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OP-Amp

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dr.power

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Hi guys,

I would like to know if OP-Amps have a starting threshold voltage for their inputs? I mean is there any threshold for the inputs so that the op-amp starts to amplifying?

Somebody asked me if Tl07X op-amp is able to amplify a 500uV by a gain of 1000 or not. I do not sure what parameter I must take a look in the datasheet to answer the said question.

Any help plz?

Thanks a lot
 
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It may depend on the op-amp. AN-106 from Analog states that the inputs of the OP-50 can distinguish 0.3 uV.

analog-op-amp-apps-an106-png.58924


John
 

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All opamps have an allowed input voltage range called "common-mode input voltage range". It is listed on the datasheet.
For the TL07x opamp it is typically from 3V more positive than its negative supply to all the way up to the positive supply but some must have their inputs from 4V more positive than the negative supply to 4V more negative than the positive supply. If you keep the input voltage near half the total supply voltage then it works fine.

It has a typical gain of 200,000 from DC to 15Hz then its gain drops at higher frequencies like most opamps. At 1kHz its typical gain is 3200.
It can easily amplify a 500uV signal but it can amplify with a gain of 1000 only up to 3200Hz. Two of them can each have a gain of 31.7 for a total gain of 1005 up to 100kHz.

It might be too noisy to give a good signal to noise ratio with an input of only 500uV. It has much less noise than a lousy old 741 opamp but there are opamps available with much less noise.
 
Thanks for all inputs,

Guru good infos, thanks for them,
But actually I would like to know the MINIMUM input voltage in which the op-amp STARTS to amplification by the gain the Rf and Ri determine, Is it clear?
I do not know what parameters I should follow in the datasheet to find the said voltage...

What I the common mode input voltage? Is it something like common mode rejection ratio or something so?

Thanks
 
The datasheet for all opamps spec's the allowed "common-mode input voltage range".
For a TL07x the datasheet assumes a plus and minus 15V supply. The typical allowed input voltage range is from -12V to +15V. Some of them are minimum spec and have an allowed input voltage range of -11V to +11V.

If you use a single polarity 9V supply then the typical allowed input voltage range is from +3V to +9V. Some ICs will be minimum spec and have an allowed input voltage range from +4V to +5V.

Here is part of the datasheet:
 

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What Is the "common-mode input voltage range" really??

It seems that you misunderstood me. I want to know for instance if I am able to have a signal input AS LOW AS say 1nV yet expect the op-amp to normally amplify it? what about 1pV input signal and so on? How can I realize this in the datasheet?

Thanks
 
The common-mode input voltage range is the input voltage range where the inputs work properly.
A TL07x and other older Fet-input opamps have a problem called "phase inversion" where the output suddenly goes high if an input voltage is lower than the minimum allowed voltage.

Of course an opamp will amplify a tiny signal if its input voltage is within the allowed input voltage range but with a high amount of gain then high frequencies will not have as much gain as DC and low frequencies as is shown on a graph in the datasheet. Also resistor noise and opamp noise may be noticeable when the gain is high.
 
Then aside from the bandwidth problems is An Op-AMP like a TL072 able to handle a dc or an ac input signal as low as pico volts or even less?
 
A problem with a low signal level and high DC gain is the input offset voltage of the opamp (typically 3mV but could be up to 10mV for a TL07x) will also be amplified and cause saturation of the output of the opamp unless it is manually nulled with a trimpot.
 
it is an ANALOG amplifier, there's really no "threshold" voltage. it will amplify whatever input is there. if the input is below the noise floor however, the actual signal will be difficult to tell from the input stage noise. TL072, (082,etc...) are relatively low noise devices, so a 500uV signal probably will not be a problem to amplify. you didn't mention what the source impedance is, and that makes a big difference in noise levels. avoid large value resistors (if you can), as they generate noise that's proportional to their resistance. use metal film resistors. also make sure your power supply is regulated and well filtered. with signal levels smaller than 1mV, you might want to look up "guarded inputs", which is not exactly the same as shielding, but can be better at reducing noise and such at the input of a high gain stage.

to minimize offset errors, terminate both inputs to ground through equal resistances (actually the noninverting input termination should be the same value as the inverting input termination in parallel with the feedback resistor. this balances the input bias currents and minimizes offset). so if the inverting input is terminated with a 1k resistor and the feedback resistor is 1Meg, the termination of the noninverting input should be 999ohms.
 
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