01 What are the contents of Kirchhoff's theors?
Kirchhoff's Current Law: At any node in a circuit, the algebraic sum of the currents flowing into and out of that node is zero.
Kirchhoff's Voltage Law: In any closed circuit, the algebraic sum of the voltages is zero.
02 Thevenin's Theorem
A two-terminal circuit containing independent sources, linear resistors, and controlled sources can be equivalently represented by an ideal voltage source in series with its internal resistance for both terminals. The value of the ideal voltage source is the open-circuit voltage between the two terminals of the active two-terminal circuit, and the series internal resistance is the equivalent resistance between the two terminals when all internal independent sources are equal to zero.
03 Transistor Curve Characteristics
04 The concept and application of feedback circuits
Feedback, in an electronic system, is the process by which part or all of the output quantity (current or voltage) of an amplifier circuit is fed back to the input circuit through a certain form of feedback sampling network in a specific manner, thereby influencing the input quantity of the amplifier circuit. Types of feedback include: voltage series negative feedback, current series negative feedback, voltage parallel negative feedback, and current parallel negative feedback.
Negative feedback has four effects on amplifier performance: improving the stability of the amplification factor, as the amplification factor will change due to changes in external conditions (T℃, Vcc, device aging, etc.), and the smaller the relative change, the higher the stability; reducing nonlinear distortion and noise; changing the amplifier's input resistance Ri and output resistance Ro; and effectively extending the amplifier's bandwidth.
The characteristic of voltage negative feedback is that the output voltage of the circuit tends to remain constant.
The general principles for introducing negative feedback are: DC negative feedback should be introduced to stabilize the static operating point of the amplifier circuit; AC negative feedback (polarity in the mid-frequency range) should be introduced to improve the dynamic performance of the amplifier circuit.
When the signal source has a low internal resistance or when it is required to increase the input resistance of the amplifier circuit, series negative feedback should be introduced; when the signal source has a high internal resistance or when it is required to reduce the input resistance, feedback should be introduced and connected.
The choice between voltage and current negative feedback depends on the load's requirements for the amplifier's output voltage or resistance. If the load requires a stable signal voltage or low output resistance, voltage negative feedback should be introduced; if the load requires a stable signal current or high output resistance, current negative feedback should be introduced.
When signal conversion is required, the appropriate configuration should be selected based on the function of the four types of negative feedback amplifier circuits. For example, when current-to-voltage signal conversion is required, voltage parallel negative feedback should be introduced into the amplifier circuit.
05. Difference between active and passive filters
Passive filter: This type of circuit is mainly composed of passive components R, L and C.
Active filters are composed of integrated operational amplifiers and resistors and capacitors (R and C), and have advantages such as no inductor required, small size, and light weight.
Integrated operational amplifiers (op-amps) have high open-loop voltage gain and input impedance, and low output resistance. When used in active filter circuits, they also provide voltage amplification and buffering. However, the bandwidth of integrated op-amps is limited, so it is difficult to make the operating frequency of current active filter circuits very high.
06 Differential mode signal and common mode signal
Two signals of equal magnitude but opposite polarity are called differential signals. When a differential amplifier circuit receives a differential signal (uil = -ui2), it is called a differential input.
Two signals of equal magnitude and polarity are called common-mode signals. When a differential amplifier circuit receives a common-mode signal (uil = ui2), it is called a common-mode input.
In a differential amplifier, the useful signal is input in differential mode and the interference signal is input in common mode, so the interference signal will be suppressed to a very small extent.
07 Comparison of Field-Effect Transistors and Transistors
Transistor: The full name should be semiconductor triode, also known as bipolar transistor or crystal triode. It is a semiconductor device that controls current. Its function is to amplify weak signals into electrical signals with larger amplitude values. It is also used as a contactless switch.
A field-effect transistor (FET) is a transistor that is not simply called a field-effect transistor. There are two main types: junction field-effect transistors (JFETs) and metal-oxide-semiconductor field-effect transistors (MOSFETs).
In environments with significant variations, field-effect transistors (FETs) are more suitable. FETs are commonly used as preamplifiers to improve the input impedance of instruments and reduce noise. FETs have lower amplification capability than transistors; however, they are simple to manufacture, occupy less chip area, and are suitable for large-scale integrated circuits. They are also more widely used in pulse digital circuits.
08 The Composition Principles of Basic Amplifier Circuits
• The emitter junction is forward biased, and the collector junction is reverse biased.
• The input circuit should be connected in a way that allows the input signal to be applied to the amplifier input terminal with minimal loss.
• The output circuit should be connected in a way that allows the output signal to be transmitted to the load as much as possible.
09 Conditions for achieving amplification
• The transistor must be biased in the amplification region. The emitter junction is forward biased, and the collector junction is reverse biased.
• Set the static operating point correctly so that the entire waveform is in the amplification region.
• The input circuit converts the changing voltage into a changing base current.
• The output circuit converts the changing collector current into a changing collector voltage, and after capacitor filtering, only an AC signal is output.
10 power amplifier requirements
• Maximize output power.
High efficiency.
• Low nonlinear distortion.
• Transistor heat dissipation and protection.
11 Frequency Compensation
This refers to increasing or decreasing the strength of a signal at a specific frequency to compensate for the weakening or strengthening of that frequency during signal processing. Common methods include negative feedback compensation, emitter capacitor compensation, and inductor compensation.
Frequency compensation of 12 amplifier circuits
The purpose of frequency compensation in amplifier circuits is twofold: first, to improve the high-frequency characteristics of the amplifier circuit; and second, to overcome the self-oscillation phenomenon that may occur due to the introduction of negative feedback, thus enabling the amplifier to operate stably. In amplifier circuits, the presence of transistor junction capacitance often leads to an unsatisfactory high-frequency response. To solve this problem, a common method is to introduce negative feedback into the circuit. However, the introduction of negative feedback introduces a new problem: self-oscillation can occur in the negative feedback circuit. Therefore, to ensure the normal and stable operation of the amplifier circuit, frequency compensation is necessary. Frequency compensation methods can be divided into lead compensation and lag compensation. These methods primarily involve using resistive and capacitive components to alter the phase-frequency characteristics of the amplifier circuit's open-loop gain in the high-frequency range. Currently, the most commonly used method is the phase-locked loop (PLL).
13 Basic Amplifier Circuits
The function of an amplifier circuit: An amplifier circuit is one of the most widely used circuits in electronic technology. Its function is to amplify weak input signals (voltage, current, power) to the value required by the load without distortion.
Types of amplifier circuits:
Voltage amplifier: When the input signal is very small, a large output voltage without distortion is required. It is also called a small-signal amplifier.
Power amplifier: When the input signal is large, the amplifier is required to output sufficient power, also known as a large-signal amplifier.
A differential circuit is a circuit that takes two signals as inputs, the difference between which is the effective input signal. The output is an amplification of this difference. Imagine a scenario where interference signals cause the same interference to both input signals. By calculating the difference between the two signals, the effective input of the interference signal becomes zero, thus achieving common-mode interference suppression.
14 Class A, Class B, and Class AB power amplifiers
15. Block diagram of a phase-locked loop and a brief description of its principle.
Phase-locked loop (PLL): Locks the phase and fixes the frequency at a fixed value.
Phase-locked loop: A circuit that locks the phase.
The components of a phase-locked loop (PLL) include: a phase detector (PD), a frequency divider, a loop filter (LPF), and a voltage-controlled oscillator (VCO).
The working principle of a phase-locked loop:
• The output of the voltage-controlled oscillator is acquired and frequency-divided;
• The reference signal is input to the phase detector simultaneously;
The phase detector compares the frequency difference between the two signals and then outputs a DC pulse voltage.
• Control the VCO to change its frequency;
After a very short time, the output of the VCO will stabilize at a certain expected value.
Reference signal
The phase detector is a phase comparison circuit that compares the phase of the input reference signal and the output signal of the VCO, and outputs an error signal representing the phase difference. After passing through a loop filter, the harmonic and noise components in the error signal are filtered out, and the error voltage is used to control the VCO, so that the frequency of the voltage-controlled oscillator changes in the direction of reducing the frequency difference and phase difference between the two signals, and finally makes the frequency of the VCO output signal equal to the frequency of the reference signal.
16 Zero Drift
This refers to the situation where, when the input terminal of an amplifier circuit is short-circuited, a slowly changing voltage is still generated at the output terminal, meaning that the output voltage deviates from its original starting point and fluctuates up and down.
Methods to suppress zero-point drift generally include:
• Use constant temperature measures;
• Compensation method: This method uses a thermistor to offset the changes in the amplifier tube or uses amplifier tubes with the same characteristics to construct a differential amplifier circuit.
• DC negative feedback is used to stabilize the static operating point;
• Use RC coupling or specially designed modulation/demodulation DC amplifiers between stages.
17 Frequency Response
Also commonly referred to as frequency response, frequency characteristics are a technical indicator that measures the ability of an amplifier circuit to adapt to input signals of different frequencies. In an amplifier circuit, due to the presence of reactive components (such as capacitors and inductors) and the inter-electrode capacitance of transistors, the amplification factor of the amplifier circuit will decrease when the frequency of the input signal is too low or too high, and phase lead or lag will also occur. In other words, the amplification factor (or gain) of an amplifier circuit and the frequency of the input signal have a functional relationship, which we call the frequency response or frequency characteristics of the amplifier circuit. Essentially, frequency response refers to the relationship between the amplifier's gain and frequency. Generally speaking, a good amplifier should not only have sufficient amplification factor but also good fidelity performance. That is, the amplifier should have low nonlinear distortion and a good frequency response. "Good" means that the amplifier should amplify signals of different frequencies equally.
The reasons for frequency response are twofold: first, the frequency of the signal actually amplified is not singular; second, the amplifier has reactive components and reactive factors. Due to the presence of reactive components in the amplifier circuit (such as the inter-electrode capacitance of the transistor, the load capacitance, distributed capacitance, coupling capacitance, emitter bypass capacitance, etc.), the amplifier may amplify different frequency signal components with different factors and phase shifts. If the amplifier circuit amplifies different frequencies of signals differently, it will cause amplitude distortion; if the amplifier circuit produces different phase shifts for different frequencies of signals, it will cause phase distortion. Amplitude distortion and phase distortion are collectively referred to as frequency distortion. Since this distortion is caused by the linear reactive components of the circuit (resistors, capacitors, inductors, etc.), it is not called linear distortion. To achieve distortion-free signal amplification, it is necessary to study the frequency response of the amplifier. The frequency response of an amplifier circuit can be described by amplitude-frequency characteristic curves and phase-frequency characteristic curves. If the amplitude-frequency characteristic curve of an amplifier circuit is a straight line parallel to the x-axis (or parallel to the x-axis within the frequency range of interest), then the frequency response is considered to be positively correlated. If the phase frequency response curve is a straight line passing through the origin, or a straight line passing through the origin within the frequency range of interest, then the frequency response is stable.
The main methods for changing the frequency response are:
• Change the component parameters of the amplifier circuit;
• Introduce new components to improve the frequency response of existing amplifier circuits;
• Connect a new amplifier circuit in series with the existing amplifier circuit to form a multi-stage amplifier circuit.
18 transistors are operating in the amplification region
Why is an AGC circuit added to the 19 receiver?
• The received signal has varying strengths and large differences. Without AGC, the output will fluctuate greatly, affecting the performance.
• In order to receive weak signals, the amplifier of the receiver is always made large, that is, the sensitivity is high. However, when receiving strong signals, if the amplifier of the channel is not adjusted, it will have adverse consequences.
20LC Sine Wave Oscillator
For articles on inductive three-point oscillators and capacitive three-point oscillators, please refer to: Easy to Understand! Animated Explanation of the Working Principle of LC Oscillators.
Phase compensation using 21 differential op-amps
As the operating frequency increases, the amplifier will generate an additional phase shift, which may turn negative feedback into positive feedback and cause self-oscillation. Phase compensation can eliminate high-frequency self-oscillation. The principle of phase compensation is: in the intermediate stage with high amplification, a small capacitor C (tens to hundreds of picofarads) is used to form a voltage parallel negative feedback circuit. Capacitor correction and RC correction can be used to modify the phase frequency characteristics and amplitude frequency characteristics, respectively.
22. Find the common-mode and differential-mode components of a differential circuit.
Let the common-mode component be Yc and the differential-mode component be Yd, then the output is: Y+=Yc+Yd
In an amplifier circuit, a high input resistance is generally desirable because it minimizes the impact on the signal source. Looking at the amplifier circuit from its output, it can be considered an equivalent signal source with a certain internal resistance, which is the output resistance. Ideally, this output resistance should be as low as possible to improve the amplifier's load-driving capability.
23 DC Regulated Power Supply Principle
Function: Converts AC voltage into a stable DC voltage of appropriate magnitude.
Power transformer: Converts the AC mains voltage u1 into a suitable AC voltage u2.
Rectifier circuit: Converts AC voltage u2 into pulsating DC voltage u3.
Filter circuit: Converts the pulsating DC voltage u3 into a smooth DC voltage u4.
Voltage regulator circuit: Eliminates the effects of power grid fluctuations and load changes, maintaining a stable output voltage uo.
24. Composition of Integrated Operational Amplifier Circuit
Bias circuit: Sets a suitable static operating point for each stage of the amplifier circuit, and often uses a constant current source circuit.
Input stage: Usually a differential amplifier circuit, requiring large Ri, large Ad, and small Ac, high input voltage withstand, and has two input terminals: non-inverting and inverting.
Intermediate stage: The main amplification stage is often a common-emitter amplifier circuit, which often uses composite transistors and requires sufficient amplification capability.
Output stage: Power stage, often employing complementary power amplifier circuits or emitter followers, requiring low Ro, maximum undistorted output, and the largest possible output voltage. 26. Active filters: First-order active low-pass filter and first-order active high-pass filter.
Composition and working principle of 25RC oscillator
Composition of a sinusoidal oscillator circuit:
• Amplifier circuit: Amplifies the signal
• Feedback network: It must be positive feedback, and the feedback signal is the input signal of the amplifier circuit.
• Frequency-selective network: Ensures the output is a sine wave of a single frequency, even if the circuit only meets its oscillation condition at a specific frequency.
• Amplitude stabilization stage: Enables the circuit to transition from ½AuF½ > 1 to ½AuF½ = 1, thereby achieving stable amplitude oscillation.
