The reason of self-excited oscillation in negative feedback amplifying circuit should not be eliminated

The reason of self-excited oscillation in negative feedback amplifying circuit should not be eliminated

First of all, the principle of negative feedback is that the polarity of the input is shifted 180 ° at the output, and then returned to the input. The polarity is just the opposite, and the input and output become smaller. Then, the reason for self excitation is that the phase shift is 360 ° or the difference is not big. When the feedback is sent to the input, the polarity is the same, and then the input and feedback become larger. Finally, self excitation occurs. I hope my answer can help you

If a single transistor common emitter amplifier introduces negative feedback through resistance, will self-excited oscillation occur when the resistance value is changed?

The premise of self-excited oscillation is that there is a phase shift of more than 180 degrees to form a positive feedback

Influence of negative feedback on amplifier circuit
It's urgent to summarize and explain

1、 Negative feedback can improve the stability of gain
Although the introduction of negative feedback into the amplifier will lead to the decrease of the closed-loop gain, it can improve the performance of the amplifier. For example, it can improve the stability of the gain, expand the pass band, reduce the nonlinear distortion, change the input resistance and output resistance, etc
The gain of amplification circuit may be unstable due to the changes of component parameters, ambient temperature, power supply voltage, load size and other factors. The stability of closed-loop gain can be improved by introducing appropriate negative feedback
When deep AC negative feedback is introduced into the amplifier circuit, that is, the closed-loop gain is almost only determined by the feedback network. The feedback network is usually composed of passive linear components (such as R, C, etc.) with relatively stable performance, so the closed-loop gain is relatively stable, It is usually measured by the relative change of gain with and without feedback. The relative change of open-loop and closed-loop gain is expressed by sum respectively. In this case, the modulus of sum is expressed by positive real numbers a and f respectively, and the expression of closed-loop gain becomes zero
How to get the derivative of the above formula
（1）
（2）
Divide the two sides of equation (2) respectively to obtain the form of relative variation, i.e
（3）
It can be seen from equation (3) that after adding negative feedback, the relative change of closed-loop gain is that of open-loop gain, that is, the relative stability of closed-loop gain is improved. The larger the feedback is, the deeper and smaller the feedback is, the better the stability of closed-loop gain is
2、 Negative feedback can enlarge the passband
Negative feedback has the function of stabilizing the closed-loop gain, that is, after introducing negative feedback, the gain change caused by various reasons, including the change of signal frequency, will be reduced
In order to simplify the analysis, it is assumed that the feedback network consists of pure resistors, and the basic amplifier circuit has only one inflection point in the high frequency band and the low frequency band, and the expression of its high frequency gain is
Where is the open-loop if gain and the open-loop upper limit frequency
After introducing negative feedback, the expression of closed loop gain in high frequency band is as follows
Divide the numerator and denominator by 1 + to get
Where is the closed loop gain in the intermediate frequency region and the upper frequency in the closed loop
Similarly, the lower limit frequency of the closed loop can be obtained as
From the above results, it can be seen that the if closed-loop gain decreases to, and the upper limit frequency expands to, that is, the passband expands to times of that without feedback
If the basic amplifier circuit has multiple inflection points and the feedback network is not a pure resistance network, the problem will be more complicated, but the trend of bandwidth broadening will not change
3、 Negative feedback can reduce nonlinear distortion
Triode, FET and other active devices have nonlinear characteristics, so the voltage transmission characteristics of the basic amplifier circuit composed of them are also nonlinear, as shown in curve 1 in Figure 1. When the amplitude of the input sine signal is large, the output waveform will produce nonlinear distortion
Figure 1
After the introduction of negative feedback, the closed-loop voltage transmission characteristic curve of the amplifier circuit will be flattened and the linear range will be widened obviously. Under the condition of deep negative feedback, if the feedback network is composed of pure resistors, the closed-loop voltage transmission characteristic curve is close to a straight line in a wide range, as shown in curve 2 in Figure 1, the nonlinear distortion of the output voltage will be significantly reduced
It should be noted that after adding negative feedback, if the size of the input signal remains unchanged, the net input signal output signal of the basic amplifier circuit will also drop to that of the open-loop gain because the closed-loop gain drops to that of the open-loop gain. Obviously, the working range of the transistor and other devices will become smaller, and its non-linear distortion will also be reduced accordingly, In order to explain that the reduction of nonlinear distortion is the result of negative feedback, it is necessary to ensure that the working range of active devices is the same (the amplitude of output waveform is the same) in both closed-loop and open-loop situations. Therefore, the amplitude of input signal in closed-loop should be increased to times of that in open-loop, as shown in a and B in Figure 1, If the input signal itself is distorted, negative feedback can do nothing
4、 Negative feedback can suppress the noise and interference in the feedback loop
Noise or interference is harmful to the amplifying circuit. The principle that negative feedback can suppress noise is introduced below. In Fig. 1 (a), there is input signal and noise or interference voltage at the input end of the amplifying circuit with gain. At this time, the signal-to-noise ratio of the circuit is
Figure 1
In order to improve the signal-to-noise ratio of the circuit, a pre stage with gain of is added on the basis of Fig. 1 (a), which is considered to be noiseless, and then a feedback network with feedback coefficient of is added to the whole circuit, as shown in Fig. 1 (b)
So we can get a new SNR of
It is more than twice of the original signal-to-noise ratio. It must be noted that the noiseless amplifying circuit is difficult to achieve in practice, but it can make its noise as small as possible, such as selecting devices, adjusting parameters, improving process, etc
For example, the power output stage of a loudspeaker often has AC hum, which comes from the 50 Hz interference of the power supply. Its front stage or voltage amplification stage is powered by a stable DC power supply, with less noise or interference. When a negative feedback loop is added to the back stages of the whole system, it has obvious effect on improving the signal-to-noise ratio of the system
If the noise or interference comes from outside the feedback loop, then adding negative feedback will not help
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① Generally speaking, the frequency spectrum of noise voltage VN is widely distributed. Strictly speaking, it is improper to use it to express it. This is just to explain the principle that negative feedback can suppress noise and improve signal-to-noise ratio
5、 Influence of negative feedback on input resistance of amplifier circuit
The influence of negative feedback on the input resistance depends on the connection mode of the feedback network and the basic amplifier circuit in the input circuit, but it has no direct relationship with the sampling mode of the feedback in the output circuit (the sampling mode only changes the specific meaning), RI is the input resistor of the closed loop amplifier (RI 1)
Figure 1
1. The series negative feedback increases the input resistance
Compared with the open-loop, in the series negative feedback amplifier circuit, the net input signal of the basic amplifier circuit decreases and the input current is smaller as a result of the series comparison between the feedback signal and the input signal in the input circuit. Therefore, the closed-loop input resistance RI is higher than the open-loop input resistance ri. The deeper the feedback is, the more RIF increases
The closed loop input resistance with negative feedback is
and
therefore
Therefore, the input resistance RIF is (1 +) times of the open-loop input resistance RI
It should be pointed out that in some negative feedback amplifying circuits, some resistors are not in the feedback loop, such as the base resistor Rb in the common emitter circuit, and feedback has no effect on it. The block diagram of such circuits is shown in Fig. 1 (b)
And the input resistance of the whole circuit
Therefore, it is more accurate to say that the equivalent resistance of the feedback branch is increased to (1 +) times of the input resistance of the basic amplifier circuit by introducing negative feedback in series. However, in any case, the input resistance will be increased by introducing negative feedback in series
2. Parallel negative feedback reduces the input resistance
It can be seen from Fig. 1 (c) that in the parallel negative feedback amplifier circuit, the feedback network is in parallel with the input resistance of the basic amplifier circuit, so the closed-loop input resistance RIF is less than the open-loop input resistance RI
,
and
therefore
The formula shows that the closed-loop input resistance is 1 / (1 +) times of the open-loop input resistance after introducing the parallel negative feedback
6、 Influence of negative feedback on output resistance of amplifier circuit
The influence of negative feedback on the output resistance depends on the sampling mode of the feedback network in the output circuit of the amplifier circuit, and has no direct relationship with the connection mode of the feedback network in the input circuit (the specific meaning of the input connection mode only changes). Because the sampling object is a stable object, this paper analyzes the influence of negative feedback on the output resistance of the amplifier circuit, As long as it is stable output signal voltage or stable output signal current
1. Negative voltage feedback reduces the output resistance
When the input signal is fixed, the output of the voltage negative feedback amplifier tends to be a constant voltage source, and its output resistance is very small. It can be proved that the closed-loop output resistance with voltage negative feedback is 1 / (1 +) of the open-loop output resistance without feedback. The deeper the feedback, the smaller the rof
2. Negative current feedback increases output resistance
When the input signal is fixed, the output of the current negative feedback amplifier tends to be a constant current source, and its output resistance is large. It can be proved that the closed-loop output resistance with current negative feedback is (1 +) times that of the open-loop output resistance without feedback. The deeper the feedback, the greater the rof
7、 The general principle of introducing negative feedback
It can be seen from the above analysis that the reason why negative feedback can improve the performance of the amplifier circuit in many aspects is that the output (or) of the circuit is led back to the input for comparison with the input (or), Thus, the net input (or) and output can be adjusted at any time. The improvement of gain constancy, the reduction of nonlinear distortion, the suppression of noise, the expansion of passband and the influence on input resistance and output resistance can be explained by automatic adjustment. The deeper the feedback is, the stronger the adjustment is, In addition, different types of negative feedback have different effects on the amplifier circuit
In order to improve the performance of the circuit or electronic system, it is often required to introduce appropriate negative feedback into the amplifier circuit according to the actual needs
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 middle 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, the series negative feedback should be introduced; when the internal resistance of the signal source is large or the input resistance is required to be decreased, the feedback should be introduced and contacted
3. According to the requirements of the load on the output power or output resistance of the amplifier circuit, decide whether to introduce voltage or current negative feedback. If the load requires stable signal voltage or low output resistance, voltage negative feedback should be introduced; if the load requires stable signal current or high output resistance, current negative feedback should be introduced
4. When signal conversion is needed, the appropriate configuration should be selected according to the functions of four types of negative feedback amplification circuit. For example, when current voltage signal conversion is required, the amplification circuit should be used