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Analog Voltage Adder

  1. Feb 19, 2009 #1

    I've learnt about voltage adder circuits using op-amp that add two or more DC voltages.
    But is it possible to add AC voltage with a DC voltage? What i mean is can we add (for eg 2V DC with a AC of 5v peak value,50hz) so that the output is again AC with peak of 7V and frequency is 50hz
    If so How?
    Last edited: Feb 19, 2009
  2. jcsd
  3. Feb 19, 2009 #2


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    That would be more considered an amplifier. A circuit can be designed that uses a DC control voltage to control the gain of an amplifier.
  4. Feb 19, 2009 #3
    No, I don't think he's talking about an amplifier. Is this what you mean? The AC has peaks of +5V and -5V and you want to add 2V DC to get an output of +7Vpk and -3Vpk?

    What do you think you'd get if you added them exactly the same way you added two DC voltages?
  5. Feb 19, 2009 #4
    as long as you don't "hit the rails" with your summed signal, and the frequency is within the limits of your circuit, the opamp doesn't really care if it's AC or DC or any combination.
  6. Feb 20, 2009 #5
    No i'm not talking about amplifier.
    Yeah you got it right. But just a small variation. I dont want the output to be 7V and -3V, instead it should be 7Vpeak and -7Vpeak. Is that possible? because it is a sinusoidal signal and should be symmetrical.
  7. Feb 20, 2009 #6
    If the OP and only add DC voltages it would be pretty useless. The OP Amp will of course have band width limitations and range limitations.
  8. Feb 20, 2009 #7
    Then you are talking about an amplifier. You're multiplying the original signal by a factor of 1.4.
  9. Feb 20, 2009 #8
    Depends on the OP amp and what voltage you connected across the power terminals. What is the application. If you are doing signal processing you might want to use an instrumentation amplifier, if you are doing control systems you might want to use a power amplifier. Power amplifiers are often based on, on and off switching.
  10. Feb 20, 2009 #9
    Oh!. Now i get it. Thank you Skeptic2.

    @John Creighto

    The application is i'll be having a DC voltage and 2 sine waves which are 180 degrees out of phase. Now i'm required to add the DC voltage to one of them and subtract it with the other sine wave. Just like an amplifier of gain 1.4 goes the addition part,can an attenuator do the subtraction part? I dont know what an attenuator is or how it works. its my friend's idea.
  11. Feb 20, 2009 #10
    why don't you start by writing out the equation(s) you want to implement?
  12. Feb 20, 2009 #11
    Both amplifiers and attenuators work like multipliers. Amplifiers usually have a gain of more than one, attenuators have gains of less than one.

    When you add a DC voltage to an AC waveform you shift the whole waveform up such as I posted above. It's exactly the same as adding that DC voltage to every point on the waveform. Subtraction would be the same but it shifts the waveform down.
  13. Feb 23, 2009 #12
    Thank u everyone. Now i got a clear idea of the difference between amplification and shifting the waveform. I'll be needing only the amplification part.
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