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TL084 Op amp giving high output voltage

smijesh

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I am using TL084 TSSOP 14, as an amplifier need gain of 20(40mV DC to 800mV DC) for current sense application. But output of op amp always giving Vcc voltage.
Please advice how to tune this op amp
 

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You are violating the input CM range :

1716281944260.png


There are OpAmps that common mode to their rails (RRIO type), ones that CM to just - rail, and ones
that CM to + rail.

You can still use this part if you supply a negative V to the - rail of <= -1.5V


Regards, Dana.
 
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...or if you can accept some frequency dependency, add a capacitor between R3 and ground. Without a negative supply the output can't drop below zero to provide the required negative feedback.

Brian.
 
You are violating the input CM range :

View attachment 190946

There are OpAmps that common mode to their rails (RRIO type), ones that CM to just - rail, and ones
that CM to + rail.

You can still use this part if you supply a negative V to the - rail of <= -1.5V


Regards, Dana.
It is working when dual power supply used. Any other option to use the same op amp without dual power supply?
 
Hi,
Any other option to use the same op amp without dual power supply?
How?
* We neither know your input signal, and how it may be "manipulated"
* nor do we know the options for the output.

All we could do is guess.

In my eyes Electronics design has nothing to do with guessing, hoping and so on. If quite well follows the rules of physics and thus it´s rather well calculable. But to be able to calculate ... one first needs numbers.

Klaus
 
It is working when dual power supply used. Any other option to use the same op amp without dual power supply?
You could use a charge pump to develop - supply, or if there is a clock in system (depending on its
frequency) us a diode inverter to generate the negative supply.

Regards, Dana.
 
The TL07x and TL08x MUST have input voltages of at least 4V more positive than the negative supply. Your circuit will work with an opamp that works when its inputs are at the negative supply. See the datasheet input common mode voltage spec.
Your simple circuit will work fine with your TL084 if you add a negative supply (-4V for an old opamp or -1.5V for a new opamp).
 
Hi,

short answer: of course you can.

More detailed: it depends.
The LM324 has benefits. So for your case above it´s common mode input voltage range goes down to GND.
But it also has disadvantage. No idea whether they violate the expected operation or not.

some examples: (as raw comparison TL084 vs LM324)
* can it amplify 40mV to 800mV? Yes. But because of internal errors it won´t be exactly 800.000mV
* can it amplify 40mV 10MHz to 800mV? probably not
* can it amplify 4mV to 80mV? generally yes, but with expectable errors
* can it amplify 1mV to 20mV? I´m not sure.... at least one can expect huge errors
* can it amplify high impedance input signals? With it´s several thousand times higher input current: surely way worse
* is it low noise for high input impedance: probaly not
* is it low noise for low input impedance: probaly yes
* does it overshot on step resonse: probably worse

We don´t know nothing about your input signal and it´s impedance, same is with output. We don´t know what´s the requirements of your application.

Klaus
 
The LM324 OpAmp output cannot swing to ground :

1716462908474.png


As you can see the offset contributes significant error as well.


You need a RRIO part for the application with low offset.

What is load at output the the LPF RC filter you show on OpAmp output ?


Regards, Dana.
 
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The LM324 quad and LM358 dual are very noisy and produce crossover distortion so they should not be used for low level signals.
They do not produce high level signals above only 2kHz but produce 30kHz at 3V peak-to-peak.
Their input impedance is not high.
In your circuit, their lowest output is about 0.02V but only with very low output current (your output filter might cause a higher minimum output voltage with AC signals).
 
It may be the current sense R, it's value, negates any concern over Zin of OpAmp, that would
have to be investigated.

Crossover distortion, given output close to ground, should not be a factor, if so Bob Pease
has a good recommendation for that on the LM324, LM358 :


So in short the number have to be run to determine OpAmp choice.

But as posted earlier by several, get a good RRIO OpAmp.....


Regards, Dana.
 
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