The role and symbols of the components in the circuit diagram were introduced earlier. A circuit diagram usually has dozens or even hundreds of components, their lines cross each other horizontally and horizontally, in many different forms, beginners often do not know where to start, how to read it. In fact, the electronic circuit itself has a very strong regularity, no matter how complex the circuit, after analyzing the circuit can be found, it is composed of a few unit circuits. Like children playing with blocks, although only ten or twenty or thirty kinds of blocks, but in the hands of children can be built into dozens or even hundreds of plane graphics or three-dimensional model. By the same token, even more complex circuits, after analyzing the circuit can be found, it is also composed of a few unit circuits. Therefore, the beginner as long as the first familiar with the common basic unit circuit, and then learn to analyze and break down the circuit of the skills, to understand the general circuit diagram should not be difficult.
By the function of the unit circuit can be divided into a number of categories, each of which has a variety of all unit circuits, there are probably hundreds of them. Here we choose the most commonly used basic unit circuits to introduce. Let's start with the power supply circuit.
I, the function and composition of the power supply circuit
Every electronic device has a power supply circuit to supply energy. Power supply circuit has a rectifier power supply, inverter power supply and inverter three. Most of the common household appliances to use DC power supply. The simplest way to supply DC power is to use batteries. But batteries have the disadvantages of high cost, large size, need to be replaced from time to time (batteries need to be charged often), so the most economical, reliable and convenient is to use the rectifier power supply.
The power supply in electronic circuits is generally low-voltage DC, so in order to convert from 220-volt mains to DC, it should first turn 220-volt AC into low-voltage AC, and then turn it into pulsating DC with a rectifier circuit, and finally filter out the AC component of pulsating DC with a filter circuit in order to get DC. Some electronic devices require high quality of power supply, so it is sometimes necessary to add a voltage regulator circuit. Therefore, a rectifier power supply generally consists of four main parts, see Figure 1. One of the transformer circuit is actually an iron core transformer, need to introduce only the latter three unit circuits.
Two, rectifier circuit
Rectifier circuit is the use of semiconductor diode unidirectional conductivity of alternating current into unidirectional pulsating direct current circuit.
(1) half-wave rectifier
Half-wave rectifier circuit requires only one diode, see Figure 2 (a). In the positive half-cycle of the alternating current VD conductive, negative half-cycle VD cut-off, the load R is pulsating DC
(2) full-wave rectifier
Full-wave rectifier to use two diodes, and the transformer is required to have a center tap with the same number of turns of the two secondary coils, see Figure 2 (b). A pulsating full-wave rectified current is obtained on the load R L and the output voltage is higher than that of a half-wave rectifier circuit.
(3) Full-wave bridge rectifier
The bridge rectifier circuit with 4 diodes can be used with a transformer that has only a single secondary coil, see Fig. 2 (c). The current waveform and output voltage values at the load are the same as for a full-wave rectifier circuit.
(4) Multiplier Rectification
Higher DC voltages can be obtained with multiple diodes and capacitors. Figure 2 (d) shows a doubled voltage rectifier circuit. When U2 is negative half-cycle VD1 on, C1 is charged, C1 on the highest voltage can be close to 1.4U2; when U2 positive half-cycle VD2 on, C1 on the voltage and U2 superimposed on C2 charging, so that the voltage on the C2 is close to 2.8U2, C1 on the voltage is two times, so it is called doubling the voltage rectifier circuit.
Three, filter circuit
The rectifier is a pulsating DC, if you add a filter circuit to filter out the pulsating DC in the AC component, you can get a smooth DC.
(1) capacitor filter
The capacitor and the load are connected in parallel, as shown in Figure 3 (a), the capacitor is charged during the positive half-cycle, and the capacitor is discharged during the negative half-cycle, so that the load can get a smooth DC current.
(2) inductive filtering
Inductors and loads connected in series, such as Figure 3 (b), can also filter out the pulsating current in the AC component.
(3) L, C filter
A filter circuit consisting of an inductor and a capacitor is called L-type because it resembles an inverted letter "L", see Figure 3 (c). The filter circuit with 1 inductor and 2 capacitors is called π-type because it resembles the letter "π", see Figure 3 (d), which is a better filter circuit.
(4) RC filter
Inductors are costly and large, so resistors are often used to replace inductors in electronic circuits where the current is not too large and form RC filter circuits. Similarly, there are L-type, see Figure 3 (e), and π-type, see Figure 3 (f).
Fourth, the voltage regulator circuit
The fluctuation of the AC grid voltage and load current changes will make the rectifier power supply output voltage and current changes, so the higher requirements of electronic circuits must be used to regulate the power supply.
(1) Voltage regulator parallel voltage regulator circuit
A voltage regulator and load parallel circuit is the simplest voltage regulator circuit, see Figure 4 (a). R is the current limiting resistor. The output current of this circuit is very small and its output voltage is equal to the stabilized voltage value of the regulator, V Z .
(2) Series regulator circuit
The series regulator circuit with amplification and negative feedback is the most commonly used regulator circuit. Its circuit and block diagram are shown in Figure 4 (b), (c). It is from the sampling circuit (R3, R4) to detect changes in the output voltage, compared with the reference voltage (V Z) and amplified by the amplifier (VT2) added to the adjustment tube (VT1), so that the adjustment tube with the change in the voltage on both sides. If the output voltage decreases, the adjustment tube voltage drop is also reduced, so the output voltage is raised; if the output voltage rises, the adjustment tube voltage drop also rises, so the output voltage is lowered, and the result is that the output voltage is basically unchanged. On the basis of this circuit developed into a lot of variant circuits or increase some auxiliary circuits, such as composite tubes as a regulator, the output voltage adjustable circuits, operational amplifiers for comparative amplification circuits, as well as the increase in the auxiliary power supply and overcurrent protection circuits, and so on.
(3) switching type voltage regulator circuit
In recent years, the widely used new type of voltage regulator is switching type voltage regulator. Its regulator works in the switching state, its own power consumption is very small, so the efficiency is high, small size and other advantages, but the circuit is more complex.
Switching regulated power supply from the principle of a variety of points. Its basic principle block diagram is shown in Figure 4 (d). Figure inductance L and capacitance C is the energy storage and filtering components, diode VD is the adjustment tube in the off state for the L, C filter to provide current path of the continuity diode. Switching regulator switching frequency are very high, generally a few to tens of kilohertz, so the volume of the inductor is not very large, the output voltage in the high harmonics is not much.
Its basic principle of operation is: from the sampling circuit (R3, R4) to detect the sampling voltage by comparing the amplification to control a rectangular wave generator. The output pulse of the rectangular wave generator is to control the on and off time of the adjustment tube (VT). If the output voltage U 0 because of the grid voltage or load current changes and reduce the output pulse of the rectangular wave generator will be widened, so the adjustment tube conduction time increases, so that the L, C energy storage circuit to get more energy, the result is to make the output voltage U 0 was raised, to stabilize the output voltage of the purpose.
(4) integrated voltage regulator circuit
In recent years, there have been a large number of integrated voltage regulator products, many varieties, different structures. Currently used more three-terminal integrated voltage regulator, the output voltage of the CW7800 series and the output voltage of the negative CW7900 series and other products. Output current from 0.1A ~ 3A, output voltage 5V, 6V, 9V, 12V, 15V, 18V, 24V and so on.
This integrated voltage regulator has only three terminals, all parts of the voltage regulator circuit, including high-power regulator and protection circuitry have been integrated into the chip. When you use it, just add a heat sink and connect it to the back of the rectifier and filter circuit. With fewer peripheral components, higher precision voltage stabilization, and reliable operation, there is no need for debugging.
Figure 4 (e) is a three-terminal voltage regulator circuit. Figure C is the main filter capacitor, C1, C2 is to eliminate parasitic oscillation capacitance, VD is to prevent the input short-circuit burned the integrated block and the use of protection diodes.
V, power supply circuit reading points and examples
Power supply circuit is a relatively simple electronic circuit, but is the most widely used circuit. When you get a power supply circuit diagram, you should: ① First, according to the "rectifier - filter - regulator" order to break down the entire power supply circuit, level by level to analyze in detail. ② level by level analysis to distinguish between the main circuit and auxiliary circuits, major components and minor components, to clarify their role and parameter requirements. For example, switching regulated power supply, inductance and capacitance and current diode is its key components. ③ Because the transistor has two types of NPN and PNP type, some integrated circuits require dual power supply, so a power supply circuit often includes a different polarity of different voltage values and several groups of output. When reading the diagram, you must distinguish the value and polarity of the output voltage of each group. When assembling and repairing, the polarity of transistors and electrolytic capacitors should also be carefully separated to prevent errors. Familiarize yourself with some of the customary and simplified drawings. ⑤ Finally, synthesize the whole power supply circuit from front to back. This power supply circuit diagram is also read.
Example 1 Temperature Control Circuit for Electric Blanket
Figure 5 is an electric blanket circuit. The switch is in the "1" position for low temperature. The 220-volt utility power is connected to the electric blanket through the diode, because it is a half-wave rectifier, the electric blanket is added to the ends of the pulsating DC power of about 100 volts, the heat is not high, so it is a heat preservation or low-temperature state. Switch to the "2" position, 220 volts of utility power directly to the electric blanket, so it is a high-temperature gear.
Example 2 high-voltage electronic mosquito and fly killer
Figure 6 is the use of doubled voltage rectifier principle to get a small current DC high-voltage mosquito and fly killer. The 220-volt AC output voltage can reach 1100 volts after quadruple-voltage rectification, and this DC high voltage is added to the parallel wire mesh. Bait is placed under the wire mesh and when a fly stops on the wire, it causes a short circuit and the high voltage on the capacitor discharges through the fly's body and kills the fly. When the fly's body falls, the capacitor is recharged and the grid returns to high voltage. This high voltage grid current is very small, so it is harmless to people.
Since insects are phototropic at night, if you put a 3-watt fluorescent light or a small black light behind this grid, you can trap mosquitoes and harmful insects.
Example 3 Practical Regulated Power Supply
Figure 7 shows a practical regulated power supply. The output voltage is adjustable from 3 to 9 volts, and the maximum output current is 100 mA. This circuit is a series regulated power supply circuit. It is important to note that: ① The rectifier bridge is drawn differently from Fig. 2 (c), but it is actually a bridge rectifier circuit. ② This circuit uses a PNP germanium tube, so the output is negative voltage and the positive terminal is grounded. ③ Two ordinary diodes are used instead of a voltage regulator. The forward voltage drop of any diode is essentially constant, so a diode can be used instead of a voltage regulator. The forward voltage drop of a 2AP-type diode is about 0.3 volts, a 2CP-type is about 0.7 volts, and a 2CZ-type is about 1 volt. Two 2CZ diodes are used as reference voltages. The ④ sampling resistor is a potentiometer, so the output voltage is adjustable.
A circuit that amplifies a weak signal is called an amplifier circuit or amplifier. For example, the key component in a hearing aid is an amplifier.
Uses and Components of Amplifier Circuits
Amplifiers are AC amplifiers and DC amplifiers. AC amplifiers can be divided into low-frequency, medium-source and high-frequency according to the frequency; the strength of the output signal is divided into voltage amplification, power amplification and so on. In addition, there are integrated operational amplifiers and special transistors for device amplifiers. It is the most complex and variable electronic circuits. But beginners often encountered only a few of the more typical amplifier circuits.
Read the amplifier circuit diagrams are still in accordance with the "level by level decomposition, seize the key, detailed analysis, comprehensive synthesis" principle and steps. First of all, the entire amplifier circuit according to the input and output level by level, and then level by level to seize the key to analyze and get through the principle. Amplified circuit has its own characteristics: First, there are static and dynamic two states of operation, so sometimes it is often necessary to draw its direct flow path and AC path to analyze; Second, the circuit is often added with negative feedback, this feedback is sometimes in the level, sometimes from the back level feedback to the front level, so when analyzing the level, but also to be able to "look forward and backward! Therefore, when analyzing this level, you should also be able to "look forward and backward". After getting through the principle of each level, you can connect the whole circuit for a comprehensive synthesis.
Below we introduce a few common amplifier circuits.
Low-frequency voltage amplifier
Low-frequency voltage amplifier refers to the operating frequency of 20 Hz to 20 kHz between the output requirements of a certain voltage value, but does not require a very strong current amplifier.
(1) *** emitter amplifier circuit
Figure 1 (a) is *** emitter amplifier circuit. C1 is the input capacitance, C2 is the output capacitance, transistor VT is the amplifying device, RB is the base bias resistor, RC is the collector load resistance. Terminals 1 and 3 are inputs, and terminals 2 and 3 are outputs. Terminal 3 is the public **** point, usually grounded, also known as "ground". The static DC path is shown in Figure 1 (b), and the dynamic AC path is shown in Figure 1 (c). Circuit is characterized by voltage amplification from more than a dozen to more than a hundred, the phase of the output voltage and the input voltage is the opposite, the performance is not stable enough to be used for general occasions.
(2) voltage divider bias **** emitter amplifier circuit
Figure 2 than Figure 1, 3 more components. The base voltage is obtained by dividing the voltage between RB1 and RB2, so it is called voltage divider bias. In the emitter, a resistor RE and a capacitor CE are added, CE is called AC bypass capacitor, which is short-circuited to AC; RE has a DC negative feedback effect. Feedback means that changes in the output are somehow sent to the input as part of the input. If the sent back portion is subtracted from the original input portion, it is negative feedback. The real input voltage to the base in the diagram is the difference between the voltage on RB2 and the voltage on RE, so it is negative feedback. As a result of the above two measures to improve the stability of the circuit is the most widely used amplifier circuit.
(3) Emitter output
Figure 3 (a) is an emitter output. Its output voltage is from the emitter. Fig. 3 ( b ) is a diagram of its AC path and it can be seen that it is *** collector amplifier circuit.
The real input to the transistor in this diagram is the difference between V i and V o, so this is a circuit with deep AC negative feedback. Because of the deep negative feedback, this circuit is characterized by: voltage amplification is less than 1 and close to 1, the output voltage and the input voltage in phase, high input impedance and low output impedance, small distortion, wide bandwidth, stable operation. It is often used as an amplifier input stage, output stage or for impedance matching.
(4) low-frequency amplifier coupling
An amplifier usually has several stages, the link between the stage is called coupling. Amplifier interstage coupling there are three ways: ① RC coupling, see Figure 4 (a). The advantages are simplicity and low cost. But the performance is not the best. ② Transformer coupling, see Figure 4 (b). The advantages are good impedance matching, high output power and efficiency, but the transformer production is more difficult. ③ Direct coupling, see Figure 4 (c). Advantages are wide bandwidth, can be used as a DC amplifier, but before and after the work of the stage has a hold, poor stability, design and production of more trouble.
Power amplifier
The input signal can be amplified and provide sufficient power to the load of the amplifier called 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, T is the output transformer. Its collector load resistance Ri′ is the load resistance R L through the transformer turns ratio converted to:
RC′ = (N1 N2) 2 RL = N 2 RL
The load resistance is a low impedance speaker, with the transformer can play the role of impedance transformation, so that the load to get more power.
This circuit, regardless of whether there is no input signal, the transistor is always in the on state, the quiescent current is relatively large, trapped in this collector loss is large, the efficiency is not high, only about 35%. This operating state is called Class A operating state. This circuit is generally used in the power is not too large occasions, its input method can be transformer coupling can also be RC coupling.
(2) Class B Push-Pull Power Amplifier
Figure 6 shows a commonly used Class B push-pull power amplifier circuit. It consists of two transistors with the same characteristics of the symmetrical circuit, in the absence of an input signal, each tube is in the cut-off state, the quiescent current is almost zero, only when there is a signal input tube on, this state is known as the class B operating state. When the input signal is sinusoidal, VT1 conducts and VT2 cuts off during the positive half-cycle, and VT2 conducts and VT1 cuts off during the negative half-cycle. The alternating currents of the two tubes are synthesized in the output transformer to give a pure sine wave on the load. This form of alternating two tubes is called push-pull circuit.
Class B push-pull amplifier output power, distortion is small, high efficiency, generally up to 60%.
(3) OTL power amplifiers
Currently widely used transformerless class B push-pull amplifiers, referred to as OTL circuits, is a very good performance power amplifiers. To
easily illustrate, an OTL circuit with an input transformer and no output transformer is presented first, as in Figure 7.
This circuit uses two transistors with the same characteristics, and two sets of bias and emitter resistors with the same resistance values. At rest, the currents flowing through VT1 and VT2 are very small, and the capacitor C is charged with a DC voltage of 12 E c to ground. When there is an input signal, VT1 conducts and VT2 cuts off during the positive half-cycle, the collector current i c1 is directed as shown in the figure, and an amplified positive half-cycle output signal is obtained on the load RL. During the negative half-cycle, VT1 cuts off, VT2 conducts, the direction of collector current i c2 is 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 true OTL circuits without input transformers using triode phase inversion, complementary symmetrical OTL circuits composed of PNP tubes and NPN tubes, as well as the latest bridge push-pull power amplifiers, referred to as BTL circuits, etc.
This circuit has been used to amplify the negative half-cycle of the output signal of the DC power amplifier.
DC Amplifier
A circuit that amplifies DC signals or signals that change very slowly is called a DC amplifier circuit or DC amplifier. These amplifiers are commonly used in measurement and control.
(1) Two-tube direct-coupled amplifier
DC amplifiers can not be RC-coupled or transformer-coupled, but only direct-coupled. Figure 8 shows a two-stage direct-coupled amplifier. The direct-coupling method will bring about the front and rear stages of the operating point of each other, the circuit in the emitter of VT2 add resistance R E to increase the emitter potential of the rear stage to solve the front and rear stage of the tie. Another more important issue for DC amplifiers is zero drift. Zero drift refers to the slow change in static potential caused by the unstable operating point of the amplifier in the absence of an input signal, which is amplified stage by stage, resulting in a spurious signal at the output. The more amplifier stages, the more serious zero drift. Therefore, this double-tube direct-coupled amplifier can only be used for less demanding applications.
(2) differential amplifier
The solution to the zero drift is to use a differential amplifier, Figure 9 is a widely used emitter coupled differential amplifier. It uses a dual power supply, where the characteristics of VT1 and VT2 are the same, the two sets of resistors have the same value, R E has negative feedback. It is actually a bridge circuit, with the two R Cs and the two tubes being the four bridge arms, and the output voltage, V 0, is taken from the diagonal of the bridge. When there is no input signal, the bridge is balanced and the output is zero because RC1=RC2 and the two tubes have the same characteristics. Since it is connected as a bridge, the zero drift is also very small.
Differential amplifiers have good stability and are therefore widely used.
Integrated operational amplifiers
Integrated operational amplifiers are a multi-stage DC amplifiers made in an integrated chip, as long as a small number of external components can be connected to complete a variety of functions of the device. It is called an operational amplifier because it was used as an adder and multiplier in analog computers in the early days. It has more than ten pins, which are generally represented by a triangle symbol with three terminals, as shown in Fig. 10. It has two inputs, one output, the top of the input is called inverting input, with "-" as a mark; the following is called the same phase input, with "+" as a mark.
Integrated operational amplifiers can complete the addition, subtraction, multiplication, division, differentiation, integration and other analog operations, can also be connected to AC or DC amplifier applications. In the application of amplifiers:
(1) in-phase output amplifier circuit with zero adjustment
Figure 11 is an in-phase output op amp circuit with zero adjustment. Pins 1, 11, 12 is the zero end, adjust the RP can make the output terminal (8) in the static output voltage is zero. Pins 9 and 6 are connected to positive and negative power supply respectively. The input signal is connected to the in-phase input (5), so the output signal and the input signal are in phase. The negative feedback of the amplifier is connected to the inverting input (4) through the feedback resistor R2. The voltage amplification of in-phase input connection is always greater than 1.
(2) Inverted output op amp circuit
It is also possible to make the input signal from the inverted input, as shown in Figure 12. If the circuit requirements are not high, you can not need to zero, then you can short-circuit the three zero terminals.
The input signal is connected to the inverting input from the coupling capacitor C1 via R1, while the inverting input is connected to ground via resistor R3. The voltage amplification of the inverting input connection can be greater than 1, equal to 1 or less than 1.
(3) in-phase output high input impedance op amp circuit
Figure 13 does not have access to R1, the equivalent of R1 resistance to infinity, the circuit's voltage amplification is equal to 1, the input impedance of up to several hundred kilohms.
Amplified circuit reading points and examples
Amplified circuits are more varied and more complex circuits in electronic circuits. When you get an amplified circuit diagram, first of all, it should be broken down level by level, and then level by level to analyze and understand the principle, and then finally comprehensive synthesis. Read the diagram to pay attention to: ① in the level by level analysis to distinguish between the main components and auxiliary components. There are many auxiliary components used in the amplifier, such as temperature compensation components in the bias circuit, voltage and current regulator components, anti-vibration components to prevent self-excited oscillation, de- coupling components, and protection components in the protection circuit, etc. ② In the analysis of the most important and the most important components, it is necessary to distinguish between the main components and auxiliary components. ② in the analysis of the most important and difficult is the analysis of feedback, to be able to find the feedback path, to determine the polarity and type of feedback, especially multi-stage amplifiers, often after the negative feedback will be added to the front stage, so it is necessary to analyze in detail. ③ General low-frequency amplifiers commonly used RC coupling; high-frequency amplifiers are often associated with LC tuned circuits, either single-tuned or double-tuned circuits, and the capacitor capacity used in the circuit is generally smaller. ④ Pay attention to the polarity of the transistor and power supply, amplifiers often use a dual power supply, which is the specificity of the amplifier circuit.
Example 1 Hearing Aid Circuit
Figure 14 shows 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, while RC coupling is used between VT2 and VT3. In order to improve the sound quality, there is a parallel voltage negative feedback (R2 and R7) on this stage of VT1 and VT3. Since high impedance headphones are used, it is possible to connect the headphones directly into the collector circuit of VT4. R6 and C2 are decoupling circuits, and C6 is the power supply filter capacitor.
Example 2 Radio Low-Load Circuit
Figure 15 shows the low-load circuit of a popular radio. The circuit has 3 stages, the first stage (VT1) is a preamplifier, the second stage (VT2) is a driver stage, and the third stage (VT3, VT4) is a push-pull amplifier. VT1 and VT2 direct coupling, VT2 and VT3, VT4 between the input transformer (T1) coupling and phase inversion is completed, and finally the output transformer (T2) output, the use of low-resistance speakers. In addition, VT1 level has a parallel voltage negative feedback (R1), T2 secondary through R3 back to VT2 series voltage negative feedback. The role of C2 in the circuit is to enhance the negative feedback in the treble region, attenuate the treble to enhance the bass. R4, C4 for the decoupling circuit, C3 for the power supply filter capacitor. The entire circuit is simple and straightforward.
Oscillatory circuit use and oscillation conditions
No need for external signals can be automatically converted to DC energy with a certain amplitude and a certain frequency of the AC signal circuit is known as oscillatory circuits or oscillators. This phenomenon is also called self-excited oscillation. Alternatively, a circuit that can generate an AC signal is called an oscillating circuit.
An oscillator must include three parts: an amplifier, a positive feedback circuit, and a frequency selection network. The amplifier amplifies the input signal added at the input of the oscillator so that the output signal maintains a constant value. The positive feedback circuit ensures that the feedback signal to the input of the oscillator is in phase so that the oscillation can be maintained. The frequency selector network allows only a specific frequency f 0 to pass, so that the oscillator produces a single frequency output.
The ability of the oscillator to oscillate and maintain a stable output is determined by two conditions; one is the feedback voltage u f and the input voltage U i to be equal, which is the amplitude balance condition. Second, u f and u i must be the same phase, which is the phase balance condition, that is to say, must ensure that the positive feedback. In general, the amplitude balance condition is often easy to do, so in the judgment of an oscillator circuit can be oscillated, the main point is to see whether its phase balance condition is established.
The oscillator can be divided into ultra-low frequency (20 Hz or less), low-frequency (20 Hz ~ 200 kHz), high-frequency (200 kHz ~ 30 MHz) and ultra-high-frequency (10 MHz ~ 350 MHz) and so on. According to the oscillation waveform can be divided into sinusoidal oscillation and non-sinusoidal oscillation two categories.
Sine wave oscillators can be divided into LC oscillators, RC oscillators and quartz crystal oscillators according to the components used in the frequency selection network. Quartz crystal oscillator has a very high frequency stability, only in the high demand occasions. In general household appliances, a large number of various LC oscillators and RG oscillators.
LC oscillator
LC oscillator frequency selection network is the LC resonant circuit. They all oscillate at a high frequency, and there are three common circuits.
(1) transformer feedback LC oscillation circuit
Figure 1 (a) is a transformer feedback LC oscillation circuit. Transistor VT is *** emitter amplifier. The primary of transformer T is an LC resonant circuit for frequency selection, and the secondary of transformer T provides a positive feedback signal to the amplifier input. When the power supply is turned on, there is a weak transient current in the LC loop, but only a current with the same frequency as the resonant frequency of the loop, f 0, can generate a high voltage at both ends of the loop, which is coupled through the primary stage of the transformer, L1, L2, and then sent back to the base of the transistor V. As can be seen in Fig. 1 (b), as long as there are no errors in the connection, this feedback signal voltage is in phase with the input signal voltage, i.e., it is positive feedback. Therefore, the oscillation of the circuit is rapidly strengthened and finally stabilized.
Transformer feedback LC oscillator circuit is characterized by: a wide frequency range, easy to vibrate, but the frequency stability is not high. Its oscillation frequency is: f 0 = 1 / 2π LC. Commonly used to generate tens of kilohertz to tens of megahertz sine wave signals.
(2) Inductive three-point oscillation circuit
Figure 2 (a) is another commonly used inductive three-point oscillation circuit. In the figure, inductors L1, L2 and capacitor C form a resonant circuit for frequency selection. A feedback voltage is taken from L2 and added to the base of transistor VT. As can be seen in Fig. 2 (b), the input and feedback voltages of the transistor are in phase, which satisfies the phase balance condition, and the circuit can be resonated. Because the three poles of the transistor are connected to the three points of the inductor, so it is called inductive three-point oscillator circuit.
Inductive three-point oscillator circuit is characterized by: a wide frequency range, easy to vibrate, but the output contains more high frequency modulation waveforms, the waveform is poor. Its oscillation frequency is: f 0 =1/2π LC, where L = L1 + L2 + 2M. It is commonly used to generate sine wave signals below tens of megahertz.
(3) capacitive three-point oscillation circuit
There is also a commonly used oscillation circuit is a capacitive three-point oscillation circuit, see Figure 3 (a). Inductor L and capacitors C1 and C2 form a resonant circuit for frequency selection, and the feedback voltage from capacitor C2 is added to the base of transistor VT. As seen in Fig. 3 (b), the input voltage of the transistor and the feedback voltage are in phase, which satisfies the phase balance condition, and the circuit can be resonated. Because the circuit of the transistor's three poles are connected to the capacitor C1, C2 of the three points, so it is called capacitive three-point oscillator circuit.
Capacitive three-point oscillator circuit is characterized by: high frequency stability, good output waveforms, the frequency can be as high as 100 MHz or more, but the frequency adjustment range is small, so it is suitable for fixed-frequency oscillator. Its oscillation frequency is: f 0 =1/2π LC, where C= C 1 C 2 C 1 +C 2.
The amplifiers in the above three oscillator circuits are all *** emitter circuits. The oscillator with *** emitter connection has higher gain and is easy to oscillate. It is also possible to connect the amplifier in the oscillator circuit as a *** base circuit. The *** base connection of the oscillator oscillation frequency is higher, and the frequency stability is good.