Basic hardware concepts-Analog Electronic Circuits

In electronic circuits, power supply, amplification, oscillation, and Modulation circuits are called Analog Electronic Circuits because they process and process continuously changing analog signals.

1. Feedback

Feedback refers to sending output changes to the input end in some way as part of the input. If the return part and the original input part are subtracted, It is negative feedback.

2. Coupling

An amplifier usually has several levels, and the association between levels is called coupling. There are three inter-stage coupling modes for the amplifier: ① RC coupling (see figure A): the advantage is simple and the cost is low. However, performance is not optimal. ② Transformer coupling (see figure B): the advantage is good impedance matching, high output power and high efficiency, but the transformer production is troublesome. ③ Direct Coupling (see Figure C): the advantage is the frequency bandwidth, which can be used as a dc amplifier. However, the front and back-level operations are restrained, and the stability is poor, which makes the design and production troublesome.

 

3. Power Amplifier

A power amplifier that can zoom in the input signal and provide enough power to the load. For example, the last amplifier of the radio is the power amplifier.

3.1 Class A single-tube Power Amplifier

The load resistance is a low-impedance speaker. Using a transformer can act as an impedance conversion, resulting in a high load power.
No matter whether the circuit has any input signal, the transistor is always in the conduction state, and the static current is relatively large, so the collector loss is large and the efficiency is not high, which is only about 35%. This kind of working state is called a kind of working state. This type of circuit is generally used in scenarios with low power. Its input mode can be transformer coupling or RC coupling.

3.2 Class B push-pull power amplifier

Is a commonly used Class B push-pull power amplifier circuit. It is a symmetric circuit consisting of two transistors with the same characteristics. When there is no input signal, each pipe is in the cutoff state, and the static current is almost zero. The pipe is turned on only when a signal is input, this status is called the Class B working status. When the input signal is a sine wave, vt1 is switched on to VT2 at half a week, And VT2 is switched on to vt1 at half a week. The alternating current of the two tubes is synthesized in the output transformer to generate a Pure sine wave on the load. This form of alternating two tubes is called a push-pull circuit.

 

3.3 OTL Power Amplifier

Currently, the Class-B transformer-free push-pull amplifier (OTL Circuit for short) is a power amplifier with good performance. For ease of description, first introduce an OTL Circuit with an input transformer without an output transformer, as shown in.

 

4. DC amplifier

A circuit capable of amplifying a DC signal or a signal that changes slowly is called a dc amplifier or a dc amplifier. This amplifier is often used in measurement and control.

4.1 dual-tube direct coupling Amplifier

DC amplifiers cannot use RC coupling or transformer coupling, but can only be directly coupled. Is a two-level direct coupling amplifier. The direct coupling method can cause mutual restraint between front and back-level work points. In the circuit, the emission pole of VT2 is added with the resistance r e to increase the rear-Level Emission pole potential to solve the issue.

Another more important issue of DC amplifier is zero point drift. The so-called zero-point drift means that when the amplifier does not have an input signal, the static Potential changes slowly due to unstable work points. This change is amplified step by step to generate false signals at the output end. The higher the amplifier level, the more severe the zero point drift. Therefore, this type of Dual-tube direct coupling amplifier can only be used in scenarios with low requirements.

4.2 differential amplifier

The difference amplifier is used to solve zero point drift. It is a widely used shot-pole coupled difference amplifier. It uses a dual-power supply, where vt1 and VT2 share the same characteristics, the two groups of resistance values are the same, and r e has a negative feedback effect. In fact, this is a bridge circuit. Two r c and two tubes are four bridge arm, and the output voltage V 0 is extracted from the diagonal line of the bridge. When there is no input signal, because RC1 = RC2 has the same characteristics as the two tubes, the bridge is balanced and the output is zero. Because it is connected to a bridge, zero point drift is also very small. The difference amplifier has good stability, so it is widely used.

5. Integrated Operational Amplifier

An integrated operational amplifier is a kind of device that makes a multilevel dc amplifier on an integrated chip and can complete various functions as long as a small number of components are connected externally. Because it was used in the early days as a multiplier and multiplier in a simulated computer, it is called an operational amplifier.

 

6. Oscillator

A circuit that automatically converts DC power into an AC signal with a certain amplitude and a certain frequency is called an oscillating circuit or an oscillator. This phenomenon is also called self-oscillation. Or the circuit that can generate an AC signal is called an oscillating circuit.

An oscillator must consist of three parts: an amplifier, a positive feedback circuit, and a frequency selection network. The amplifier can enlarge the input signal added to the oscillator input to keep the output signal constant. The positive feedback circuit ensures that the feedback signal provided to the oscillator input is of the same phase. Only in this way can the oscillation be maintained. The frequency selection network only allows a specific frequency f0 to pass, so that the oscillator generates a single frequency output.

Whether the oscillator can oscillate and maintain stable output is determined by the following two conditions. One is that the feedback voltage UF is equal to the input voltage UI, which is the amplitude balancing condition. Second
UF and UI must have the same phase, which is a phase equilibrium condition, that is, positive feedback must be ensured. Generally, amplitude balancing conditions are easy to achieve. Therefore, to determine whether an oscillating circuit can oscillate, it mainly depends on whether its phase equilibrium conditions are true.

The oscillator can be divided into ultra-low frequency (less than 20Hz) and low frequency (20Hz ~ 200 kHz), high frequency (200 kHz ~
30 MHz) and ultra high frequency (10 MHz ~ 350 MHz. It can be divided into two types: Sine Wave Oscillation and non-sine wave oscillation.

The sine wave oscillator can be divided
LC oscillator, RC oscillator and Z crystal oscillator. The Z crystal oscillator has a high frequency stability and is only used in scenarios with high requirements. In general household appliances, various LC and RC oscillator are widely used.

6.1 LC Oscillator

The Frequency Selection Network of the LC oscillator is the LC resonance circuit. Their oscillation frequency is relatively high, and there are three common circuits.

1) Transformer feedback LC oscillating circuit

 

Figure (a) shows the transformer feedback LC oscillating circuit. Transistor VT is a co-emission pole amplifier. The primary component of the transformer T is the LC resonance circuit for selecting frequency. The sub-component of the transformer t provides positive feedback signal to the amplifier input. When the power is switched on, the LC circuit has a weak transient current, but only the current with the same frequency and the circuit resonance frequency F 0 can generate a high voltage at both ends of the circuit, this voltage is sent back to the base pole of the transistor V through the coupling of L1 and L2 at the beginning of the transformer. As shown in figure (B), as long as the method is correct, the voltage of the feedback signal is the same as the voltage phase of the input signal, that is, it is positive feedback. Therefore, the circuit oscillates rapidly and finally becomes stable.
The LC oscillator circuit provided by transformer feedback is characterized by wide frequency range and easy to vibrate, but the frequency stability is not high. Its oscillation frequency is: F 0 = 1/2 π LC. It is often used to generate a sine wave signal from tens of thousands to dozens of megahertz.
2) inductor three-point oscillator circuit

Figure (a) is another common inductance three-point oscillator circuit. In the figure, the inductance L1, L2, and capacitor C constitute a frequency-selecting resonance circuit. Extract the feedback voltage from L2 to the base pole of the transistor vt. As shown in figure (B), the input voltage and feedback voltage of the transistor are in the same phase and meet the phase equilibrium conditions, so the circuit can start to vibrate. Because the three poles of the transistor are connected to three points of the inductance, they are called the inductance three-point oscillator circuit.
The inductance three-point oscillating circuit has the following characteristics: wide frequency range, easy to vibrate, but the output contains high frequency tuning, poor waveform. Its oscillation frequency is: F 0 = 1/2 π LC, where L = l1 + l2 + 2 m. It is usually used to generate a sine wave signal below dozens of megahertz.
3) capacitor three-point oscillating circuit

There is also a commonly used oscillating circuit, which is a capacitive three-point oscillating circuit, as shown in figure (). In the figure, the inductance L and the capacitor C1 and C2 constitute a frequency-selecting resonance circuit. The feedback voltage is obtained from the capacitor C2 and added to the base pole of the transistor vt. As shown in figure (B), the input voltage of the transistor is in the same phase as the feedback voltage, meeting the phase equilibrium conditions, so the circuit can start vibration. Because the three poles of the transistor are connected to the three points of the capacitor C1 and C2 respectively, they are called the capacitive three-point oscillator circuit.
The characteristic of the capacitive three-point oscillating circuit is that the frequency stability is high, the output waveform is good, and the frequency can be as high as 100 MHz or above, but the frequency adjustment range is small, so it is suitable for a fixed frequency oscillator. Its oscillation frequency is: F 0 = 1/2 π LC, where C = C 1 + C 2.
All the amplifiers in the above three kinds of oscillating circuits use the co-emission pole circuit. The oscillator with the co-emission pole method has a high gain and is easy to vibrate. The amplifier in the oscillating circuit can also be connected to a common base circuit. The resonant frequency of the common base electrode method is relatively high, and the frequency stability is good.

6.2 RC oscillator

The RC oscillator's frequency selection network is an RC Circuit, and their frequency of oscillation is relatively low. There are two commonly used circuits.
1) RC phase shifting oscillator circuit

RC Phase-Shifting oscillator circuit is characterized by simple and economical circuit, but its stability is not high and its adjustment is inconvenient. It is generally used as a fixed-frequency oscillator and has low requirements. Its oscillation frequency is: when the parameters of the three RC networks are the same: F 0 = 1 2 π 6rc. Generally, the frequency is several thousand.

2) RC bridge oscillator circuit

 

RC bridge oscillator circuit has better performance than RC phase shifting oscillator circuit. It features high stability, low nonlinear distortion, and easy frequency adjustment. Its oscillation frequency is: When R1 = R2 = r, C1 = C2 = C, F 0 = 1 2 π RC. Its frequency ranges from 1Hz ~ 1 MHz.

7. Amplitude Modulation and detection circuit
Radio communication uses modulation technology to add low-frequency sound signals to high-frequency signals for transmission. The restoration process in the receiver is called demodulation. Low-frequency signals are called modulation signals, while high-frequency signals are called carriers. There are two common methods of continuous wave modulation: Amplitude Modulation and frequency modulation. The corresponding demodulation method is called detection and frequency authentication.

7.1 am Circuit
Amplitude Modulation changes the amplitude of the carrier signal with the amplitude of the modulation signal, and the frequency and phase of the carrier remain unchanged. The circuit that can complete the amplitude modulation function is called the amplitude modulation circuit or the amplitude regulator.
Amplitude Modulation (AM) is a nonlinear frequency conversion process. Therefore, it must use diode, transistor, and other non-linear devices. According to the circuit in which the modulation process is implemented, the transistor amplitude modulation circuit can be divided into three types: collector amplitude modulation, baseline amplitude modulation, and emitter amplitude modulation. The following describes the collector Amplitude Modulation Circuit.

It is a collector amplitude modulation circuit. The Equal-width carrier generated by the high-frequency carrier oscillator is added to the Transistor Base through T1. Low-frequency modulation signals are coupled to the Collector through T3. C1, C2, and C3 are high-frequency bypass capacitors, while R1 and R2 are bias resistors. The LC parallel loop of the collector is resonant on the carrier frequency. If you click the static operation of the transistor on the curved part of the characteristic curve, the transistor is a non-linear device. Because the collector current of the transistor changes with the modulation voltage, the two signals in the collector realize the amplitude modulation due to non-linearity. Because the LC resonant circuit is tuned to the carrier's fundamental frequency
The second level of T2.

7.2 detection circuit
The function of the detection circuit or detector is to retrieve the low-frequency signal from the modulation wave. Its working process is the opposite of AM. The detection process is also a frequency conversion process, but also the use of non-linear components. Commonly used diode and transistor. In addition, in order to obtain useful low-frequency signals, a filter must be used to filter out the high-frequency components. Therefore, the detection circuit usually contains two parts: Nonlinear Components and filters. The following uses a diode detector as an example to describe how it works.

It is a diode detection circuit. VD is the detection component, and C and R are low-pass filters. When the input modulation signal is large, the diode VD works intermittently. During half a week, the diode is turned on and charged to C. When the negative half week is used and the input voltage is small, the diode ends and C is discharged to R. The voltage obtained at both ends of R contains a lot of frequency components, which are filtered by capacitor C to remove the high-frequency part and then subjected to the DC-separated capacitor C0, the restored low-frequency signal can be obtained at the output end.

8. Frequency Modulation and frequency authentication Circuit

Frequency Modulation changes the carrier frequency with the amplitude of the modulated signal, while the amplitude remains unchanged. The frequency authentication is to extract the original low-frequency signal from the frequency modulation wave, and its process is the opposite of the frequency modulation.
8.1 FM Circuit
A circuit that can complete the frequency modulation function is called a frequency regulator or frequency modulation circuit. The common frequency modulation method is direct frequency modulation, that is, the method that uses the modulation signal to directly change the frequency of the carrier oscillator. The general idea is shown. In the figure, a variable Impedance Component is used in parallel on the resonant circuit. Low-frequency modulation signals are used to control the changes in Variable-Frequency Impedance parameters so that the frequency of the carrier oscillator changes.

8.2 frequency authentication Circuit
The circuit that can complete the frequency authentication function is called the frequency divider or frequency authentication circuit, sometimes also called the frequency detector. The frequency authentication method is usually divided into two steps. The first step is to change the frequency modulation wave of the same amplitude to the frequency adjustment wave, and the second step is to use a general detector to detect the amplitude change, returns a low-frequency signal. Commonly used frequency validators include phase and proportional frequency validators.