The problem of positive feedback of op amps, the role of negative feedback of op amps

I'm sorry, it's been a long time since I've solved the knowledge, so I saw it.

This is a comparator, a comparator with feedback, a hysteresis comparator, when V- is greater than V+, the output is low, and V+ is greater than V- and the output is low and high.

When V2 is in the air, the V- voltage is 1V, the V+ voltage starts at 3V, the output level is high, and the VO output voltage is 12V.

Due to the positive feedback, V0 and R4 R3 form a loop, which determines the voltage value of V+ next time when compared, 3V+*(R3 R3+R4)=, that is, when V- voltage is greater than the next time, a low level of 0V is output

That is, when V2 is not suspended, the voltage increases to V- greater than before the output is low.

In the same way, 0V and R3 R4 form a loop, and the V+ reference is also changed, the voltage is 3V*100 103=, and when V- is less than and outputs high.

Summary: After the first time V+ is greater than V- output high level 12V (close to the power supply voltage), the V+ reference will be changed to, when the V- voltage rises to greater than and the output low level is 0, the V+ reference will become, that is, when the V- voltage is lower than the output high level of 12V (close to the power supply voltage) again.

1. Improve gain stability: under the condition of deep negative feedback, the closed-loop gain is not affected by the change of peripheral component parameters or has little influence, so as to improve the gain stability;

2. Reduce nonlinear distortion: Deep negative feedback has nothing to do with open-loop gain, so it has little to do with the nonlinear change in open-loop transmission, so as to reduce nonlinear distortion.

3. Noise suppression: mainly suppress the noise of peripheral devices.

4. Extended bandwidth: less affected by frequency changes.

The DC working point of the amplification circuit with and without negative feedback is basically the same; After the introduction of negative feedback, the basic performance of the circuit has been greatly improved, but the amplification factor is reduced. Negative feedback can greatly suppress the nonlinear effects caused by temperature.

The deeper the feedback depth, the smaller the voltage amplification factor, and the stronger the ability to regulate the circuit. Under certain conditions, negative feedback may become positive feedback and destroy the normal performance of the amplification circuit.

An empirical method for determining whether it is voltage feedback or current feedback.

Load short-circuit method: make the output of the amplification circuit AC short-circuit. If the feedback signal xf disappears, it means that the feedback signal is sampled at the output voltage, which is voltage feedback (xf fv0).

If the feedback signal persists, the feedback signal is sampled from the output current, which is current feedback (XF FI0).

General law method: the feedback amount is taken from the voltage signal at the output end of the signal, which is the voltage feedback; The feedback signal is taken from the current signal at the output end of the signal, which is current feedback. Specifically, the load resistor and the feedback network are regarded as double-ended networks (one end of the feedback amplification circuit is usually a common ground terminal), and if the load resistor and the feedback network are connected in parallel, the feedback amount samples the output voltage and is voltage feedback.

If it's an amplifier, it must be negative feedback. If an op amp is an inverting proportional amplifier, it is a voltage series negative feedback.

Sending part or all of the output signal of a system back to the input end of the system as part of the input signal in a certain way and path is called feedback. According to the signal polarity of the feedback, feedback can be divided into positive feedback and negative feedback.

The operational amplifier is an electronic integrated circuit containing a multi-stage amplifier circuit, and its input stage is a differential amplifier circuit, which has high input resistance and the ability to suppress zero drift; The intermediate stage is mainly used for voltage amplification, with high voltage amplification factor, and is generally composed of a common emitter amplification circuit; The output pole is connected to the load, which has the characteristics of strong load capacity and low output resistance. Operational amplifiers are used in a wide range of applications.

The sampling mode of negative feedback amplification circuit from the output can be divided into voltage feedback and current feedback, and the mode of access circuit from the input end can be divided into series feedback and parallel feedback. Chunchong only distinguishes the method of judging the skin:

1. If the feedback sampling point of the output is at the same point as the output, it is voltage feedback, and if it is not at the same point, it is current feedback;

2. At the input end, if the feedback signal and the input signal are connected to the same input terminal, it is to participate in the calculation in the form of current, which is a negative feedback of current, such as the feedback signal and the input signal are connected to different terminals of the amplification circuit, then it participates in the operation in the form of voltage, which is a negative feedback of voltage.

The returned information is in the same direction as the original information, and develops in the same direction.

Generally, the gain of the amplifier A is about ten to the 5th power, which can be considered infinite, and the gain is too large, so it is very unstable, that is, a small voltage change will output a large voltage change, so add negative feedback here, the system is closed-loop, so that the transfer function becomes a 1+L, L is the closed-loop gain, from the process point of view, the actual amplifier gain becomes the original 1 1+L, the amplification is reduced, and the stability is improved.

1. Stabilize the output voltage gain.

2. Increase the input impedance.

3. Reduce the input impedance.

The output signal of a system is sent back to the system in a certain way and path.

The incoming end is part of the input signal, and this process is called feedback. According to the signal polarity of the feedback, feedback can be divided into positive feedback and negative feedback.

The operational amplifier is an electronic integrated circuit containing a multi-stage amplifier circuit, and its input stage is a differential amplifier circuit, which has high input resistance and the ability to suppress zero drift; The intermediate stage is mainly used for voltage amplification, with high voltage amplification factor, and is generally composed of a common emitter amplification circuit; The output pole is connected to the load, which has the characteristics of strong load capacity and low output resistance. Operational amplifiers are used in a wide range of applications.

There are two types of power supply modes: dual power supply and single power supply. In the case of dual-supply op amps, the output can vary on both sides of the zero voltage, and the output can also be set to zero when the differential input voltage is zero. In a single-supply op amp, the output varies within a certain range between the power supply and ground.

The input potential of the op amp is usually required to be higher than a certain value for the negative supply and a certain value for the positive supply. The op amp is specially designed to allow the input potential to vary from negative to positive, even slightly above the positive supply or slightly below the negative supply. This op amp is called a rail-to-rail input op amp.

The op amp has a positive phase input and an inverting input end, and the introduced feedback input inputs to different ends form positive and negative feedback, and the output will also change in amplitude and phase.

Feedback refers to the input signal is loaded into the input through the circuit, and the input signal is strengthened, which is usually called positive feedback, and vice versa, negative feedback. Usually positive feedback is used to generate self-oscillations and negative feedback is used to stabilize gain. Refer to "Analog Electronic Technology".

If the output voltage or current of the op amp is to be stabilized at a fixed value within the supply voltage range, it needs to operate in a negative feedback state.

Otherwise, it does not operate in negative feedback mode, so that the output voltage is close to the maximum and minimum value of the supply voltage. Voltage comparators work in such a way without feedback. Forward feedback should refer to the way in which the voltage fed back to the inverting terminal decreases or increases after the voltage at the inverting terminal increases or decreases, so that the output voltage is at the maximum or minimum close to the supply voltage.

It's not that the feedback received at the positive end is positive feedback, and the opposite end is negative feedback, this must be understood!

It can be said that if the op amp does not work in negative feedback mode, it must work in positive feedback mode. If there is feedback, the voltage comparator is the case when there is no feedback. Schmitt triggers, window comparators, etc., all work in a positive feedback mode.

The maximum voltage of an op amp is twice that of a single power supply as it is when used with a dual power supply.

However, if the input signal voltage range is 1V 45V, then the power supply voltage must not be less than 45V, but also higher than this, because TLE2142 and all high voltage op amps are not full power amplitude output, its maximum output high voltage is 1V lower than the power supply voltage, and the lowest output voltage is higher than the power supply ground. However, the supply voltage of the TLE2142 is up to 44V, so if the input signal voltage range cannot be reduced, you are better off choosing another type of op amp. As far as I know, there are very few op amps with a supply voltage higher than 44V, but TI has a high voltage op amp OP445 that can operate up to 90V (or 45V) with a supply voltage of up to 90V (or 45V) that you can consider.

Your input voltage amplitude is 44V, and your op amp power supply is also 44, which is obviously not suitable. Please change to something else**.

The single power supply usage you said is OK, don't forget to pay attention to the bias at the input end, and follow the single power supply method.

The operating voltage is +-20V or higher, and the single-supply type is an operational amplifier that can amplify signals from 0V.

Since it is a single power supply type, it can be connected like this, and the 44V power supply may only be able to follow up to about 42V. I've used it, rest assured.

For example, the LM324 can be +- 16V, or it can be a single power supply of +3V---32V.

High-voltage op amp ICs are not only hard to buy but also very expensive, I wonder if it's cost-effective? Your requirements are not high, suggestion:

1。With LM324, +30V single power supply, the input voltage divider is 1 2, and the output can reach 0 --V.

2。If the voltage division is not possible, then use the triode emitter to follow the circuit, the supply voltage is +50V, and the output is 1 --45 V. It's just that.

The output impedance is a little higher.