Analysis of Difficulties in the Basis of Analog Electronic Technology

Analysis of several difficulties in bookmark0 in the foundation of analog electronic technology bookmark1 Yelin Fujian University of Engineering, Department of Electronic Information and Electrical Engineering 350108 The purpose of the oscillator is to generate oscillation, the intermediate frequency band is positive feedback, no additional phase shift is required to meet the phase balance conditions and The amplitude balance condition produces self-excited oscillation. If the oscillator produces an additional phase shift and reaches 180 °, it becomes a negative feedback amplifier and does not oscillate. Therefore, when the oscillator does not oscillate and the negative feedback amplifier generates self-oscillation, special attention should be paid to whether the circuit generates additional phase shift.

2 Judgment of the phase balance condition of the oscillator bookmark2 This article makes a more in-depth analysis of some difficulties in the foundation of analog electronic technology in order to better understand the basic concepts and principles of electronic technology and master the basic analysis methods of electronic technology.

Key solid analog electronic technology; difficulty analysis; feedback; phase balance condition analog electronic technology foundation is a professional basic course for electrical and electronic majors, and also an introductory course for learning electronic technology. There are some difficulties in the learning process that are not easy to understand. This article analyzes and discusses some difficult issues, hoping to help understand the basic concepts, basic principles and basic analysis methods of electronic technology. It lays a good theoretical foundation for eliminating electronic circuit faults in practical work.

1 Oscillator and negative feedback amplifier self-oscillation bookmark3 In electronic equipment, the oscillator and negative feedback amplifier are commonly used unit circuits, but the oscillator may not vibrate, and the negative feedback amplifier may produce self-oscillation. This is an undesirable circuit failure. The difficulty in analyzing this type of circuit failure is obvious. The working principle of the oscillator can be expressed by the box as shown in Figure 1. The net input signal is the feedback signal, i. For the output signal, Yi is the input signal. For the device, it is a feedback amplifier that satisfies the self-excited oscillator. When the feedback signal of the feedback amplifier is the same as the net input signal iv in amplitude and phase, even if the added signal oscillation conditions are different, the conditions of the circuit are: 4 = 0, I = I = 丨 Q, I, the amplitude is flat, where the gas is The phase shift of the basic amplifier circuit is the phase shift of the feedback network.

For the negative feedback device: ff = -then: Ar f = ~ V dichotomy, where A is the feedback coefficient.

It can be seen that for the negative feedback amplifier to oscillate, an additional phase shift of 180 ° must be generated to make the negative feedback positive.

For the oscillator: Af instead of Ai, it can be seen that there is no additional phase shift required for the oscillator to oscillate.

13 The purpose of introducing negative feedback in the key point amplifier is to improve the performance of the amplifier. Under normal operating conditions, it is not allowed to produce oscillation, so it is negative feedback in the middle frequency band. When the amplifier is at high frequency or low frequency, due to the influence of the reactive element, the additional phase shift becomes positive feedback, which causes self-oscillation, so it must be eliminated.

The method of oscillator oscillation must meet the conditions of amplitude balance and phase balance. Amplitude balance conditions can generally be met, the key is to know how to judge the phase balance conditions.

There are various kinds of oscillating circuits, and grasping the judgment method is a difficult problem. We must grasp the main points and conclude a simple, easy to understand, and easy to remember judgment method.

The method is to open the circuit by i, and introduce the input ", use the instantaneous B shed property method to see whether the reverse signal and the input signal)> are in phase, if in phase, it means that gas + 1 = ± meets its phase balance condition. "Loop" assumes that the feedback loop is broken and the amplifier is a loop amplifier.

Add an input signal, and the output terminal has an output signal r, t pan "a phase difference between L and L. Anti-feedback network junction, from the output terminal of the feedback network junction to get Qin, compare with)>. The phase difference ¢ .. If Qin is in phase with 1> 1, it means that gas + 1 = ± meets the phase piece. The more a key point is: determine the disconnection point. If the disconnection point is wrong, the correct junction transformer coupling LC oscillator cannot be obtained. The application is very extensive, and there are many types, but the judgment of the phase balance condition is also difficult.

Example 1 Figure 3 is a transformer-coupled common-shot LC oscillation continued on page 147

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