(Dry goods exchange) Eight basic circuits of analog circuits

In electronic circuits, power, amplification, generation, and modulation circuits are called analog electronic circuits because they process continuously changing analog signals.

01 Feedback

Feedback refers to sending changes in output to input as part of input. If returned part is subtracted from original input part, this is negative feedback.

02 Connection

An amplifier usually has multiple stages, and connection between stages is called coupling. There are three ways of interstage communication of an amplifier:

①RC connection (see fig. a): simplicity and low cost are advantages. But performance is not optimal. ② Transformer connection (see Fig. b): The advantages are good impedance matching, high output power and high efficiency, but production of transformers is more problematic. ③ Direct-coupling (see Fig. c): The advantage is that frequency band is wide, and it can be used as a DC amplifier, but operation of front and rear stages is limited, stability is poor, and design and production is troublesome.

03 power amplifier

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

Single tube 3.1 class A power amplifier

The load resistor is a low impedance speaker and a transformer can be used to convert impedance so that load can get more power.

In this circuit, whether there is an input signal or not, transistor is always on and quiescent current is relatively large, so collector loss is relatively large and efficiency is low, only about 35%. This operating state is called class A operating state. This type of circuit is usually used in cases where power is not too large, and its input mode can be transformer-coupled or RC-coupled.

3.2 Class B push-pull power amplifier

The figure below shows a commonly used class B push-pull power amplifier circuit.

It consists of two transistors with same characteristics, forming a symmetrical circuit. In absence of an input signal, each lamp is in off state, and quiescent current is practically zero. Only when there is an input signal The tube is on This state is called It is in class B operating state. When input signal is a sine wave, VT1 is on and VT2 is off during positive half cycle, and VT2 is on and VT1 is off during negative half cycle. The alternating currents of two lamps are synthesized in output transformer so that a pure sine wave is obtained at load. This form of two lamps operating alternately is called a push-pull circuit.

3.3 OTL Power Amplifier

The widely used class B transformerless push-pull amplifier, called OTL circuit, is a power amplifier with good performance. For ease of explanation, first imagine an OTL circuit with an input transformer and no output transformer, as shown in figure below.

04 DC Amplifier

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

4.1 Direct coupled two-tube amplifier

DC amplifiers cannot be RC or transformer coupled, only direct coupled. The figure below shows a two-stage feed-forward amplifier. The direct connection method will lead to mutual containment of operating points of front and rear stages. In circuit, a resistor R E is added to emitter VT2 to increase emitter potential of last stage to solve containment of front and rear stages.

Another important problem with DC amplifiers is zero drift. The so-called zero drift means that when amplifier has no input signal, static potential changes slowly due to operating point instability, and this change is stepped up, causing false signals at output. The more amplifier stages, more serious zero drift. Therefore, this two-tube feed-forward amplifier can only be used in low-demand applications.

4.2 Differential amplifier

The way to eliminate zero drift is to use a differential amplifier. The figure below shows a widely used emitter-coupled differential amplifier. It uses dual power supplies where characteristics of VT1 and VT2 are same and values ​​of two sets of resistors are also same, and R E has a negative feedback effect. In fact, this is a bridge circuit, two RC and two tubes are four arms of bridge, output voltage V 0 is taken from diagonal of bridge. When there is no input signal, since RC1=RC2 has same characteristics as two lamps, bridge is balanced and output is zero. Since it is connected in form of a bridge, zero point drift is also very small. Differential amplifiers have good stability, so they are widely used.

05 Built-in operational amplifier

An integrated operational amplifier is a device in which a multi-stage DC amplifier is integrated on an integrated circuit and can perform various functions when a small number of external components are connected to it. Because it was used as an adder and multiplier in an analog computer in early days, it is called an operational amplifier.

06 Oscillator

A circuit that can automatically convert DC power into an AC signal with a certain amplitude and frequency without an external signal is called an oscillator circuit or generator. This phenomenon is also called self-oscillations. In other words, a circuit that can generate an AC signal is called an oscillator circuit.

The generator should consist of three parts: an amplifier, a positive feedback circuit and a frequency selection circuit. The amplifier can amplify input signal applied to input terminal of generator so that output signal maintains a constant value. The positive feedback circuit ensures that feedback signals applied to input of generator are in same phase, only in this way can oscillation be maintained. The frequency selection network only allows a certain frequency f0 to pass through, so oscillator outputs only one frequency.

The ability of oscillator to oscillate and maintain a stable output signal is determined by following two conditions: first, feedback voltage Uf and input voltage Ui must be equal, which is amplitude balance condition. Secondly, Uf and Ui must have same phase, which is a phase balance condition, that is, positive feedback must be guaranteed. In general, amplitude balance condition is often easily achievable, therefore, when judging whether an oscillating circuit can oscillate, it mainly depends on whether condition for its phase balance is established.

The generators can be divided into ultra-low frequency (below 20 Hz), low frequency (20 Hz - 200 kHz), high frequency (200 kHz - 30 MHz) and ultra high frequency (10 - 350 MHz). according to frequency of oscillations. ) and so on. According to shape of oscillation wave, it can be divided into two types: oscillations of a sine wave and oscillations of a non-sinusoidal wave.

According to components used in frequency selection network, sine wave oscillator can be divided into three types: LC oscillator, RC oscillator and crystal oscillator. Crystal oscillators have high frequency stability and are used only in difficult cases. Various LC and RC generators are widely used in household appliances.

6.1 LC Oscillator

The frequency selection network of LC oscillator is an LC resonant circuit. Their oscillation frequencies are relatively high, and there are three general patterns.

1) LC oscillating feedback loop of transformer

Figure (a) is a diagram of an LC oscillator with transformer feedback. The transistor VT is a common emitter amplifier. The primary winding of transformer T is a frequency selective LC resonant circuit, and secondary winding of transformer T provides a positive feedback signal to input of amplifier. When power is turned on, a small transient current appears in LC circuit, but only a current with same frequency as resonant frequency of circuit f0 can generate a higher voltage at both ends of circuit, and this voltage associated with primary and secondary windings of transformer L1 and L2 returns to base of transistor V. From figure (b) it can be seen that as long as there is no error in connection, feedback signal voltage is in same phase as input signal voltage, that is, it is positive feedback. Therefore, oscillations of circuit rapidly increase and finally stabilize.

Characteristics of LC oscillation circuit with transformer feedback: wide frequency range, easy start of generation, but frequency stability is not high. The frequency of its oscillations: f 0 =1/2π LC. It is often used to generate sinusoidal signals ranging from tens of kilohertz to tens of megahertz.

2) Inductive three-point oscillatory circuit

Figure (a) is another commonly used inductive three-point generator circuit. In figure, inductance L1, L2 and capacitance C form a resonant circuit for frequency selection. The feedback voltage is removed from L2 and fed to base of transistor VT. From figure (b), it can be seen that input voltage and feedback voltage of transistor are in phase and satisfy phase balance condition, so circuit may begin to oscillate. Since three poles of transistor are respectively connected to three points of inductor, it is called an inductive three-point oscillator circuit.

The characteristics of inductive three-point oscillation circuit are: wide frequency range, easy start of generation, but output signal contains higher-order modulation, and waveform is poor. The frequency of its oscillations is equal to: f 0 =1/2π LC , where L=L1 + L2 + 2M . It is often used to generate sinusoidal signals below tens of megahertz.

3) Scheme of a capacitive three-point generator

Another commonly used oscillating circuit is a capacitive three-point oscillating circuit, as shown in figure (a). In figure, inductance L and capacitors C1 and C2 form a frequency-selective resonant circuit, and feedback voltage is removed from capacitor C2 and fed to base of transistor VT. From figure (b), it can be seen that input voltage of transistor and feedback voltage are in phase, satisfying phase balance condition, so circuit may begin to oscillate. Since three poles of transistor in circuit are respectively connected to three points of capacitors C1 and C2, it is called a capacitive three-point oscillatory circuit.

Characteristics of capacitor three-point oscillation circuit: high frequency stability, good output waveform, frequency can reach 100MHz or more, but frequency adjustment range is small, so it is suitable for fixed frequency oscillator. The frequency of its oscillations: f 0 \u003d 1/2π LC, where C \u003d C 1 + C 2.

All amplifiers in above three tank circuits use common-emitter circuits. A common-emitter oscillator has a higher gain and is easy to start lasing. It is also possible to connect amplifier via an oscillatory circuit to a common base circuit. The common base oscillator has a relatively high oscillation frequency and good frequency stability.

RC Generator 6.2

The frequency selection network of the RC oscillator is an RC circuit, and their oscillation frequency is relatively low. There are two widely used schemes.

1) RC oscillator circuit with phase shift

The characteristics of phase shift RC resonant circuit: circuit is simple and economical, but stability is not high, adjustment is inconvenient. It is usually used as a fixed frequency generator and in applications where requirements are not too high. The frequency of its oscillations is: with same parameters of a 3-section RC network: f 0 = 1 2π 6RC. The frequency is usually tens of kilohertz.

2) RC oscillator bridge

The performance of RC bridge oscillator circuit is better than that of RC phase oscillator circuit. It has high stability, small non-linear distortion and convenient frequency adjustment. The frequency of its oscillations: f 0 = 1 2πRC, when R1=R2=R and C1=C2=C. Its frequency range is from 1 Hz to 1 MHz.

07 AM and detector layout

Broadcast and radio communications are transmitted by adding low frequency audio signals to high frequency signals using modulation techniques. The recovery process at receiver is called demodulation. Among them, low frequency signal is called modulation signal, and high frequency signal is called carrier signal. Common continuous wave modulation methods include amplitude modulation and frequency modulation, and corresponding demodulation methods are called wave detection and frequency discrimination.

Scheme 7.1am

AM must cause amplitude of carrier signal to change with amplitude of modulating signal, while frequency and phase of carrier remain unchanged. A circuit that can perform an amplitude modulation function is called an amplitude modulation circuit or an amplitude modulator.

AM is a non-linear frequency conversion process, so its key is to use non-linear devices such as diodes and triodes. According to circuit in which modulation process is carried out, triode amplitude modulation circuit can be divided into three types: collector amplitude modulation, base amplitude modulation and emitter amplitude modulation. Let's take a collector amplitude modulation circuit as an example.

The figure above shows a collector amplitude modulation circuit, a carrier wave of equal amplitude generated by a high frequency carrier generator is added to base of transistor through T1. The low frequency modulation signal enters collector via T3. C1, C2, C3 - high-frequency bypass capacitors, R1, R2 - bias resistors. The LC collector parallel circuit resonates at carrier frequency. If static operating point of triode is chosen on curved part of characteristic curve, triode is a non-linear device. Since collector current of transistor depends on modulation voltage, two signals in collector achieve amplitude modulation due to non-linear effects. Since LC resonant circuit is tuned to fundamental frequency of carrier, an amplitude modulated wave output can be obtained in secondary of T2.

7.2 Detection scheme

The function of detection circuit or detector is to extract low frequency signal from AM wave. Its workflow is exact opposite of AM. The detection process is also a frequency conversion process, and non-linear components are also used. Diodes and triodes are commonly used. In addition, to isolate low-frequency useful signals, it is necessary to use filters to filter out high-frequency components, so detection circuit usually includes two parts: nonlinear components and filters. Let's take a diode detector as an example to illustrate how it works.

The picture above shows a schematic of a diode detector. VD - detection element, C and R - low-pass filters. When input modulated wave signal is large, diode VD works intermittently. During positive half-cycle, diode is on and charges C, when negative half-cycle and input voltage are low, diode turns off and C discharges R. The voltage received at both ends of resistor R contains many frequency components, high-frequency part is filtered by capacitor C, and then by blocking direct current effect of capacitor C0, which blocks direct current, a restored low-frequency signal is obtained at output.

08 FM and frequency discrimination circuit

Frequency modulation is to change carrier frequency depending on amplitude of modulating signal, while amplitude remains unchanged. Frequency discrimination consists in demodulating original low frequency signal from frequency modulation wave, and this process is in direct contrast to frequency modulation.

8.1 FM channel

A circuit that can perform a frequency modulation function is called a frequency modulator or frequency modulation circuit. The commonly used frequency modulation method is direct frequency modulation method, that is, method of directly changing frequency of carrier oscillator using a modulation signal. The figure below shows its general idea. In figure, a variable reactance element is connected in parallel with resonant circuit. The change in parameters of element of variable reactance is controlled by a low-frequency modulation signal, due to which frequency of carrier generator changes.

8.2 Frequency Discrimination Circuit

A circuit that can perform function of frequency discrimination is called a frequency discriminator or a frequency discrimination circuit, sometimes also called a frequency detector. The frequency discrimination method is usually divided into two stages. The first step is to convert constant amplitude FM wave into an FM-AM wave whose amplitude depends on frequency. The second step is to use a conventional detector to detect change in amplitude. and restore it to a low frequency signal. Commonly used discriminators include phase discriminators, ratio discriminators, etc.