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problem gain 741


teragiga
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Hi all!
This is my first post so, sorry if I would not be proficient in discussing electronics :)

I am stuck in a practical experiment, I studied how amplifier for obtaining gain work. And I created a small circuit with an astable multivibrator, and operational, 2 resistors, two batteries of 9v:
Something like this:
circuitgain.jpg
Where the settings are :
non inverting input.
Rf = 300 K, Rg=150K
And the expected gain should be : Av= 1 + Rf/Rg = 3.
Doing a simulation I obtain something like this
gain.jpg
Where the output in volts of the operational is effectively augmented.
But when I build the real circuit it doesn't happen:
The oscillator generates a square wave that I can detect with
oscilloscope and even with a tester (between -4.80v +4.80v ), the output of the amplifier is instead stuck to 7.8v.
I have done this with a LM741, and also using a LM386 (one with
single supply) and nothing.
The crazy thing is that I obtain constant 7.8 v independently from  the value of the feedback resistors (like they don't matter) and even if I completely disconnect the input.
So, it is like if whatever I do with this configuration
146px-Operational_amplifier_noninverting
the amplifier saturates to 7.8v and end of the story.
I also tried the inverting input configuration and nothing.
Where is the mistake ? Or I completely miss something in what "gain" means ?

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The LM74 needs a dual supply for this circuit to work properly.

Even if the 741 is correctly powered, the demonstration won't work as expected because the simulation program's models are probably not very accurate:

The 555's output saturates at 1.4V so with a 9V supply you can expect a 7.8V waveform.

The 741'a output a saturates at 1.4V below the supply so the maximum output voltage you can expect with a 9V supply is 7.8V.

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  • 1 month later...

... The oscillator generates a square wave that I can detect with
oscilloscope and even with a tester (between -4.80v +4.80v )...

This sounds incomprehensibly. Owing to 555's single supply there is no way its output could go negative.
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The output could be negative 4v depending on the design, or it may be negative 200mV depending on it's assembly with the current design.


We have got the only one design presented here so far. However, since we can see now strict figures perhaps it is also worth showing where the above mentioned -200mV may come from in the current design.
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