At present, there are many kinds of operational amps in the market, and in the face of different conditions and environments, can we choose the same operational amps? It doesn't matter, this is a lot of electronic engineers will be confused about the problem, next for you to uncover the mystery of the amp selection. The input is really short circuited.

First, how to analyze the operational amplifier circuit?

    Before learning the op amp selection, we need to first learn the internal structure and principle of the op amp circuit through the test. For us, the op amp is a very important component in the analog circuit, which can form various circuits such as amplification, addition, subtraction, and conversion. We can use the "virtual short" and "virtual break" of the op amp to analyze the circuit, and then apply Ohm's law and other current and voltage relations. The input and output magnification relationship can be obtained.

    Due to the large voltage amplification of the op amplifier, the open-loop voltage amplification of the general operational amplifier is more than 80 dB. The output voltage of the op amplifier is limited, generally in 10 V ~ 14 V. Therefore, the differential mode input voltage of the op amp is less than 1 mV, and the two input terminals are approximately equal potential, which is equivalent to "short circuit". The larger the open-loop voltage amplification, the more equal the potential of the two input terminals. "Virtual short" means that when analyzing the operational amplifier in a linear state, the two input terminals can be regarded as equal potential, which is called false short circuit, referred to as virtual short. Obviously you can't really short the two inputs.

    In the inverted phase amplifier circuit, the signal voltage is added to the inverted phase input of the OP-amp through the resistor R1, and the output voltage Vo is fed back to the inverted phase input of the OP-amp through the feedback resistor Rf, forming a voltage parallel negative feedback amplifier circuit.

    The in-phase terminal of the op amp is grounded =0V, the inverting terminal and the in-phase terminal are virtual short, so it is also 0V, the inverting input terminal input resistance is high, the virtual break, there is almost no current injection and flow out, then R1 and Rf are equivalent to series, the current flowing through each component of a series circuit is the same, that is, the current flowing through R1 and the current flowing through Rf is the same.

According to Ohm's law:

    Is= (Vs- V-)/R1................（1）
    If= (V- - Vo)/Rf...............（2）
    V- = V = 0 ....................（3）
    Is= If ........................（4）

    The solution may be Vo== (-Rf/R1)*Vi
    In the process of analyzing the circuit, the other characteristics of the amplifier are not needed for the time being, and the characteristics of the virtual short and virtual break are analyzed. Of course, if the op amp does not work in the amplification area, it does not meet the virtual short and virtual break conditions, and it cannot be analyzed using this method, such as the comparator.

    Figure 1-2 below is the adder implemented by the operation amplifier, and the method of virtual short and virtual break is used to analyze this circuit.

    Because the differential mode input resistance of the op amp is very large, the input resistance of the general purpose op amp is above 1MΩ. Therefore, the current flowing into the op-amp input is often less than 1uA, which is much smaller than the current of the external circuit at the input end. Therefore, the two inputs of the op amp can usually be regarded as open circuit, and the greater the input resistance, the closer the two inputs are to the open circuit. "Virtual break" means that when analyzing the operation amplifier in a linear state, the two input ends can be regarded as equivalent open circuit, this feature is called false open circuit, referred to as virtual break. Obviously, you can't really disconnect the two inputs.

    The following paper uses virtual break and virtual break methods to analyze the actual circuit, as shown in Figure 1-1, which is a common inverse-phase proportional operation amplifier circuit:

    Because there is a virtual short circuit, the net input voltage vI=0 of the op amp, and the inverting end is a virtual ground.

    vI=0，vN=0.......................（5）

    Inverting the concept that the input current iI=0, the sum of the current through R2 and R1 is equal to the current through Rf

    (Vs1 – V-)/R1 (Vs2 – V-)/R2

    = (V- –Vo)/Rf.......（6）

    If we take R1=R2=R3, we can solve it by the equations a and b

    -Vout=Vs1 Vs.......................（7）

    In formula (7), the negative sign is caused by the inverting input, and if the inverting circuit is connected again, the negative sign can be eliminated.

    In short, the virtual short is that the voltage of the positive input and the negative input of the op amp is equal, which is approximately short circuit; A virtual break means that the current flowing into the positive and negative inputs is 0. As long as we master this point, and then use Ohm's law, we can easily analyze the in-phase proportional amplifier circuit, reverse proportional amplifier circuit and other commonly used operational amplifier circuit.


Second, how to choose the specific operation amplifier?

    The following categories introduce what kind of op amp to choose under what circumstances!

1. Universal operational amplifier

    General-purpose operational amplifiers are designed for general-purpose purposes. The main features of this kind of device are low price, large product quantity and wide range, and its performance index can be suitable for general use. For example, mA741 (single op amp), LM358 (double op amp), LM324 (four op amp), they are currently widely used integrated operational amplifiers.

2. Precision operational amplifier

    Precision operational amplifiers generally refer to the offset voltage below 1mV of the op amp, for DC input signals, input offset voltage (VOS) and its temperature drift is small, but for AC input signals, we must also consider the input voltage noise and input current noise of the op amplifier, in many applications input voltage noise and input current noise is more important. The choice of an op amp becomes particularly difficult when the sensor type and/or the environment in which it is used impose many special requirements, such as ultra-low power consumption, low noise, zero drift, rail-to-rail input and output, reliable thermal stability, and reproducibility for thousands of readings and/or consistent performance under harsh operating conditions. The precision amplifier circuit will have more specially designed circuits such as power decoupling and filtering. The main difference is that the performance of precision operational amplifiers is much better than that of general operational amplifiers, such as open loop magnification, CMRR is larger, the speed is relatively slow, GBW, SR is generally small. Offset voltage or offset current is relatively small, small temperature drift, low noise and so on. The performance of a good precision op amp is far from the general operational amplifier can be compared, the general op amp imbalance is often several mV, and the precision op amp can be as small as 1uV level. To amplify a small signal, you must use a precision op amp, with a general op amp, it will itself bring a lot of interference. To improve through the peripheral circuit, small or fine tuning can be, but can not be drastically or completely changed. The commonly used precision operational amplifier is OP07, and its family, OP27, OP37, OP177, OPA2333. There are many others, such as the products of the United States AD company, many of which are spearheaded by OPA.

3. High resistance integrated operational amplifier

    The high-resistance integrated operational amplifier is characterized by a very high differential mode input impedance, a very small input bias current, a general rid> (109~1012) W, IIB for a few picoamps to dozens of picoamps. The main measure to achieve these indexes is to make use of the high input impedance of the field effect tube (FET) to form the differential input stage of the op AMP. Using FET as input stage not only has high input impedance and low input bias current, but also has the advantages of high speed, wide band and low noise, but also has large input offset voltage. Common integrated devices are LF356, LF355, LF347 (four op-amp) and higher input impedance CA3130, CA3140 and so on.

4. Low-temperature bleached operational amplifier

    In automatic control instruments such as precision instruments and weak signal detection, it is always hoped that the offset voltage of the operational amplifier is small and does not change with the change of temperature. Cryobleached operational amplifiers are designed for this purpose. The commonly used high and low temperature drift operational amplifiers are OP-07, OP-27, AD508 and the chopped zero stable low drift device ICL7650 composed of MOSFETs.

5. High-speed op amp

    The high-speed type is placed in the fast A/D and D/A converters and video amplifiers, requiring the conversion rate SR of the integrated operational amplifier to be high, and the unit-gain bandwidth BWG to be large enough, like the universal integrated operational amplifier is not suitable for high-speed applications. The high speed operational amplifier is characterized by high conversion rate and wide frequency response. Common operational amps are LM318, mA715, etc., whose SR=50~70V/us, BWG>20MHz.

6. Low power amplifier

    Low power amplifier Because the integration of electronic circuits can make complex circuits small and lightweight, so with the expansion of the application range of portable operational amplifiers, it is necessary to use low power supply voltage and low power consumption of operational amplifiers. The commonly used operational amplifiers are TL-022C, TL-060C, etc., whose operating voltage is ±2V~±18V, and the consumption current is 50~250mA. At present, the power consumption of some products has reached the microwatt level, such as the power supply of ICL7600 is 1.5V, power consumption is 10mW, and can be powered by a single battery.

7. High voltage and high power operational amplifier

    The output voltage of the high voltage and high power operational amplifier is mainly limited by the power supply. In ordinary operational amplifiers, the maximum output voltage is generally only tens of volts, and the output current is only tens of Ma. In order to increase the output voltage or increase the output current, an auxiliary circuit must be added outside the integrated op amp. High voltage and high current integrated operational amplifiers can output high voltage and high current without any additional circuit. For example, the power supply voltage of the D41 integrated op amp can reach ±150V, and the output current of the mA791 integrated op amp can reach 1A.

    I believe that through the above introduction, it is obviously more clear whether the same operational amplifier can be used under different conditions of use, and other factors should be considered when actually choosing an integrated operational amplifier. For example, the nature of the signal source, whether it is a voltage source or a current source; The nature of the load and whether the output voltage and current of the integrated op amp meet the requirements; Environmental conditions, integrated operation amplifier allowable operating range, operating voltage range, power consumption and volume and other factors meet the requirements.