching, high output power and efficiency, but the transformer production is more troublesome. ③ Direct coupling, see Figure 4 (c). The advantage is that it has a wide frequency range and can be used as a DC amplifier, but the front and rear stages are restricted in their work, have poor stability, and are troublesome to design and manufacture.

power amplifier

An amplifier that can amplify the input signal and provide sufficient power to the load is called a power amplifier. For example, the final amplifier of a radio is a power amplifier.

(1) Class A single tube power amplifier

Figure 5 is a single-tube power amplifier, C1 is the input capacitor, and T is the output transformer. Its collector load resistance Ri′ is converted from the load resistance RL through the transformer turns ratio:

RC′= (N1 N2) 2 RL=N 2 RL

The load resistor is a low-impedance speaker, and a transformer can be used to transform the impedance so that the load can obtain greater power.

In this circuit, regardless of whether there is an input signal or not, the transistor is always on.

, the quiescent current is relatively large, the collector loss is large, and the efficiency is not high, only about 35%. This working status is called Category A working status. This kind of circuit is generally used in situations where the power is not too high, and its input method can be transformer coupling or RC coupling.

(2) Class B push-pull power amplifier

Figure 6 is a commonly used Class B push-pull power amplifier circuit. It consists of two transistors with the same characteristics to form a symmetrical circuit. When there is no input signal, each tube is in a cut-off state, and the quiescent current is almost zero. The tube is turned on only when there is a signal input. This state is called Class B working status. When the input signal is a sine wave, VT1 is on and VT2 is off during the positive half cycle, and VT2 is on and VT1 is off during the negative half cycle. The alternating currents of the two tubes are combined in the output transformer to obtain a pure sine wave on the load. This form of two tubes working alternately is called a push-pull circuit.

The output power of Class B push-pull amplifier is larger, the distortion is smaller, and the efficiency is higher, generally up to 60%.

(3) OTL power amplifier

The currently widely used transformerless Class B push-pull amplifier, referred to as OTL circuit, is a power amplifier with very good performance. for

For ease of explanation, let's first introduce an OTL circuit with an input transformer but no output transformer, as shown in Figure 7.

This circuit uses two transistors with identical characteristics, and the two sets of bias and emitter resistors have the same resistance. In the static state, the current flowing through VT1 and VT2 is very small, and the capacitor C is charged with a DC voltage of 1 2 E c to ground. When there is an input signal, VT1 is turned on during the positive half cycle, VT2 is turned off, the direction of the collector current i c1 is as shown in the figure, and the amplified positive half cycle output signal is obtained on the load RL. During the negative half cycle, VT1 is cut off, VT2 is turned on, the direction of the collector current i c2 is as shown in the figure, and an amplified negative half cycle output signal is obtained on RL. The key component of this circuit is capacitor C, the voltage on it is equivalent to the supply voltage of VT2.

Based on this circuit, there are also real OTL circuits that use transistors for inversion without input transformers, complementary symmetrical OTL circuits composed of PNP tubes and NPN tubes, and the latest bridge push-pull power amplifiers, referred to as BTL circuits, etc.

DC amplifier

A circuit that can amplify a DC signal or a signal that changes slowly is called a DC amplifier circuit or DC amplifier. This type of amplifier is commonly used in measurement and control applications.

(1)Two-tube direct-coupled amplifier

The DC amplifier cannot use RC coupling or transformer coupling, and can only use direct coupling. Figure 8 is a two-stage direct-coupled amplifier. The direct coupling method will cause mutual restraint of the working points of the front and rear stages. In the circuit, a resistor RE is added to the emitter of VT2 to increase the potential of the emitter of the latter stage to solve the restraint of the front and rear stages. Another more important problem with DC amplifiers is zero drift. The so-called zero point drift means that when there is no input signal to the amplifier, the static potential changes slowly due to unstable operating point. This change is amplified step by step, causing false signals to be generated at the output end. The more amplifier stages there are, the more severe the zero point drift will be. Therefore, this kind of two-tube direct-coupled amplifier can only be used in situations with low requirements.

(2) Differential amplifier

The solution to zero drift is to use a differential amplifier. Figure 9 shows a widely used emitter-coupled differential amplifier. It uses dual power supplies, in which VT1 and VT2 have the same characteristics, the two sets of resistor values ​​are also the same, and RE has a negative feedback effect. In fact, this is a bridge circuit. Two RCs and two tubes are the four bridge arms. The output voltage V 0 is taken from the diagonal line of the bridge. When there is no input signal, because RC1=RC2 and the characteristics of the two tubes are the same, the bridge is balanced and the output is zero. Because it is connected in a bridge shape, the zero point drift is also very small.

Differential amplifiers have good stability and are widely used.

Integrated operational amplifier

An integrated operational amplifier is a device in which a multi-stage DC amplifier is built on an integrated chip and can complete various functions as long as a small number of external components are connected. Because it was used as adders and multipliers in analog computers in the early days, it was called an operational amplifier. It has more than ten pins, generally represented by a triangle symbol with three terminals, as shown in Figure 10. It has two input terminals and one output terminal. The upper input terminal is called the inverting input terminal and is marked with "—"; the lower input terminal is called the non-inverting input terminal and is marked with "+".

Integrated operational amplifiers can complete various analog operations such as addition, subtraction, multiplication, division, differentiation, and integration, and can also be connected to AC or DC amplifier applications. When used as amplifier applications:

(1) Non-inverting output amplifier circuit with zero adjustment

Figure 11 is a non-inverting output op amp circuit with zero adjustment terminal. Pins 1, 11, and 12 are zero adjustment terminals. Adjusting RP can make the output terminal (8) output voltage zero when in static state. 9 and 6 are connected to the positive and negative power supplies respectively. The input signal is connected to the non-inverting input terminal (5), so the output signal is in phase with the input signal. The negative feedback of the amplifier is connected to the inverting input terminal (4) through the feedback resistor R2. The voltage amplification factor of non-inverting input connection is always greater than 1.

(2)Inverting output op amp circuit

The input signal can also be connected from the inverting input terminal, as shown in Figure 12. If the circuit requirements are not high, zero adjustment is not needed. In this case, the three zero adjustment terminals can be short-circuited.

The input signal is connected to the inverting input terminal from the coupling capacitor C1 via R1, while the non-inverting input terminal is connected to ground through resistor R3. The voltage amplification factor of the inverting input connection can be greater than 1, equal to 1, or less than 1.

(3) Non-inverting output high input impedance op amp circuit

In Figure 13, R1 is not connected, which means that the resistance of R1 is infinite. At this time, the voltage amplification factor of the circuit is equal to 1, and the input impedance can reach hundreds of kiloohms.

Key points and examples for reading diagrams of amplifier circuits

Amplification circuits are the most varied and complex circuits in electronic circuits. When you get an enlarged circuit diagram, you must first decompose it step by step, then analyze it step by step to understand its principles, and finally comprehensively synthesize it. When reading pictures, please pay attention to: ① Distinguish between main components and auxiliary components when analyzing step by step. There are many auxiliary components used in amplifiers, such as temperature compensation components in the bias circuit, voltage and current stabilizing components, anti-vibration components to prevent self-oscillation, decoupling components, protection components in the protection circuit, etc. ② The most important and difficult part in the analysis is the analysis of feedback. It is necessary to find the feedback path and judge the polarity and type of feedback. Especially for multi-stage amplifiers, negative feedback is often added to the previous stage from the later stage, so it must be more meticulous. analyze. ③ Generally, low-frequency amplifiers commonly use RC coupling; high-frequency amplifiers are often related to LC tuning circuits, either with single tuning or double tuning circuits, and the capacitor capacity used in the circuit is generally relatively small. ④ Pay attention to the polarity of the transistor and power supply. Dual power supplies are often used in amplifiers. This is the particularity of the amplifier circuit.

Example 1 Hearing aid circuit

Figure 14 is a hearing aid circuit, which is actually a 4-stage low-frequency amplifier. Direct coupling is used between VT1 and VT2 and between VT3 and VT4, and RC coupling is used between VT2 and VT3. In order to improve the sound quality, the current stages of VT1 and VT3 have parallel voltage negative feedback (R2 and R7). Since high-impedance headphones are used, the headphones can be directly connected to the collector circuit of VT4. R6 and C2 are decoupling circuits, and C6 is the power supply filter capacitor.

Example 2 Radio low amplifier circuit

Figure 15 is the low amplifier circuit of a popular radio. The circuit has a total of 3 stages. The first stage (VT1) is a pre-voltage amplifier, the second stage (VT2) is a push stage, and the third stage (VT3, VT4) is a push-pull power amplifier. Direct coupling is used between VT1 and VT2, input transformer (T1) is used between VT2, VT3, and VT4 to couple and phase invert, and finally the output transformer (T2) is used for output, using a low-impedance speaker. In addition, VT1 has parallel voltage negative feedback (R1) at its own stage, and T2 secondary is sent back to VT2 via R3 to have series voltage negative feedback. The function of C2 in the circuit is to enhance the negative feedback in the treble area and weaken the treble to enhance the bass. R4 and C4 are decoupling circuits, and C3 is the filter capacitor of the power supply. The whole circuit is simple and straightforward.