Rail to rail opamp problem

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#1
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Hi,

I got this signal coming from a source. Vpp = 0.65V.

This signal feeds the non inverting input of an opamp (TL974).

I supply the opamp with +5V and -5V.

The circuit is this one :

I want to amplify the signal with a gain of 3, so I choosed

Rf = 2000R

R2 = 1000R

So that G = 1 + 2 = 3

The problem is that I don t get an AC output signal but this :

Notice that when I feed the opamp with a positive AC signal (with amplitude from 0 to 0.65V rather then -0.325 to +0.325),  I get the amplified positive AC signal at the output (with amp from 0 to 2V).

 

 

Last Edited: Mon. Mar 16, 2020 - 09:35 AM
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More detail required.

 

Please provide a full schematic - including power supplies, pin numbers, etc.

 

Details of the source.

 

Some good, clear, in-focus photographs of your setup would also help.

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But the immediate thought is that the signal isn't attached to the same ground as the opamp input. Or rather, looking at your example diagram (which is really insufficient in itself), you are using the negative supply to your opamp as the ground to your signal.

 

Your power supply, if it has a positive and negative, should also have a midpoint voltage, AKA 0v - use that to go to the ground side of the signal source.

 

Alternatively, capacitively couple the signal.

 

Neil

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Show the setup, especially, how you supply the negative supply voltage to the opamp (you do want the opamp to put out a negative voltage for 180degrees, correct?)

A high frequency opamp like this certainly requires caps on the supply pins---do you have them?

 

Also, NEVER hook a sig generator directly to an opamp pin...include at least 1K ohm in series (use 1k).

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Last Edited: Tue. Mar 17, 2020 - 01:17 AM
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jowell88 wrote:

I want to amplify the signal with a gain of 3, so I choosed

Rf = 2000R

R2 = 1000R

So that G = 1 + 2 = 3

 

Thats not the formula for gain:

 

Av = (Rf / R2) + 1

 

In your case, your math worked, but if you use other values it will not.

 

 

As others have said use a 1K resistor in series with your input, place a 10 resistor to ground.  As an added precaution against the input being offset add a 1uF non-polarised capacitor in series with the input to remove any DC bias from teh signal source.

 

JIm

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It appears to be saturated at the upper rail. That suggests one of the following:

 

(1) the most negative input voltage is higher than 5V/3 = 1.67V.

 

(2) Rf is not properly connected

 

(3) R2 is shorted (negative input is grounded)

 

(4) No or improper negative supply voltage.

 

JIm

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Thanks for you replies.

 

I did this and it's working :

 

I wasn't using resistors before. I found the circuit on the net after typing (ac coupled amplifier).

I changed the values of the gain resistors to my needs.

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That's a good start...however, do you always only want to amplify AC signals?  Your post is a bit vague.

Don't forget caps on your power supply pins!!!!

 

Add a gnd symbol & wire at the in & out so you can denote your in/out signals are referred (relative) to gnd

 

Try to avoid drawing any 4-way dotted connections...use two 3-ways instead...it is then impossible to confuse or accidentally overlook a dot....doing so can have dire consequences & wreak havoc when a technician overlooks it & you can't figure out why the elevator keeps going.  Dots tend to get obscured by dust, fading, or making copies of copies.   A 3way connection will never suffer.

 

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Tue. Mar 17, 2020 - 09:55 PM
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Yes I only want to amplify the AC signal.

Ok for the caps, the two 3-ways instead of 4-ways, and the GND.

 

Thanks!

Last Edited: Fri. Mar 20, 2020 - 08:57 AM