[Multisim] problems and solutions of Multisim Simulation "zero crossing comparator"

Problems arise

A few days ago, I wanted to make a sine wave to square wave PCB, Simple, of course “ Zero crossing comparator ” Realized . Painting PCB Before , I first drew the circuit according to the traditional zero crossing comparator , And then use Multisim Simulated it , So there is a problem ：


The voltage at the output has reached an astonishing thousands of volts , And did not play the role of comparator , The output is still a sine wave ！

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Problem analysis

After checking some information , To realize the root of the problem ：

The ideal op amp is without negative feedback , Its open-loop gain is close to infinity .

Multisim This software operates completely according to the ideal operational amplifier during simulation , The result is that the output voltage calculated by the software is very, very large ！
But this situation does not appear in the actual circuit , One 5V It's ridiculous that the double ended power supply can produce thousands of volts output .

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The solution to the problem

The solution is to add a “ Bidirectional zener diode ”.

The function of bidirectional zener diode is ：
Working in reverse breakdown state , No matter how much the current changes , The voltage at both ends can always be maintained in the state just when it breaks down , And remain the same .

natural “ Zero crossing comparator ” The design tutorial will not emphasize the need to add regulators , I will only emphasize some properties of the operational amplifier in the comparison state , So it will make Xiaobai encounter such problems for the first time ！ But this regulator is very important , Although in the actual circuit, there will be no abnormal situation of thousands of volts without a regulator （ The output voltage is powered by the operational amplifier ±Vcc It's limited ）, But there will be problems in the simulation .

Now modify the circuit ：

Now the output is the square wave of the normal voltage value ：

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Summary of experience

In the future, the design work is in the positive “ Compare ” State op amp , It is best to add “ Bidirectional zener diode ”.