Summary of experience in selecting operational amplifiers

fish001

Summary of experience in selecting operational amplifiers [Copy link]

Operational amplifiers (op amps) are the cornerstone of the entire analog circuit design. Choosing an appropriate amplifier is crucial to achieving system design specifications. Considerations: 1. Op amp power supply voltage and mode selection; 2. Op amp package selection; 3. Op amp feedback mode, that is, VFA (voltage feedback op amp) or CFA (current feedback op amp); 4. Op amp bandwidth; 5. Slew rate, which determines the full-power signal bandwidth; 6. Offset voltage and offset current selection; 7. Offset voltage drift with temperature, that is, ΔVoffset/ΔT; 8. Op amp input impedance selection; 9. Op amp output drive capability selection; 10. Op amp static power consumption, that is, ICC current selection; 11. Op amp noise selection; 12. Op amp load stabilization time. When designing analog circuits for switching power supplies, some people don't know how to choose op amps at all. They just use whatever they have on hand. Maybe you have done this 100 times and succeeded every time, but what will happen the 101st time? Others are just the opposite. They flip through five or six original manufacturer's materials, and finally find the "dream lover" but can't buy it. I would like to recommend some common op amps to you, which are definitely available and can adapt to most occasions. 1. Low speed requirement, or DC amplification: LF441 (single), LF442 (dual), LF444 (quad), TL084 (quad) (the above op amps are JFET input, with extremely high impedance, no need to consider the impedance balance of the input end) OP07 (single, high precision, with zero adjustment terminal, but very slow speed, good for DC amplification) 2. Relatively high speed, audio range, multiple not exceeding 100: LF356 (single), LF353 (dual), LF347 (quad), TL074 (quad) (the above op amps are JFET input, with extremely high impedance, no need to consider the impedance balance of the input end) OP27 (single, high precision, with zero adjustment terminal, faster than LF356) NE5534 (used for audio amplification, good sound quality, but low input impedance) 3. High-speed OP37 (unit frequency response 50MHz, but must not be used as a follower! It will self-excite when the closed-loop gain is less than 5) 4. Low voltage or single power supply LM324 (too slow) It is recommended to use Maxim products. For other special occasions, such as video amplification, ultra-linear amplification, low drift and other requirements, you still need to check the information first. "The op amp you soldered on the circuit board is not the ideal op amp in the textbook!" When designing a circuit, after considering all the issues you have considered, please pay attention to the following issues. 1. Output voltage swing Do not expect the output voltage of a general op amp to reach the power supply voltage, even if your load resistance is 10M. The peak-to-peak value of the output voltage of a general-purpose op amp is 1~3V different from the power supply. 2. Common-mode input voltage range Do not let the potential of your op amp's input terminal be very close to its power supply voltage, otherwise you will be in trouble. For example, if you choose an LF347 op amp (most JFET op amps are similar), the power supply voltage is positive and negative 12V, the positive input terminal potential is -11V, and the negative input terminal is -11.5V. What do you think the output will be? Maybe you guessed wrong, it is -10V. This is the result of using it beyond the common-mode voltage range. Of course, if you change to LM324, there will be no such effect. Fortunately, Maxim and NS have now launched Rail to Rail op amps, and their common-mode voltage range is the same as the power supply voltage. 3. Output voltage slew rate SR If you are using an op amp to amplify high-frequency and large-amplitude signals, you must not ignore the SR parameter, which indicates the maximum change in output voltage per microsecond. For example, the unit bandwidth of uA741 is 1MHz, SR=0.7V/us, if you connect it in a follower form (gain=1), at this time, if you input a square wave with an amplitude of -5V~+5V and a frequency of 200KHz, then the output result will definitely disappoint you. Its output is actually a strange triangle wave with an amplitude of only about 2V. A few supplements: (1) For low-potential amplification circuits, offset, temperature drift and input noise must also be considered. (2) For high-precision circuits, attention should be paid to the common-mode rejection ratio. Generally speaking, OPs with high common-mode rejection ratios have better linearity. (3) Pay attention to the input resistance. Bipolar OPs are generally in the range of several hundred K to several tens of M. There are many possible causes of self-excitation of op amps: 1. Insufficient compensation. For example, in the design of OP37 and other operational amplifiers, in order to improve the high-frequency response, the compensation amount is small, and self-excitation will occur when the feedback is deep. By measuring the BODE diagram of its open-loop response, it can be seen that as the frequency increases, the open-loop gain of the operational amplifier will decrease. If the phase lag exceeds 180 degrees before the gain drops to 0db, the closed-loop use will inevitably self-excite. 2. Power supply feedback self-excitation. From the internal structure of the operational amplifier, it is a multi-stage amplifier circuit. Generally, the operational amplifier is composed of more than 3 stages of circuits. The front stage completes high gain amplification and potential movement, the second stage completes the phase compensation function, and the final stage realizes power amplification. If the internal resistance of the power supply to the operational amplifier is large, the power consumption of the final stage will cause power supply fluctuations. This fluctuation will affect the operation of the circuit of the previous stage and be amplified by the previous stage, causing greater fluctuations in the circuit of the next stage. This vicious cycle will cause self-excitation. 3. External interference. To be exact, this is not considered self-excitation, but the phenomenon is similar to self-excitation. The output generates a signal that is unrelated to the input. Because we are in an environment covered by electromagnetic waves, there are 50Hz and 100Hz power frequency interference, hundreds of Hz medium wave broadcast interference, several MHz short wave interference, tens to hundreds of Hz TV broadcast and FM broadcast interference, and wireless communication interference around 1GHz. If the circuit design is not well shielded, the interference will naturally be introduced into the circuit and amplified. If the circuit has self-excitation, we should first determine what caused it. The first type of self-excitation occurs when the op amp is used in a closed loop and the gain is low. Generally, it can only occur when the gain is less than 10. In fact, this kind of self-excitation is the easiest to solve. Just choose the op amp correctly. For some high-speed op amps, the manufacturer's manual will indicate the minimum closed-loop gain. On the contrary, the latter two situations occur under high gain conditions, which is very important and can accurately determine the cause of self-excitation. Relatively speaking, the latter two self-excitations are more difficult to solve. I am not modest to say that only with certain experience in analog circuit design can the above situations be avoided. The basic principle is to increase the area of the ground wire as much as possible. In the vicinity of the operational amplifier power supply pin, a high-frequency decoupling capacitor must be added nearby, and high-frequency shielding and other methods are used to eliminate self-excitation and reduce interference. The difference between operational amplifiers and comparators: Operational amplifiers and dedicated comparators are more common in the control circuit of the inverter main control board. I don’t need to describe its role. People in this industry know it better than me. 1. The operational amplifier can be connected to become a comparison output, and the comparator is a comparison. So why are the two products sold separately on the market? What are their similarities and differences? 2. The comparator output is generally OC for level conversion; the comparator has no frequency compensation, and the SLEW RATE is larger than the same level operational amplifier, but it is easy to self-excite when connected to an amplifier. The open-loop gain of the comparator is much higher than that of a general amplifier, so the small difference between the positive and negative ends of the comparator causes the output end to change. 3. Frequency response is one aspect. On the other hand, when the op amp is used as a comparator, its output is unstable and may not meet the requirements of the subsequent logic circuit. 4. The comparator has an open collector output and is easy to output TTL level, while the op amp has a saturation voltage drop, which is inconvenient to use. The differences between operational amplifiers and dedicated comparators can be divided into the following points: 1. The comparator has a fast flip speed, which is about the NS order of magnitude, while the op amp flip speed is generally the US order of magnitude (except for special high-speed op amps). 2. The op amp can input a negative feedback circuit, while the comparator cannot use negative feedback. Although the comparator also has two input terminals, the in-phase and the inverting input terminals, because it does not have a phase compensation circuit inside, if negative feedback is input, the circuit cannot work stably. There is no phase compensation circuit inside, which is why the comparator is faster than the op amp.