Hi everyone,

I want to build a comparator circuit by using LM311 but there is a problem while applying input signal in the pin 3(-Vin) of LM311. I normally have 2 khz sinus signal which does not include clipped(picture1). When I start to the circuit, I see clipped signal(picture2).
Can anybody help me?

 

How, pray tell, are we supposed to tell you what is wrong with a circuit that you won't show?

What does it mean to "start the circuit"?

 

It means that when I give VCC to circuit

 

Let's see the schematic. Comparators normally use positive feedback to create a hysterisis. Depending on the impedance of your source, this can cause some apparent distortion.

 

thats all

 

Add a 2.2K resistor in between your input signal and pin 3. See if that improves matters.

Oh, and you need a pull-up on the output of that LM311 - it has an open collector output.

 

Thanks a lot Ylli. But now I have another problem. I want to square wave in the output. Because it is neccesary that if the input greater than 9v, output will go low. if the input lower than 6v, output will go low again. In that case, output will be high between 6V and 9V. Also can you tell me how do you know putting 2.2k in there? thanks a lot

 

When I put a pull up resistor in the output, the output seems like that

 

Use a 10kΩ resistor to pin 3 (in place of the 2.2k) and a diode (preferably a Schottky) from pin 3 (cathode) to ground (anode).

 

Also can you tell me how do you know putting 2.2k in there?

It isn't always required. You are direct coupling the input and it must be swinging negative. The comparator input was clamping the input signal to one diode drop below VEE (ground in your case).

 

It isn't always required. You are direct coupling the input and it must be swinging negative. The comparator input was clamping the input signal to one diode drop below VEE (ground in your case).

I did not understand. can you clarify that?

 

I did not understand. can you clarify that?

That resistor would normally be used to minimize the effect of input bias current.

In your case, you were exceeding the negative input voltage range and were forward biasing the CB junction of that input transistor. Adding the resistor limited the current and removed the distortion you saw on the input signal, but you're still violating input voltage range.

Adding a Schottky diode clamp to that input as mentioned by @crutschow would clean up the output.

 

But now I have another problem. I want to square wave in the output. Because it is neccesary that if the input greater than 9v, output will go low. if the input lower than 6v, output will go low again. In that case, output will be high between 6V and 9V.

You'll need another comparator to do that.

Since you're only interested in the range from 6-9V, you can replace the clamp diode with a signal diode in series with the input.

 

The 2.2k resistor is just equivalent to the parallel combination of all the resistors on the positive input. Compensates for offset currents and isolates the input from loading of the comparator a bit.

Looks like the square portion of that waveform is just about right. Don't know where the sine portion of that output waveform is coming from. Is your input signal going below ground? If so, the LM311 will get mad at you and do strange things.

 

Is your input signal going below ground?

The input is about 16V P-P. I had to look closely at the pictures to find where zero was for each trace:


I usually use an analog scope and have to keep track of vertical offsets myself...

 

thats allView attachment 132270

You input signal is going below the negative supply rail for the device. Internal input protection circuits are attempting to clamp the input signal to the rails.

Are you specifically attempting to create hysteresis?

Are you aware that the LM311's output is both open collector AND open emitter? You need external circuitry/connections to both pull it HI and pull it LO.

 

Are you aware that the LM311's output is both open collector AND open emitter? You need external circuitry/connections to both pull it HI and pull it LO.

Since you are taking the output from pin 7 (the collector) you need to ground pin 1 (the emitter).
And dont forget the pullup resistor from pin 7 to +12 volts.