At present, there are various types of op amps on the market. Can we choose the same op amp for different conditions of use and environment? It does not matter, this is a problem that many electronic engineers will be confused.
How to analyze the op amp circuit?
Before learning the op amp selection, we need to learn the internal structure and principle of the op amp circuit first. For us, the op amp is an important component of the analog circuit. It can form the amplification, addition, subtraction, conversion For various circuits, we can analyze the circuit by using the "virtuality" and "virtuality" of the op amp, and then apply Ohm's law and other current-voltage relationships to obtain the input-output amplification relationship.
Due to the large voltage amplification of the op amp, the open-loop voltage amplification of the general-purpose op amp is more than 80 dB. The output voltage of the op amp is limited, typically between 10 V and 14 V. Therefore, the differential amplifier input voltage of the operational amplifier is less than 1 mV, and the two input terminals are approximately equal potential, which is equivalent to a "short circuit." The larger the open-loop voltage amplification, the closer the potentials of the two input terminals are. "Virtual short" means that when the analysis operational amplifier is in a linear state, the two input terminals can be regarded as equipotential. This characteristic is called false short circuit, short for short. Obviously it is not possible to really short the two inputs.
Due to the large differential input resistance of the op amp, the input resistance of a general-purpose op amp is above 1 MΩ. Therefore, the current flowing into the input terminal of the op amp is often less than 1uA, which is much less than the current of the external circuit of the input terminal. Therefore, the two input terminals of the op amp are usually regarded as open circuits, and the greater the input resistance is, the closer the two input terminals are to the open circuit. "Virtually broken" means that the two input terminals can be regarded as equivalent open circuits when analyzing the operational amplifier in a linear state. This feature is called false open circuit, or short circuit. Obviously the two inputs cannot be truly open circuited.
In the following text, the actual circuit is analyzed by the method of virtual break and virtual break. As shown in Figure 1-1, it is a common inverse scale operation and amplification circuit:
Figure 1-1. Directional Ratio Operational Amplifier Circuit
In the inverting amplifying circuit, the signal voltage is added to the inverting input terminal of the operational amplifier through the resistor R1, and the output voltage Vo is fed back to the inverting input terminal of the operational amplifier through the feedback resistor Rf to constitute a voltage parallel negative feedback amplifying circuit.
The non-inverting terminal of the op amp is grounded = 0V, the inverting terminal and the non-inverting terminal are shorted, so it is also 0V. The input resistance of the inverting input is very high, and the input is broken. There is almost no current inflow and outflow. Then R1 and Rf are connected in series. The current flowing through each component in a series circuit is the same, ie the current flowing through R1 is the same as the current flowing through Rf.
According to Ohm's Law:
Is= (Vs- V-)/R1...............(1)
If= (V- - Vo)/Rf.................(2)
V- = V+ = 0 ..................... (3)
Is= If ..............................(4)
Possible Vo== (-Rf/R1)*Vi after solving
In the process of analyzing the circuit, temporarily ignore the other characteristics of the op amp, and analyze it based on the characteristics of virtual short and virtual disconnect. Of course, if the op amp does not work in the amplification area, it does not satisfy the conditions of virtual short and false disconnection and cannot be analyzed using this method, such as a comparator.
As shown in Figure 1-2 below, the adder implemented by the op amp uses virtual short and virtual broken methods to analyze this circuit.
Figure 1-2. Adder implemented by op amp
Due to the short circuit in the circuit, the net input voltage of the op amp is vI = 0, and the inverting terminal is a virtual ground.
vI=0, vN=0................................................. (5)
The concept of the inverting input current iI=0, the sum of the currents passing through R2 and R1 is equal to the current passing through Rf.
(Vs1 - V-)/R1 + (Vs2 - V-)/R2 = (V- -Vo)/Rf....(6)
If you take R1=R2=R3, it can be solved by a, b
-Vout=Vs1+Vs.................................................(7)
In equation (7), the negative sign is caused by the inverting input. If the inverting circuit is further connected, the negative sign can be eliminated.
In short, imaginary short is equal to the voltage at the positive input and negative input of the op amp, and is approximately short-circuited; imaginary-break is that the current flowing to the positive and negative inputs is zero. As long as you master this, and then use Ohm's law, you can easily analyze the analog amplifier circuit, the reverse proportional amplification circuit and other commonly used op amp amplification circuit.
How to choose the op amp?
The following classification describes what kind of opamp to choose.
1. General-purpose operational amplifier
General purpose operational amplifiers are designed for general purpose purposes. The main features of these devices are low price, wide range of products, and their performance indicators can be suitable for general use. Examples are mA741 (single op amp), LM358 (double op amp), and LM324 (quad op amp), which are currently the most widely used integrated op amps.
2, precision operational amplifier
Precision op amps generally refer to op amps with offset voltages below 1mV. For DC input signals, the input offset voltage (VOS) and its temperature drift are small, but for AC input signals, we must also consider the op amp's input voltage noise and Input current noise, input voltage noise and input current noise are more important in many applications. In the sensor type and/or its use environment brings many special requirements, such as ultra-low power consumption, low noise, zero drift, rail-to-rail input and output, reliable thermal stability, and thousands of readings ( Or) to provide reproducible consistent performance under harsh operating conditions, the choice of operational amplifiers becomes particularly difficult. Precision amplification circuits will have more power supply decoupling, filtering and other specially designed circuits.
The main difference is that in operational amplifiers, the performance of precision operational amplifiers is much better than that of general op amps. For example, the open-loop amplification is larger, the CMRR is larger, and the speed is slower. GBW and SR are generally smaller. Offset voltage or offset current is small, temperature drift is small, noise is low, and so on. The performance of a good precision op amp is far less than that of a typical op amp. The offset of a typical op amp is often several mV, while a precision op amp can be as small as 1 uV. To amplify a tiny signal, you must use a precision op amp and use a general op amp. It will bring a lot of interference to itself. To improve through the peripheral circuit, small or fine tuning can be, but can not be drastically or completely changed. The most commonly used precision op amp is OP07, as well as its family, OP27, OP37, OP177, OPA2333. There are many others, such as the products of American AD company, many of which are led by OPA.
2-1. Dual Precision Operational Amplifier
3, high resistance integrated operational amplifier
The characteristic of the high-resistance type integrated operational amplifier is that the input impedance of the differential amplifier is very high, and the input bias current is very small. Generally, rid>(109~1012)W, IIB is several picoamps to several tens of picoamps. The main measure to realize these indexes is to use the characteristics of high input impedance of the field effect transistor, and use field effect transistors to form the differential input stage of the operational amplifier. Using FETs as the 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 the input offset voltage is large. Common integrated devices include the LF356, LF355, and LF347 (four op amps) and higher input impedances such as CA3130 and CA3140.
4, low temperature drift type operational amplifier
In precision instruments, weak signal detection, and other automatic control instruments, it is always desirable that the offset voltage of the operational amplifier be small and does not change with changes in temperature. Low-temperature drift type operational amplifiers are designed for this purpose. Commonly used high-precision, low-temperature drift op amps are OP-07, OP-27, AD508, and chopper-stabilized low-drift devices ICL7650 composed of MOSFETs.
5, high-speed op amp
High-speed op amps are used in fast A/D and D/A converters and video amplifiers. The SR of the integrated op amp must be high, and the unity-gain bandwidth BWG must be large enough. For example, a general-purpose integrated op amp is not suitable. For high speed applications. High speed op amps feature high slew rate and wide frequency response. Common operational amplifiers are LM318, mA715, etc., with SR=50~70V/us and BWG >20MHz.
6, low-power op amp
The low-power op amp due to the greatest advantages of electronic circuit integration is able to make the complex circuit small and lightweight, so with the expansion of the scope of application of portable op-amp instruments, the use of low power supply voltage, low power consumption op amp phase . Commonly used operational amplifiers are TL-022C, TL-060C, etc. The operating voltage is ±2V~±18V and the current consumption is 50~250mA. At present, the power consumption of some products has reached microwatt level, for example, the power supply of the ICL7600 is 1.5V, and the power consumption is 10mW, which can be powered by a single battery.
7, high voltage and high power operational amplifier
The output voltage of a high voltage, high power operational amplifier is mainly limited by the power supply. In an ordinary operational amplifier, the maximum value of the output voltage is generally only tens of volts, and the output current is only tens of milliamperes. To increase the output voltage or increase the output current, an auxiliary circuit must be added outside the integrated op amp. The high voltage and high current integrated operational amplifier 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.
It is believed that through the above introduction, whether the same op amp can be used under different conditions of use is clearly clear. When actually selecting an integrated op amp, other factors should also be considered. For example, the nature of the signal source is a voltage source or a current source; the nature of the load, whether the integrated op amp output voltage and current meet the requirements; environmental conditions, whether the operating range, operating voltage range, power consumption, and volume of the integrated op amp are allowed fulfil requirements.
Finally, give everyone a little experience in evaluating the operational amplifier. The merits and demerits of evaluating the performance of the integrated op amp should be based on its comprehensive performance. SR is the conversion rate and the unit is V/ms. The larger the value, the better the AC characteristics of the op amp; Iib is the input bias current of the op amp and the unit is nA; VOS is the input offset voltage and the unit is mV. The smaller the Iib and VOS values, the better the DC characteristics of the op amp. Therefore, for circuits that amplify audio signals, video, and other AC signals, selecting an op amp with a large SR (Slew Rate) is appropriate. For a circuit that handles a weak DC signal, it is appropriate to use a high-op-amp with a relatively high accuracy (both offset current, Offset voltage and temperature drift are relatively small. When there are no special requirements, try to use a general-purpose integrated op amp. This will not only reduce costs, but also make it easier to guarantee supply. When using multiple op amps in a system, use multiple op amp integrated circuits where possible. For example, the LM324, LF347, and so on are all integrated circuits that encapsulate four op amps.
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