How to choose an op-amp?
Op-amp is the cornerstone of the whole analog circuit design, and choosing a proper amplifier is crucial to achieving the system design specifications.
1. Op-amp supply voltage size and mode selection;
3.Op-amp feedback, i.e. VFA (voltage feedback op-amp) or CFA (current feedback op-amp);
5. Slew rate size, which determines the full power signal bandwidth;
6. Offset voltage and offset current selection;
7. Offset voltage drift with temperature, i.e. ΔVoffset/ΔT size;
8. Op-amp input impedance selection;
9.Op-amp output drive capability size selection;
10. op-amp quiescent power consumption, i.e. ICC current size selection;
12. op-amp drive load stabilization time.
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In the design of switching power supply analog circuit, some people simply do not know how to choose the op-amp, what is on hand, maybe you have done so 100 times, are lucky to succeed, but the 101st time what will happen? Some other people are just the opposite, holding the five or six original information to flip around, the result is good to find the dream girl, and can not buy.
Not only do you recommend some commonplace op amps, you can definitely buy them and adapt to most occasions.
1. speed requirements are not high, or DC amplification.
LF441 (single), LF442 (double), LF444 (four), TL084 (four)
(The above op-amp for JFET input, impedance is very high, do not need to consider the impedance balance of the input)
OP07 (single, high-precision, with zero terminal, but the speed is particularly slow, for DC amplification is good)
2. relatively high speed, audio range, multiples of not more than 100: the
LF356 (single), LF353 (double), LF347 (four), TL074 (four)
(The above op-amp for JFET input, impedance is very high, do not need to consider the input impedance balance)
OP27 (single, high precision, with zero terminal, faster than LF356)
NE5534 (for audio amplification, very good sound quality, but low input impedance)
OP37 (unit frequency response 50MHz, but must not be used as a follower! In the closed loop gain of less than 5 will self-excitation)
4. low voltage or single power supply
Recommend the use of Maxim products
Other special occasions, such as video amplification, ultra-linear amplification, low drift and other requirements, or to look up on the Internet said
"The op-amp you solder on the 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.
Do not expect the output voltage of a general-purpose op-amp to reach the supply voltage, even if your load resistance is 10M. The output voltage of a general-purpose op-amp
The peak-to-peak value of the general-purpose op amp output voltage is 1~3V different from the power supply.
2. common mode input voltage range
Do not let the potential of the input of your op-amp is very close to its supply voltage, otherwise you will be confused. For example, you choose the
LF347 op amp (most JFET op amps are similar), supply voltage of positive and negative 12V, positive input potential of -11V, negative input of -11.5V, you guess
What will the output be? Perhaps you guessed wrong, is -10V. This is the result of your use of the common mode voltage range beyond. Of course, if you switch to LM324,
there is no such effect. Fortunately, now Maxim and NS have introduced Rail to Rail op amps, their common-mode voltage range and power
3. output voltage swing rate SR
If you are using an op-amp to amplify high-frequency large-value signals, do not ignore the SR parameter, which represents the maximum change in output voltage per microsecond. For example, the unit bandwidth of uA741 is 1MHz, SR = 0.7V/us, if you connect him as a follower (gain = 1), at this time, if you input amplitude of -5V ~ +5V, frequency of 200KHz square wave, then, the output results must make you disappointed, his output is actually a strange amplitude of only about 2V triangle wave.
1. for low potential amplification line, but also take into account the detuning, temperature drift and input noise.
2 for high-precision lines, should pay attention to the common mode rejection ratio, generally speaking, the high common mode rejection ratio of the OP its linearity is better.
3. pay attention to the input resistance, bipolar OP generally in a few hundred K to tens of M.
Op-amp self-excitation may be caused by a variety of:
For example, OP37 and other op amps, in order to improve the high frequency response, its compensation is small, when the feedback is deeper, self-excitation phenomenon will occur. By measuring the BODE diagram of its open-loop response, we can see that as the frequency increases, the open-loop gain of the op-amp will drop, and if the phase lag is more than 180 degrees before the gain drops to 0db, the closed-loop use is bound to self-excitation.
2. power feed-back self-excitation
From the analysis of the internal structure of the operational amplifier, it is a multi-stage amplifier circuit, the general op-amp is composed of more than 3 stages of circuit, the first stage to complete the high gain amplification and potential shift, the second stage to complete the phase compensation function, the last stage to achieve power amplification. If the internal resistance of the power supply to the op-amp is large, the power consumption of the final stage will cause fluctuations in the power supply, which will affect the work of the front stage circuit and be amplified by the front stage, resulting in greater fluctuations in the rear stage circuit, and so on a vicious circle, resulting in self-excitation.
To be exact, this is not self-excitation, but the phenomenon is similar to self-excitation. The output produces a signal that is unrelated to the input. Because we are in an electromagnetic envelope of the environment, there are 50Hz and 100Hz industrial frequency interference, hundreds of Hz of medium-wave broadcast interference, several MHz of short-wave interference, tens to hundreds of Hz of television broadcast and FM broadcast interference, about 1GHz of wireless communication interference, etc.. If the circuit is not well shielded, the interference will be introduced into the circuit and amplified. If self-excitation occurs in a circuit, the first step is to determine what causes it. The first type of self-excitation occurs when the op-amp is used in closed loop and the gain is low, usually only if the gain is less than 10. In fact, this self-excitation is the best solution,
For some high-speed op amps, the manufacturer's manual will specify the minimum closed-loop gain. In contrast, the latter two cases occur in the case of high gain, which is very important to accurately determine the cause of self-excitation. Relatively speaking, the latter two self-excitation is more difficult to solve, I am not modest to say that only with a certain degree of experience in analog circuit design, it is possible to avoid the occurrence of the above situation. The basic principle is to increase the area of the ground as much as possible, in the op-amp power supply near the pin, must be near the increase in high-frequency bounce capacitors, the use of high-frequency shielding and other methods to eliminate self-excitation, reduce interference.