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  #1  
Old 09-27-2011, 08:05 PM
MatheusLPS MatheusLPS is offline
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Default TL071 do not working with single supply?

So, I am making a little project that I need to compare a 25Khz triangle wave with a 60 Hz sine wave.

Everything is working fine if I use symmetrical power supply for the TL071.

Look:

http://i.imgur.com/TjXAN.jpg

And with single supply:

http://i.imgur.com/wPgR6.jpg

Why is this happening?

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Old 09-27-2011, 08:15 PM
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Bill_Marsden Bill_Marsden is offline
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You need a bias, which is equivalent to a split power supply. When you get too close to one power supply or the other a phenomena called phase inversion happens, where the phase of the signal that is too close to the power supply rail is abruptly inverted. It is common for FET type op amps.

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Creating a Virtual Power Supply Ground
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Old 09-27-2011, 08:16 PM
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Hello,

In a single supply circuit the signals can not go negative.
You will have to create an offset on the AC signals, in such a way that they can not go negative (about 1/2 Vcc).

Bertus
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Old 09-27-2011, 08:44 PM
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The TL07x, TL08x, and LF35x series of opamps cannot "see" within about 3v of the negative supply rail, nor within about 1.5v of the positive rail. You will have strange results if you try to get the inputs or outputs to go closer to the rail than that.
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Old 09-28-2011, 12:47 AM
MatheusLPS MatheusLPS is offline
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The output do not need to be at the rail voltage.

I will put The output on a mosfet gate driver like the IR2112.

So 5V on the output is very enough for me.

If I understood correct, i can not put AC signals into the comparator with the single suppy?

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Old 09-28-2011, 01:39 AM
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Quote:
Originally Posted by MatheusLPS View Post
The output do not need to be at the rail voltage.

I will put The output on a mosfet gate driver like the IR2112.
Just logic levels won't work very well for those inputs. You'll likely need to exceed 10v for the logic input when high.

Quote:
So 5V on the output is very enough for me.
Nope.

Quote:
If I understood correct, i can not put AC signals into the comparator with the single suppy?
The TL07x, TL08x series are opamps, not comparators.

If you need a comparator, then use a comparator.
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Last edited by SgtWookie; 09-28-2011 at 01:52 PM. Reason: Typoe correction
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Old 09-28-2011, 02:08 AM
MatheusLPS MatheusLPS is offline
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Ok, I understood now....about the comparators....

Can you sugest any one?

One that can drive at least 25khz.. and is easy to find.

I am from brazil and need to order every component from internet.

It is better that the comparator is on stock on the www.farnell.com.br.

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Old 09-28-2011, 03:26 AM
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LM311:
http://www.farnellnewark.com.br/ciam...8,4442493.aspx
These are single comparators. They have open-collector outputs.

LM293:
http://www.farnellnewark.com.br/cico...9,4442493.aspx
These are dual comparators with open-collector outputs.

LM339:
http://www.farnellnewark.com.br/cico...3,4442493.aspx
These are quad comparators; they also have open-collector outputs.

The LM293 and LM339 can only sink up to about 6mA until their output saturation voltage starts going up. I'll suggest no more than 5mA. So, if you are using a 12v supply for the digital side of your IR2112, then you will need to calculate 12v / 5mA to get the approximate value of your pull-up resistor; 2.4k Ohms.
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Old 09-28-2011, 05:00 AM
MatheusLPS MatheusLPS is offline
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I will simulate it here and post the results...

After that, i will mount the circuit on a bread board an test it.

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