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Trying to create an attenuated inverted circuit...

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si2030

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Hi there,

I am having some trouble with this.

I have a sin wave signal, very slow, 0 to 30 volts which I want to invert and make 0 to 3 volts... how do I do this?

Tried an opamp approach (Used LF353 as I have it already) as well as transistors in spice but I am not very good with either..

Hoping someone might give me some direction on this...


Kind Regards

Simon
 
I have something working...

Here is a picture of the circuit and simulation as its working...

I am not sure what I have done but it works... could someone explain how this biasing works..

..this is a bit of a fluke and would be nice to know how to tweak this intelligently.

Simon

View attachment 61636

Further, the file is called single rail amplification which is where I started... Ideally I would like to have the op amp work on a single rail rather than +/-15Volts..
 
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While the circuit you posted looks good on the surface it is not really a good design practice. Give this link a read Place the focus on Attenuation starting around page 6. You will see while your circuit may look good in a simulation it is not a good idea as well as a good way to get the results you want.

Ron
 
This might not be the best way to do it, but here is the way to calculate what you were trying to do.
 
Hi there, thanks for the responses. The problem I am facing is that the input is not necessarily an AC input. Its a variable input but might stay on a particular value for a while... so DC. This then rules out the opportunity of adding a cap in series to remove the DC component..

What might I do in this instance?

Further I did calculate the voltage value of Vso and it came to 5.45V. The outcome of this is that the output is sitting 3 volts above ground instead of on it... (bottom set of traces is for the left side and top set of traces relates to the right side).. not sure I have done this correctly.

View attachment 61642
 
Hi there, thanks for the responses. The problem I am facing is that the input is not necessarily an AC input. Its a variable input but might stay on a particular value for a while... so DC. This then rules out the opportunity of adding a cap in series to remove the DC component..

What might I do in this instance?

Further I did calculate the voltage value of Vso and it came to 5.45V. The outcome of this is that the output is sitting 3 volts above ground instead of on it... (bottom set of traces is for the left side and top set of traces relates to the right side).. not sure I have done this correctly.

View attachment 61642
An op amp's output cannot be lower than its negative rail nor higher than its positive rail. If you want the output to be able to swing above and below ground, you will need two supplies.
 
Hi there,


If you only have a single supply and you need a DC plus and minus input to DC plus and minus output then another option is to NOT ground reference neither input nor output, but reference both to say 1/2 the supply voltage (Vcc/2). You apply the input between the synthetic ground and circuit input and you take the output from between the synthetic ground and the output. This makes the output appear to swing plus and minus even though referenced to the true ground of the power supply it doesnt.
One of the key points to think about though is how much current you need on the output. If it is a small amount a resistive voltage divider may be all you need to make up the synthetic ground, but if you need more you may have to use a power supply splitter circuit, which can sometimes be made as simple as from another op amp section with the output becoming the new synthetic ground.
For example, an op amp voltage follower with non inverting input biased to Vcc/2 (one half Vcc) creates a basic synthetic ground on the output terminal.

If you have other parts of the circuit external to this circuit you probably need to reference them to the new synthetic ground also. This technique does reduce the total swing to plus Vcc/2 and minus Vcc/2 but at least you dont need two supplies.
 
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