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The operational amplifier has two input terminals and one output terminal, as shown in Figure 1-1. Among them, the input terminal marked "+" is "same-phase input terminal" instead of the positive end), and the other input terminal marked "I" is "inverse-phase input terminal" cannot be called a negative end. If you start from these two input terminals one after another If the same signal is input, an output signal with the same voltage but opposite polarity will be obtained at the output end: the signal output at the output end is in the same phase as the signal transmitted to the human end, and the signal at the inverse input end is in reverse phase.
Figure. 1-1 Circuit Symbol for Circuit Amplifier
The power supply connected to the operational amplifier can be single or dual, as shown in Figure 1-2. Operational amplifiers have some very interesting features. Flexible application of these features can achieve many unique uses. Generally speaking, these characteristics can be combined into two:
1. The amplification multiple of the operational amplifier is infinite.
2.The input resistance of the operational amplifier is infinite and the output resistance is zero.
Figure. 1-2 Two Power Supplies that Can be Connected to an Op-Amp
Now let's briefly take a look at what conclusions can be drawn from the above two characteristics.
First of all, the amplification multiple of the operational amplifier is infinite, so as long as the input voltage of its input is not zero, the output voltage at the output end will be as high as the positive or negative power supply. It should be an infinite output voltage, but it is limited by the power supply voltage. To be precise, if the voltage input by the same-phase input is higher than that of the reverse-phase input, even if it is only a little high or small, the output end of the operational amplifier will output the same voltage as the positive power supply; conversely, if the input voltage of the reverse input is higher than that of the same-phase input voltage, the operational amplifier The output end will output the same voltage as the negative power supply voltage (if the operational amplifier uses a single power supply, the output voltage is zero).
Secondly, because the amplification is infinite, the operational amplifier cannot be used directly as an amplifier. The output signal must be fed back to the inverse input (called negative feedback) to reduce its amplification. As shown in the figure on the left in Figure 1-3, the function of R1 is to return the output signal to the inverted input of the operational amplifier. Because the inverted input is opposite to the output voltage, it will reduce the amplification multiple of the circuit. It is a negative feedback circuit, and the resistance Rf is also called a negative feedback resistance.
Figure. 1-3 The Feedback Resistor Connection Method of the Operational Amplifier (Left: Anti-Phase Connection Method, Right: Non-Phase Connection Method)
In addition, because the input of the operational amplifier is infinite, the input of the operational amplifier has no current input - it only accepts voltage. Similarly, if we imagine that there is infinite resistance between the same-phase input and the reverse input of the operational amplifier, then the voltage added to both ends of the resistance cannot form a current. Without current, according to Ohm's law, there will be no voltage at both ends of the resistance, so we can also think that in the operational amplifier's The voltage of the two transfusion terminals is the same (in this case, the voltage is a bit like short-circuiting the two input terminals with wires, so we call this phenomenon "virtual shortness").
In order to better learn about operational amplifiers, the following 16 questions and answers can quickly familiarize themselves with the basics of operational amplifiers.
(1)Provide a suitable static bias for the transistor inside the chip.
The circuit inside the chip is usually directly coupled, and it can automatically adjust the static working point. However, if an input pin is directly connected to the power supply or the ground, its automatic adjustment function is abnormal, because the transistor inside the chip cannot lift the voltage of the high line or lower the voltage of the power supply, which is As a result, the chip cannot meet the conditions of shortness and breakage, and the circuit needs to be analyzed separately.
(2) Eliminate the influence of static base current on the output voltage, and the size should be balanced with the equivalent resistance value of the external DC path of the two inputs, which is also the reason for its name.
(1)Feedback resistance and capacitance form a high-pass filter, and local high-frequency amplification is particularly powerful.
(2) Prevent self-excitement.
(1) Burning the operational amplifier may damage the operational amplifier, and the resistance can play the role of voltage division.
(1)It is to obtain positive and negative feedback problems, depending on the specific connection. For example, if I connect the current input voltage signal, output voltage signal, and then take out a wire from the output end to connect it to the input segment, then because of the above resistance, part of the output signal obtains a voltage value after passing through the resistance, diverting the input voltage, making the input voltage smaller, which is a negative feedback. Because the signal output by the signal source is always unchanged, the output signal can be corrected through negative feedback.
5. When the operational amplifier is connected to an integral device, what is the function of parallel resistance RF at both ends of the integral capacitor?
(1) Discharge resistance to prevent the output voltage from getting out of control.
(1) If you are familiar with the internal circuit of the operational amplifier, you will know that no matter what operational amplifier is composed of several transistors or MOS transistors. In the absence of external components, the operational amplifier is a comparator. When the voltage at the same phase end is high, it will output a level similar to the positive voltage, and vice versa... But such an operational amplifier does not seem to be very useful. Only when the external circuit forms a form of feedback can the operational amplifier be amplified and flipped. Wait for the function...
(1) The opposite phase end is unbalanced, and there will also be output when the input is 0. When the input signal, the output value is a fixed number larger (or small) than the theoretical output value.
(2) The error caused by the input bias current cannot be eliminated.
(1) The magnification is infinite, the input impedance is infinite, and the voltage between the same input and reverse input is almost the same (not 0!!! For example, the homogeneous end is 10V, and the reverse end is 9,999999V). I just finished the electrician exam. I still remember!
(1) The actual operational amplifier open-loop gain reaches more than 100,000, which is very, very large. Therefore, the open-loop gain of the actual operational amplifier is imagined to be infinite, and the virtual ground is derived from this.
(2)The export of virtual ground is only for inverted phase amplifiers.
I saw in the book that the open-loop gain of operational amplifiers is infinite so that when designing circuits, the closed-loop gain can be not limited by the open-loop gain, but only depend on external components. It is to sacrifice large open-loop gain for the stability of closed-loop gain.
(3) The derived virtual ground is not just an inverted phase amplifier when it is operationally placed in the negative feedback method; there is no virtual ground when the positive feedback method is carried out.
(4) It is easy to understand that if the gain is very small, the difference between the voltage added to the two ends of the operational amplifier is relatively large for an output voltage. If it is connected to a negative feedback state, it will bring inconsistency between the voltages at both ends of the operational amplifier, resulting in the amplification error.
(5) There are two conditions for the realization of operational amplifier "virtual shortness":
1) The open-loop gain A of the operational amplifier should be large enough;
2)There should be a negative feedback circuit.
Let's talk about the first point first. We know that the output voltage of the operational amplifier Vo is equal to the difference between the positive input voltage and the reverse input voltage Vid multiplied by the open-loop gain A of the operational amplifier. That is, Vo = Vid * A = (VI+ - VI-) * A (1) is a limited value because the output voltage of the operational amplifier will not exceed the power supply voltage in practice. In this case, if A is very large, (VI+ - VI-) must be very small; if (VI+ - VI-) is small to a certain extent, then we can actually regard it as 0, and then there will be VI+ = VI-, that is, the voltage of the same-phase input terminal of the operational amplifier and the reverse input terminal. The voltage is equal as if connected together, which we call a "virtual short circuit". It is important to keep in mind that they are not really connected and there is the resistance between them.
In the above discussion, how did we get the result of "shortness"?
Our starting point is formula (1), which is the characteristic of the operational amplifier. There is no problem. We can rest assured. Then, we made two important assumptions. One is that the output voltage of the operational amplifier is limited, which is no problem. Of course, the output of the operational amplifier will not exceed the power supply, so this assumption is absolutely true, so we will not mention it in the future. The second is that the release ring gains A is very large. Ordinary operational amplifier A usually reaches 6th, 7th, or even higher of 10. This hypothesis is generally no problem, but don't forget that the actual open-loop gain of the operational amplifier is also related to its working state. Without the linear zone, A may not be large. Therefore, this second hypothesis is conditional, and we should also remember this first. Dot.
Therefore, we know that when the open-loop gain A of the amplifier is very large, the amplifier can have a "short". But this is only a possibility, not realized automatically. Take an operational amplifier and say that its two input terminals are "short". No one will believe it." Virtual shortness can only be realized in a specific circuit.
The condition of "shortness" is:
1) The open-loop gain A of the operational amplifier should be large enough;
2) There should be a negative feedback circuit.
After understanding the conditions of "virtual shortness", it is easy for us to judge when and when "virtual shortness" can be used for circuit analysis. In fact, condition (1) is established for the vast majority of operational amplifiers, and the key depends on the work area. If it is a circuit in the book, it can be judged by calculation; if it is an actual circuit, you can know whether the output voltage of the instrument volume is reasonable. Another situation related to "virtual shortness" is called "virtual ground", that is, "virtual shortness" when an input end is grounded, not a new situation. Some books say that "virtual shortness" can only be used under the condition of deep negative feedback. I think this is inaccurate. I think the latent thinking of this is that in the case of deep negative feedback, the operational amplifier is more likely to work in the online area. But this is not absolute. When the input signal is too large, the deep negative feedback operational amplifier still enters saturation.
Therefore, the output voltage value should be the most reliable to judge.
Question supplement: Certain conditions should be met to constitute virtual shortcomings. Then the composition of virtual land also needs to meet certain conditions? What is it? Why?
(1) In the same-phase amplifier circuit, the output is through feedback, so that U(+) automatically tracks U(-), so that U(+)-U(-) will be close to 0. It seems to be short-circuited at both ends, so it is called "virtual short".
(2) Due to the virtual short phenomenon and the high input resistance of the operational amplifier, the current flowing through the two input terminals of the operational amplifier is very small, close to 0. This phenomenon is called "false" (false is derived from virtual and short, don't think that the two are contradictory)
(3) The virtual ground is in the reverse phase operation and discharge circuit, (+) end grounding, (-) connection input, and feedback network. Due to the existence of virtual shortness, U(-) and U(+) [potential equal to 0] are very close, so it is called (-) terminal false grounding - "virtual ground"
(4)About the conditions: virtual shortness is an important feature of the closed-loop (in short, feedback) working state of the same-phase amplifier circuit, and virtuality is an important feature of the reverse-phase amplifier circuit in the closed-loop working state. Pay attention to understanding the short conditions (such as "nearly equal"), and it should be ok.
(1) The common-mode signal of the same-phase amplifier is much larger than that of the reverse-phase amplifier, which requires a higher common-mode suppression ratio.
(2)My view on the "common-mode signal suppression ability of the same and reverse amplifiers" The strength of the op-amp common-mode signal suppression ratio (dB value) mainly depends on the symmetry and gain of the internal (only internal) differential amplifier. Obviously, no operational amplifier provides its common-mode suppression ratio while attaching the structural conditions of the external circuit. For a single-end input, whether it is the same phase or the reverse phase, the equivalent common-mode value is half of the input value. However, because the input impedance of homophase amplification is usually greater than that of reverse phase amplification, its anti-interference ability is certainly poor.
As mentioned above, when the reverse phase is input, the voltage of the reverse phase end is almost zero, so the differential pair of tube collector voltage only changes by one tube. When the same phrase is input, the voltage of the reverse phase end is equal to that of the same phase end, so the common-mode voltage is equivalent to the input voltage! That is to say, in addition to the part of the two pipes that change in different directions at the same time, there is also a difference in the same direction, which is the common-mode output voltage. It is added to the voltage of one of the tubes. Therefore, it is easy to cause the tube to become saturated (or cut off). Fortunately, the amplification of the common-mode voltage is only one tens of thousands of the differential-to-analog amplification multiple.
The above does not mean that the common-mode suppression ratio of the differential mode input and the common-mode input of the amplifier is different! It should be that the same phase input will attach a common mode signal equal to the input amount! Therefore, the same phase amplification mode should be used cautiously when the input signal is large.
The major differences between the reverse input method and the homophase input method are:
Inverted-phase input method, because a balanced resistance is connected to the ground at the same phase end, and there is no current on this resistance (because the input resistance of the operational amplifier is extremely large), the same-phase end is approximately equal to the ground potential, which is called "virtual ground", while the reverse-phase end is extremely close to the potential of the same-phase end, so at the reverse-phase end There is also a "virtual ground". The virtual advantage is that there is no common-mode input signal. Even if the common-mode suppression ratio of this operational amplifier is not high, it ensures that there is no common-mode output. The same-phase input connection method has no "virtual ground". When using a single-terminal input signal, a common-mode input signal will be generated. Even if an operational amplifier with a high common-mode suppression ratio is used, there will still be a common-mode output.
Therefore, when using it, the reverse phase input method will be used as much as possible.
(1)The output of the operational amplifier has output without any input, which is caused by the asymmetry of the design structure of the operational amplifier itself, that is, the input misaligned voltage Vos, which is a very important performance parameter of the operational amplifier. VCC/2 is often used as a reference voltage in the operational amplifier because it is in the working state of a single power supply. At this time, the real reference of the operational amplifier is VCC/2, so a DC bias of VCC/2 is often provided at the front end of the amplifier, and the ground is often used as a reference when the positive and negative dual power supply is supplied.
There are many things to pay attention to in the selection of operational amplifiers. Under very strict conditions, it is often necessary to consider the working voltage, output current, power consumption, gain-bandwidth product, price, etc. of the operational amplifier. Of course, when used under special conditions, different impact factors need to be considered.
(1) The major differences between the inverse input method and the same input method are:
Inverted-phase input method, because a balanced resistance is connected to the ground at the same-phase end, and there is no current on this resistance (because the input resistance of the operational amplifier is extremely large), the same-phase end is approximately equal to the ground potential, which is called "virtual ground", while the reverse-phase end is extremely close to the potential of the same-phase end, so at the reverse-phase end There is also a "virtual ground". The virtual advantage is that there is no common-mode input signal. Even if the common-mode suppression ratio of this operational amplifier is not high, it ensures that there is no common-mode output. The same-phase input connection method has no "virtual ground". When using a single-terminal input signal, a common-mode input signal will be generated. Even if an operational amplifier with a high common-mode suppression ratio is used, there will still be a common-mode output. Therefore, when using it, the reverse phase input method will be used as much as possible.
(2) The positive phase is an oscillator, so the reverse phase can stabilize the amplifier and connect to negative feedback.
(3) In principle, it is okay to connect it to the same-phase proportional amplifier circuit. However, in practical application, the amplified signal (that is, differential mode signal) is often very small. At this time, attention should be paid to suppressing noise (usually manifested as a common mode signal). The same-phase proportional amplifier circuit has a poor ability to suppress common-mode signals, and the signals that need to be amplified will be submerged in the noise, which is not conducive to post-processing. Therefore, the inverse proportional amplifier circuit with good suppression ability is generally selected.
(1) If the voltage on both input terminals of the operational amplifier is 0V, the voltage at the output terminal should also be equal to 0V. But in fact, there is always some voltage on the output end, which is called the maladjusted voltage VOS. If the output offset voltage is divided by the noise gain of the circuit, the result is called input offset voltage or input reference offset voltage. This feature is usually given in VOS in the data table. VOS is equivalent to a voltage source connected in series with the reverse phase input terminal of the operational amplifier. Differential voltage must be applied to the two inputs of the amplifier to generate 0V output.
(2)The input impedance of the ideal operational amplifier is infinite, so there will be no current flowing into the input terminal. However, the real operational amplifier using bipolar junction transistor s (BJT) in the input stage requires some working current, which is called bias current (IB). There are usually two bias currents: IB+ and IB-, which flow into the two input terminals respectively. The range of IB values is very large. The bias current of special types of operational amplifiers is as low as 60fA (large z passes through an electron per 3μs), while the bias current of some high-speed operational amplifiers can be as high as dozens of mA.
(3) The power supply voltage range required for the normal operation of the first single-chip operational amplifier is ±15V. Nowadays, due to the improvement of circuit speed and the use of low-power power supplies (such as batteries), the operational amplifier power supply is developing towards low voltage. Although the voltage specifications of the operational amplifier are usually specified as symmetrical bipolar voltages (such as ±15V), these voltages do not necessarily require symmetrical or bipolar voltages. For operational amplifiers, as long as the input is biased in the active area (i.e. within the common-mode voltage range), the power supply of ±15V is equivalent to +30V/0V power supply, or +20V/–10V power supply. The operational amplifier does not have a grounding pin unless the negative voltage rail is grounded in a single-power supply application. Any device of the operational amplifier circuit does not need to be grounded.
The input voltage swing of high-speed circuits is smaller than that of low-speed devices. The higher the speed of the device, the smaller its geometry, which means that the lower the breakdown voltage. Because the breakdown voltage is low, the device must work at a lower power supply voltage. Nowadays, the breakdown voltage of the operational amplifier is generally about ±7V, so the power supply voltage of high-speed operational amplifier is generally ±5V, and they can also work at a single power supply voltage of +5V.
For general amplifiers, the power supply voltage can be as low as +1 or 8V. This kind of operational amplifier is powered by a single power supply, but this does not necessarily mean that a low power supply voltage must be used. The terms single power supply voltage and low voltage are two related and independent concepts.
The core of the operational amplifier is a differential amplifier. That is, two transistors are connected back to back. Share the current of a transverse current source. One transistor is the forward input of the operational amplifier and the other is the reverse input. The forward input transistor is amplified and sent to a power amplifier circuit to amplify the output. In this way, if the voltage of the forward input rises, the output will naturally become larger. If the voltage of the reverse phase input rises, because the reverse phase three-stage tube and the forward three-stage tube share a constant current source. If the current of the reverse three-stage tube is high, the forward one should be small, so the output will be reduced. Therefore, it is called reverse input. Of course, there are many other functional components inside the circuit, but the core is like this.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other 