# Help needed!

Discussion in 'General Electronics Discussion' started by CiaranM, Sep 20, 2012.

1. ### CiaranM

74
1
May 19, 2012
Hello, please look at the two pictures provided and try to answer my question...
One of the circuits is a three-rail circuit. I've used an op-amp circuit as an example. The other is a two-rail circuit. I've used a transistor circuit as an example.

Question:
How do I connect each circuit together? (I'm using batteries.) I'm not sure whether the 3-rail's '0V' should be connected to the 2-rail's '0V', or whether it should be '-V' to '0V'. See, if I connect 0V to 0V then the audio grounds are connected, which is good. However, the -V part of the battery would not be connected to the 2-rail circuit that way, so that circuit wouldn't get any power... do I connect the 3-rail's -V to the 2-rail's 0V, and connect the audio ground on the 3-rail side to -V?

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2. ### duke37

5,364
772
Jan 9, 2011
Nos da

Op-amp
The batteries should be connected to give a positive supply and a negative supply to the chip. The output voltage will be the invert of the input voltage and could go close to the two power lines. The input signal would normally be somewhere near the 0V rail

Transistor
Here, there is a large offset voltage between the input and output so to get a voltage somewhere near the op-amp 0V you could use the full battery voltage on the transistor circuit. Input will be referenced to V-.

If you connect via a capacitor (AC only) then such constraints do not apply and I would connect the 0V lines together.

3. ### CiaranM

74
1
May 19, 2012
so are you saying that, for an audio application, the op amp's +V goes to the transistor's +V and the op amp's 0V is connected to the transistor's 0V via a capacitor?
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4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
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Jan 21, 2010
Vs to +V

0V to 0V

Output of transistor amplifier to input of op-amp via a capacitor

5. ### donkey

1,300
56
Feb 26, 2011
steve, Just for my knowledge, what size cap should be used?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
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Jan 21, 2010
Well, that does depend somewhat on the impedances of the input to the op-amp (where no values are shown) and the output impedance of the common emitter amplifier.

I would probably pick a value between 1uF and 10uF. Lower value capacitors would start attenuating low frequencies -- it's easier to use a larger value where this doesn't become significant.