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Why 741 does not amlifies on period of low frequencies

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a.dehqan

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Hello , Good day all ;

question is on some posts later .

Regards dehqan
 

Why there is no gain in low frequencies e.g 15 hz ?

If you have a circuit, just attach it to the post. I cannot view your external link.

Keith
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

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Why there is no gain in low frequencies e.g 15 hz ?

I think that the problem is due to things you have not shown. What are the power connections to the opamp? What size voltage are you feeding in. My guess is that you haven't taken account of the input voltage range of the opamp.

Also, what are you trying to achieve? That is a strange circuit!

Keith
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

Vi = 0.1 v p-p
Click to expand...
you are doing simulation, AC analysis ?
i think the closed loop gain is
-G/[R4/R1+(1+R2/R1)(1+R4/R3)+GR4/R3]=-G/(249+G), isn't it?
what's the open loop gain of your amplifier at low and high frequencies, respectively?
 

Why there is no gain in low frequencies e.g 15 hz ?

The gain simulates as unity (-1 actually).

Keith.

Explanation:

Current through R1 = zero because both ends are at zero volts.
Therefore current through R2 = zero if we ignore opamp input current.
So, you are left with R3 being the feedback and R4 being the input resistor. Gain = -R3/R4

The extra resistors seem a bit pointless
:roll:
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

--------------------------------------------
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

I don't really understand why you think the gain is anything other than -1. It simulates that way and that is what I would expect (as I explained, R1 and R2 do nothing useful). You will get a very low bandwidth due to the effect of R1 and R2 forcing the opamp to amplify a severely attenuated signal on it's inverting input. If you remove R1 and short out R2 you will get the same gain but a considerably higher bandwidth.

What is the thinking behind the circuit? It is very strange.

Keith.
 

Why there is no gain in low frequencies e.g 15 hz ?

Sounds like total confusion...

For clarity, you may want to tell exactly which quantities you are looking for. In your initial post, you spcecified gain as Vo/Vid. Vid isn't present in the schematic however, so it should be assumed, that you meant Vi. Apparently you didn't.

As an additional remark, I also understood, that the closed loop gain, as given by Keith would answer your question. It should be noted however, that the gain of -1 is achieved only up to a few kHz, above this frequency, it's limited by the OP open loop gain.
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

--------------------------------------------
 

Why there is no gain in low frequencies e.g 15 hz ?

Your circuit doesn't seem to bear any relationship to the graph you have shown. Your circuit does not measure open loop gain.

You will have to clearly explain what you are measuring and how if you want some help.

Keith
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

Hello ,

Sorry , circuit is this like ,
Vcc=-+15
AC
question , why op-amp 741 does not amplify at low frequencies like 15 HZ ?
It amplifies at f=200 or 300 and upper .

Regards
 

Why there is no gain in low frequencies e.g 15 hz ?

There must be something wrong with what you have built. There is no reason why the circuit would amplify high frequencies and not low.

Keith
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

Hello

No , This question is posed by my master that has PHD in electronics .
So question is ok ,

Regards
 

Re: Why there is no gain in low frequencies e.g 15 hz ?

No , This question is posed by my master that has PHD in electronics .
So question is ok
Click to expand...
Wow! Impressive.

Unfortunately, you didn't manage to tell a clear question. You posted different circuits and unrelated diagrams, none of them supporting the assumption formulated in the thread title.
 

Hello

Clear question and it's related circuit is posed in 2 post before , if you need another info ,tell me .thanks
also other wrong posts have been edited by me.

Regards
 

It has all been said. The latest circuit is different from the previous ones only by a different gain. Now it has A=-10. This gain is achieved from DC up to about 50 kHz (for 741). Of course also for 15 Hz. So if the "does not amplify" claim is related to this circuit, it can be easily disproved. I guess, that you didn't yet perform experiments with similar OP circuits, otherwise you would know.
 

Question is about in AC not DC .
 

FvM said:
I'm out of this discussion now, I fear, you don't understand the very basics of electronics.
Click to expand...

True. There is nothing anyone can do to solve this problem.

Keith.
 

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