Hardware 100,000 why--op-amp article (a) How to prevent the amplification circuit self-excitation



How to prevent the amplification circuit self-excitation?

Personal experience, the most effective way:

1. Is the resistance of the amplifying circuit too large? For example, I've seen a circuit that amplifies 10 times times, and the resistor chooses 100k,1m. (Even the textbook is written). Intrinsic circuit current is too small, anti-jamming ability is weak, the output signal is more easily coupled in.

2, the output line is too long, not only easy self-excitation, and the signal is also susceptible to interference, it is best to go shielding, no conditions, you can consider the output string small resistance. Series of small resistors, not completely solve the phase problem, while solving the capacitive load is too large, resulting in the operation of the operation of abnormal.

3, magnification should not be too large, the output signal is too strong, the input signal is too weak, it is easy to coupling.

4, the transimpedance shunt capacitor, although theoretically is effective, but the capacitance affects the output phase consistency, capacitance discrete type is too large. In scenarios where phase consistency is emphasized, phase effects need to be considered.

5, grounding, the eternal topic.

Reasons for the self-excitation of op-amp shocks:

1, the loop gain is greater than 1 2, the signal before and after the phase difference of 360 degrees, that is to be able to form positive feedback.

Reference to "the principle of self-control" and "Circuit design based on OP amps and analog integrated circuits"

The self-excited oscillation is caused mainly because the integrated op amp is composed of multilevel DC amplifiers, because of the output of each stage amplifier and the input of the latter amplifier, there are output impedance and input impedances and distributed capacitance, so there are r-c phase shift networks in the stage. An additional phase shift is generated when the signal passes through a r-c network. In addition, the external bias resistor and op amp input capacitance in the op amp, the op amp output resistor and capacitive load feedback capacitance, and the common internal resistance of the multistage op amp through the power supply, even the distribution inductance on the power line, and the poor grounding coupling, can form additional phase shift. , the output signal of the OP amp is added to the additional phase shift of 180 degrees through the negative feedback loop, and if the feedback amount is large enough, the negative feedback will be transformed into positive feedback, which will cause oscillation.

A specific point may

1. Possible op amp is 2 of the distributed capacitance and inductance. The OP amp drives the capacitive load.

3. May be the result of too deep feedback

Workaround:

1. In-loop compensation

The op amp feedback resistor is connected with the feedback capacitor :

Small capacitance is called phase-shifted capacitance, to prevent the operation of self-excitation generally take 0 points to dozens of skin method hundreds of skin method , see the frequency of work and op amp model to set

Simply put, the larger the capacitance, the narrower the bandwidth.

The input capacitance and stray capacitance of the oscillating RF and op amp are prevented from forming poles, and if the pole is in the frequency range used for op amp, the op amp may oscillate; CF and RF generate 0 points to offset the poles after adding cf. The general value of the CF>CI,CI is the input capacitance of the OP amp and the total capacitance of the input foot stray.

2. Off-loop compensation method,

A small resistor on the output string of the op amp

Connected to the rear stage, between more than 10 and dozens of euros can be, the specific value and the input capacitance of the latter circuit, can try different resistance values, to obtain a stable output

Ps:

1. Power supply stable, preferably parallel 0.1uf, 10uf and other capacitors

2. Magnification can not be too large, the amplification series should not exceed four levels

Before the experiment or test, if the oscilloscope is connected to the OP amp output, it is sometimes possible to see a waveform with a higher frequency and approximate sine wave, and occasionally a low frequency oscillation. Depending on the principle of oscillation, different methods can be adopted to solve the problem:

(1) Feedback polarity is incorrect or negative feedback is too strong. If negative feedback is wrongly connected to positive feedback, oscillation is very easy to produce. In addition, the stronger the negative feedback, the more easily the self-excitation.

(2) If the output terminated capacitive load, because the capacitive load strengthens the phase shift of the circuit, so more self-excitation. You can use another RC link to compensate for the phase shift , and if the compensated self-oscillation will be eliminated.

(3) The wiring stray capacitance is too large. When the input circuit is high impedance, the hysteresis of the stray capacitance and resistance between the wiring to the ground or the wiring will cause the component to become unstable. For this purpose, a capacitor CF can be connected in parallel at either end of the Rf (feedback resistor) or an RC branch in parallel with the OP amp These two links belong to the nature of the advanced correction, that is, they produce phase lead effect will be possible to offset the aforementioned stray capacitance of the phase lag effect, so that the op amp stability .

(4) Power wiring bypass measures are insufficient. Power leads not only have a certain resistance, but also a certain inductance and distribution capacitance, so when there are many op-amp to the same power cord, will be through these factors affect each other, the solution is to connect the terminal of the positive and negative power supply on the PCB socket to the ground between the dozens of UF electrolytic capacitor and 0.01uF ceramic Ceramic capacitors in parallel, if the op amp as a broadband amplification, the use of low inductance capacitance.

The old man of the former Institute told me: Die electricity is an art; some people say: Die electricity is a magic, difficult to fathom; in fact, I think: mold is a gambling technique, buy big buy small just. In the hands of the process of discovering the law, theory and practice, spiral development.

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Hardware 100,000 why--op-amp article (a) How to prevent the amplification circuit self-excitation