1、What is Kirchhoff’s theorem?
Kirchhoff’s current law: At any node of 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 in a circuit, the algebraic sum of the voltages is zero.
2, davinan theorem
A two-terminal circuit containing an independent source, a linear resistor and a controlled source, for both terminals can be equated to an ideal voltage source series internal resistance model. The value of the ideal voltage source is the open-circuit voltage of the two terminals of the active two-terminal circuit, and the internal resistance of the series connection is the equivalent resistance between the two terminals when all the internal independent sources are equal to zero.
3, triode curve characteristics
4、Feedback circuit
Feedback: is in the electronic system, the amplifier circuit in the output quantity (current or voltage) of part or all, through a certain form of feedback sampling network and in a certain way to the input circuit to affect the process of amplification circuit input quantity.
Types of feedback are: voltage series negative feedback, current series negative feedback, voltage parallel negative feedback, current parallel negative feedback.
Negative feedback has four effects on amplifier performance.
1) Improve the stability of the amplification, due to changes in external conditions (T℃, Vcc, device aging, etc.), the amplification will change, and the smaller the relative change, the higher the stability.
2)Reduce nonlinear distortion and noise.
3)Changes the input resistance Ri and output resistance Ro of the amplifier.
4)Effectively extends the passband of the amplifier.
Characteristics of negative voltage feedback: The output voltage of the circuit tends to remain constant.
The general principles for introducing negative feedback are.
1) In order to stabilize the static operating point of the amplifier circuit, DC negative feedback should be introduced; in order to improve the dynamic performance of the amplifier circuit, AC negative feedback (polarity in the mid-frequency band) should be introduced.
2) When the internal resistance of the signal source is small or the input resistance of the amplifier circuit is required to be increased, series negative feedback should be introduced; when the internal resistance of the signal source is large or the input resistance is required to be reduced, parallel feedback should be introduced.
3) According to the requirements of the load on the output power or output resistance of the amplifier circuit to decide whether to introduce voltage or current negative feedback. If the load requires a stable signal voltage or output resistance to be small, then negative voltage feedback should be introduced; if the load requires a stable signal current or output resistance to be large, then negative current feedback should be introduced.
4) In the need for signal conversion, the four types of negative feedback amplifier circuit should be selected according to the function of the appropriate grouping. For example, when the current – voltage signal conversion is required, voltage parallel negative feedback should be introduced in the amplifier circuit, etc.
5、The difference between active filter and passive filter
Passive filter: this circuit mainly consists of passive components R, L and C.
Active filter: integrated op-amp and R, C composition, with the advantages of no inductor, small size, light weight, etc..
The open-loop voltage gain and input impedance of integrated op-amp are high, and the output resistance is small, which also has certain voltage amplification and buffering effect after forming active filter circuit. However, the bandwidth of integrated op-amp is limited, so it is difficult to make the operating frequency of current active filter circuit very high.
6.Differential mode signal, common mode signal
Two pairs of signals of equal size and opposite polarity are called differential mode signals. When the differential amplifier circuit inputs a differential mode signal (uil =-ui2), it is called a differential mode input.
A pair of two signals of equal size and polarity is called a common mode signal. When a common-mode signal (uil = ui2) is input to the differential amplifier circuit, it is called a common-mode input.
In the differential amplifier, the useful signal is input in differential mode and the interference signal is input in common mode, then the interference signal will be suppressed very little.
7、Comparison between field effect and transistor
a. In the occasion of changing environmental conditions, it is more appropriate to use field effect tube.
b. Field-effect tubes are often used as preamplifiers to improve the input impedance of instruments and equipment, reduce noise, etc.
c. The amplification capability of FETs is lower than that of transistors.
d. The process is simple, occupies small chip area, and is suitable for large-scale integrated circuits. It is more widely used in pulse digital circuits.
8、The principle of basic amplification circuit composition
a. Emitter junction positive bias, collector junction reverse bias.
b. The input circuit should be connected in such a way that the input signal is loaded to the input of the amplifier without loss as much as possible.
c. The output loop should be connected in such a way that the output signal is transmitted to the load as much as possible.
9.Conditions for achieving amplification
a. The transistor must be biased in the amplification region. The emitter junction is positively biased and the collector junction is reverse biased.
b. The static operating point is set correctly so that the entire waveform is in the amplification region.
c. The input loop converts the varying voltage into a varying base current.
d. The output circuit converts the changing collector current into the changing collector voltage, and outputs only the AC signal through capacitor filtering.
10、Amplifier requirements
a. The output power is as large as possible.
b. High efficiency
c. Small non-linear distortion
d. Heat dissipation and protection of transistors
11、Frequency compensation
The so-called frequency compensation is to increase or decrease the strength of a particular frequency signal, which is used to compensate for the weakening or enhancement of the frequency generated during signal processing. Commonly used are negative feedback compensation, emitter capacitance compensation, inductance compensation.
What is the purpose of frequency compensation in an amplifier circuit? What are the methods?
The purpose of frequency compensation in an amplifier circuit is twofold.
One is to improve the high frequency characteristics of the amplifier circuit.
The second is to overcome the possible self-excited oscillation due to the introduction of negative feedback, so that the amplifier can work stably.
In amplification circuits, the presence of transistor junction capacitance often makes the frequency response of the amplifier circuit unsatisfactory in the high frequency band. Then, the introduction of negative feedback introduces a new problem, that is, the negative feedback circuit will appear self-excited oscillation phenomenon, so in order to make the amplifier circuit can be normal and stable operation, the amplifier circuit must be frequency compensation.
Frequency compensation can be divided into over-compensation and hysteresis compensation, mainly by accessing some resistive components to change the open-loop gain of the amplifier circuit in the high frequency band of the phase frequency characteristics, the most used at present is the phase-locked loop.
12、The basic amplifier circuit types
The role of amplification circuits.
Amplifying circuit is one of the widely used circuits in electronics, and its function is to amplify the weak input signal (voltage, current, power) to the value required by the load without distortion.
Types of amplification circuits.
(1) Voltage amplifier: The input signal is very small and requires a large output voltage without distortion, also known as a small signal amplifier;
(2) Power amplifier: the input signal is large, requiring the amplifier to output sufficient power, also known as large-signal amplifier.
A differential circuit is a circuit with such a function. The input of the circuit is the input of two signals, the difference between these two signals, the effective input signal of the circuit, the output of the circuit is the amplification of the difference between these two input signals. Imagine a scenario in which the presence of an interfering signal will produce the same interference to both input signals, and by the difference between the two, the effective input of the interfering signal is zero, which achieves the purpose of anti-common mode interference.
13, Class A, Class B, Class A and B complementary amplifiers
14, Class A amplifier, Class B complementary symmetrical amplifier and Class A and B complementary symmetrical amplifier features and circuit diagrams.
14、Phase-locked loop
Phase-locked: lock the phase to lock the frequency to a fixed value.
Phase-locked loop: a loop that locks the phase in place.
Phase-locked loop components: phase discriminator PD + frequency divider + loop filter LPF + voltage controlled oscillator VCO, etc.
The working principle of the phase-locked loop.
1) The output of the voltage-controlled oscillator is collected and divided into frequencies;
2) and the reference signal is fed to the phase discriminator at the same time;
3) The discriminator compares the frequency difference between the above two signals and outputs a DC pulse voltage;
4) control the VCO so that its frequency is changed;
5) so that after a short time, the output of VCO will be stabilized at a certain desired value.
Base Signal
The phase discriminator is a phase comparison circuit. The input reference signal and the VCO output signal are compared in phase, and an error signal representing the phase difference is output, which is filtered by a loop filter to remove the harmonics and spurious components from the error signal, and the error voltage is obtained 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. Finally, the output signal frequency of the VCO is equal to the frequency of the reference signal.
15、Zero drift
Zero drift, that is, when the input of the amplifier circuit is short-circuited, there is a slow change in voltage at the output, that is, the output voltage deviates from the original starting point and drift up and down.
The methods to suppress zero-point drift are generally
a. The use of thermostatic measures;
b. Compensation method (the use of thermal components to offset the changes in the amplifier tube or the use of the same characteristics of the amplifier tube to form a differential amplifier circuit);
c. The use of negative DC feedback to stabilize the static operating point;
d. The use of resistance-capacitance coupling between levels or the use of special design modulation demodulation DC amplifier.
16、Frequency response
Frequency response: usually also known as frequency characteristics, frequency response or frequency characteristics is a measure of the amplifier circuit to different frequency input signal adaptability of a technical index.
In the amplifier circuit, due to the presence of reactance components (such as capacitors, inductor coils, etc.) and transistor inter-pole capacitance, when the frequency of the input signal is too low or too high, the value of the amplification of the amplifier circuit will be reduced, and will also produce the phase overrun or lag phenomenon. In other words, the amplification of the amplifier circuit (or gain) and the input signal frequency is a function of the frequency response or frequency characteristics of the amplifier circuit.
In essence, the frequency response is the relationship between the gain and frequency of the amplifier. Generally speaking, a good amplifier should not only have sufficient amplification, but also have good fidelity, i.e., the non-linear distortion of the amplifier should be small, and the frequency response of the amplifier should be good. “Good” means that the amplifier should have equal amplification of signals of different frequencies.
The reason for the frequency response: First, the actual amplified signal frequency is not a single; second, the amplifier has reactance elements and reactance factors. Due to the presence of reactance elements in the amplifier circuit (such as inter-pole capacitance of the tube, load capacitance of the circuit, distribution capacitance, coupling capacitance, emitter bypass capacitance, etc.), the amplifier may have different amplification and phase shift for different frequency signal components. If the amplification circuit amplifies the signals of different frequencies differently, it will cause amplitude distortion; if the amplification circuit produces different phase shifts for signals of different frequencies, it will cause phase distortion. The amplitude distortion and phase distortion are called frequency distortion, but since this distortion is caused by the linear reactance elements of the circuit (resistance, capacitance, inductance, etc.), it is not called linear distortion. In order 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 the amplitude-frequency characteristic curve and the phase-frequency characteristic curve. 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 in the frequency range of interest), and the phase-frequency characteristic curve is a straight line through the origin (or a straight line through the origin in the frequency range of interest), then the frequency response is stable.
The main methods to change the frequency response are.
1) Change the component parameters of the amplifier circuit;
2) introducing new components to improve the frequency response of the existing amplifier circuit;
3) Connecting a new amplifier circuit in series with the existing amplifier circuit to form a multi-stage amplifier circuit.
17、Why should the receiver add AGC circuit?
1) The received signal has strong and weak changes, with large disparity, if not add AGC will make the output fluctuate, affecting the effect.
2) in order to be able to receive weak signals, the receiver amplification is always made larger, that is, high sensitivity, but the reception of strong signals, if the channel amplification is not regulated, will have adverse consequences.
18、LC sine wave oscillator
LC sine wave oscillator has inductive three-point oscillator and capacitive three-point oscillator
19、Differential op amp how to phase compensation
As the operating frequency increases, the amplifier will produce additional phase shift, which may turn the negative feedback into positive feedback and cause self-excitation. Phase compensation can eliminate high frequency self-excitation. The principle of phase compensation is that a small capacitor C (tens to hundreds of microfarads) is used to form a voltage shunt negative feedback circuit in the intermediate stage with high amplification. The phase frequency characteristics and amplitude frequency characteristics can be modified using capacitance correction and RC correction, respectively.
20, calculate the common mode component and differential mode component of the differential circuit
Given a differential circuit with known output voltages Y+ and Y-, find the common-mode component and the differential-mode component.
Let the common mode component is Yc, the differential mode component is Yd, then the output is known as
Y+=Yc+Yd
Y-=Yc-Yd
21、Resistance of the amplifier
Do you usually want the input resistance and output resistance of the amplifier to be higher or lower? Why?
In an amplifier circuit, it is usually desirable to have a high input resistance of the amplifier circuit because it will have less effect on the signal source. When looking at the output of the amplifier circuit, the amplifier circuit can be equated to a signal source with a certain internal resistance, and the internal resistance of the signal source is the output resistance, which is usually desired to be as small as possible because it can improve the amplifier’s ability to carry a load.
22, DC voltage regulator principle
Function: To turn the AC voltage into a stable DC voltage of the right size
Power transformer: changes the AC grid voltage u1 into a suitable AC voltage u2.
Rectifier circuit: Changes the AC voltage u2 into a pulsating DC voltage u3.
Filter circuit: transform the pulsating DC voltage u3 into a smooth DC voltage u4.
Voltage regulator circuit: remove the influence of grid fluctuations and load changes, to maintain the stability of the output voltage uo.
23、Components of integrated op amp circuit
Bias circuit: set the appropriate static operating point for all levels of the amplifier circuit. Mostly constant current source circuits are used.
Input stage: often a differential amplifier circuit. Requires large Ri, large Ad, small Ac, and high voltage withstand at the input. It has two inputs: in-phase and anti-phase.
Intermediate stage: The main amplifier stage, often a common-shot amplifier circuit, mostly using composite tubes. Requires sufficient amplification capacity.
Output stage: power stage, mostly using complementary amplifier circuit or emitter output device. Requirement Ro small, the maximum undistorted output voltage as large as possible.
24、Active filter
First-order active low-pass filter and first-order active high-pass filter.
25、The composition and working principle of RC oscillator
Sine wave oscillation circuit consists of.
1)Amplification circuit: Amplify the signal
2) feedback network: must be positive feedback, the feedback signal is the input signal of the amplifier circuit
3) Frequency selection network: ensure that the output is a single frequency sine wave even if the circuit only at a particular frequency to meet their own oscillation conditions
4) Amplitude stabilization link: enables the circuit to transition from ½ AuF½ > 1 to ½ AuF½ > 1.
