I have a circuit with an op amp gain stage that needs to be powered by an assymetrical supply. The rails are +5 and -15VDC and there is no easy way to change this.
What method can be used to DC offset the input signal in a _negative_ direction so as to be centered on minus 5V, but without placing a cap in the signal path.
It would probably be useful this had a trimmer adjustment to fine tune the result.
Put the positive input of the op amp on the desired voltage (with a small trimmer) and use the same overall negative feedback for the whole frequency range from 0 - whatever the highest frequency of your signal.
You've not told us about the nature of the signal, ie what parameters matter. Possibles include: resistor to -15 rail diode string + bias R zener in nfb with bias R transistor V shift stage ac coupling with dc restoration etc
A differential op amp can be configured with an output referenced to any voltage within the supply range.
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In the simplest method, refering to figure 5, simply return R4 to the required output reference. R1=R3, R2=R4. Gain equal to R2/R1.
The inputs are connected to the two nodes that represent the signal being amplified. Normally either V1 or V2 would be the input signal ground. Impedances around the input nodes should be
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There will be a resistor from the negative input to ground. On paper, move the ground connection to +1.25V, note how things work.
Now take that resistor and split it into one going to ground and one going to +5V, with the junction at the op-amp negative input. Jigger the values until the parallel combination of the resistors is 1/4 the feedback resistance, and the unloaded voltage is +1.25V.
If you can use an op amp that drives output close to the (+5) rail, you don't need to apply offset (or capacitor-couple on the output) to get 10Vpp. Many op amps tolerate filtered but unregulated power, up to 40V, so if there's an unregulated supply (other than the +5V) you could make use of that.
If you DO have to apply a DC offset, summing amplifier tricks will do it. But, the result is no longer a ground-referenced signal, you have to take care of level shifting at the output as well as at the input.
Most Op I have worked with can handle the - rail being higher than the other.. It's the magic dust inside that makes it happen.
So using the +5 and -15 for your supplies to the Op-amp should not give you any trouble obtaining a symmetrical output, if that is what you are really shooting for? your numbers are kind of confusing.
You issue is that 5 Volts is the most you'll get on the output with out going into saturation on the + side of output. Also, you need to pick a op-amp with a rail output feature, many will not reach the supply rail. So at this point, you know you need an OP-AMP that can do Rail to Rail, at least on the output.
You specified 2 volts input bipolar ? To me that is a +/- signal. You also specified 10VPP on output? That can be confusing because if you are looking at 10VPP on a scope at center line then you get 5 volts on the + side and 5 volts on the - side of the screen = 10VPP ? If you expect this to be unipolar on the output then a 10VPP yeilds
10 volts and if this is on the + side of the output, you'll only reach
5 volts+, but you'll be able to do this on the - side output!
So, assuming what you meant was, a 2V +/- input with a 5V +/- output then you can use the standard non-inverting Op-amp configuration, that would be the one where you select two R's in the - feed back to set your desired gain. THe + input will be the input to use from your signal.
The end results would be an output with a +/ 5 Volts which equals 10VPP on the scope at center line. Just a gain point of your input..
If you are truely trying to move the output into a unipolar case, on the + side, 5 volts for the rail is not enough.
I think maybe we need a better lay out of the input voltage swing and output voltage swing to know better.
We still don't know enough about your app to know which is best for you. A common option is resistors from the supply rails to get your chosen offset, a cap from there to ground to reject psu noise, and connect the point to the + input.
I presume you mean centred on -5v. Otherwise your -ve rail isn't big enough.
Now that we finally know you have an audio signal... An easy option is to put 2 Rs in series from audio in to -ve rail, with a nice big cap from their junction to ground. The R from audio in to cap needs to be picked according to the audio signal's resistance. We can't specify it without a circuit.
Applying a shift at the op amp inputs doesn't solve the problem. Your 'shift' means the op amp output is no longer ground-referenced, so a load (from output to GND) will be DC biased.
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