Electronics > Beginners
Negative version of LP2951 LDO
Zero999:
--- Quote from: ZeroResistance on September 11, 2019, 01:12:41 pm ---
--- Quote from: Zero999 on September 11, 2019, 12:56:51 pm ---
Just AC couple all of the signal paths and bias the amplifiers to work at half the supply voltage.
The volume control should be between the two amplifier stages. I've converted the second one to a unity gain buffer and put the volume control before it.
--- End quote ---
This is still great! but would it still count as an virtual earth mixer?
The earth point has now shifted to 1/2 VCC.
From what I understood about the concept of a virtual earth mixer, is that if all the input signals see a true virtual earth. There won't be any crosstalk between them.
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I don't know what you mean by virtual earth mixer? Are you talking about using an op-amp as a rail splitter?
Note that ideally the potentiometer should be AC coupled anyway, to cut down on noise when it's adjusted, but in this case the DC current should be no more than the op-amp bias current and the gain is unity, so it shouldn't be too bad.
ZeroResistance:
--- Quote from: Zero999 on September 11, 2019, 03:16:26 pm ---
I don't know what you mean by virtual earth mixer? Are you talking about using an op-amp as a rail splitter?
--- End quote ---
I read about it here https://sound-au.com/articles/audio-mixing.htm#s3
Zero999:
--- Quote from: ZeroResistance on September 12, 2019, 12:18:41 pm ---
--- Quote from: Zero999 on September 11, 2019, 03:16:26 pm ---
I don't know what you mean by virtual earth mixer? Are you talking about using an op-amp as a rail splitter?
--- End quote ---
I read about it here https://sound-au.com/articles/audio-mixing.htm#s3
--- End quote ---
That's got nothing to do with whether the circuit is operating from a single supply rail, a bipolar supply or a virtual ground. Both of the circuits I posted use the same virtual earth principle, as described in that article. The difference is that first one I posted was AC coupled and the second one DC coupled. In the AC coupled design, the DC is blocked, so only the signal matters, the fact that the negative input is fixed at half the supply voltage vs 0V doesn't matter.
The circuit works by fixing the inverting op-amp input at a constant DC voltage. In most cases it's the same as 0V, because it's what the non-inverting input is connected to, hence virtual earth, but it doesn't really matter. Take the input resistors away and you'll see that the impedance into the inverting input is very low, due to negative feedback. If a current is injected into the input, the op-amp's output will move in the opposite direction, pulling current through the feedback resistor to counteract the change in voltage due to the input current.
The circuit is a transimpedance amplifier. It converts a constant current into a constant voltage. In this instance, with a feedback resistor of 10k, 1mA of current in, will give -10V out, because the input will stay at 0V and 1mA of curenet will give a drop of 10V across the resistor.
https://en.wikipedia.org/wiki/Transimpedance_amplifier
Kirchoff's current law states that the currents through all the resistors add together, at the node where they're all connected. Suppose you have 5V at two inputs, with 10k input resistors. The current through each resistor will be 0.5mA, giving 1mA in total, which also flows through the feedback resistor, giving an output of -10V.
Bassman59:
--- Quote from: ZeroResistance on September 09, 2019, 12:55:48 pm ---I'm looking for a negative output LDO, 100ma.
I found the positive version LP2951, however I'm unable to locate its negative version.
Like the LM317 has a LM337 version, I was expecting the same for the LP2951.
Am I missing something here or the negative version just does not exist for the LP2951?
--- End quote ---
Everyone else is pretty much on the money about negative LDOs.
But I should ask: does your circuit need an LDO? I mean, I understand why you might want one, but unless you're really trying to minimize heat dissipation in your regulators, the 317/337 will do ya fine.
That said, I just did a power supply that needed ±15 V, +5 V and +3.3 V, and I used LT1963A for the positive rails and LT1175 for the negative rail. And yes, the 1175 is frightfully expensive.
ZeroResistance:
--- Quote from: Bassman59 on September 13, 2019, 01:19:00 am ---
Everyone else is pretty much on the money about negative LDOs.
But I should ask: does your circuit need an LDO? I mean, I understand why you might want one, but unless you're really trying to minimize heat dissipation in your regulators, the 317/337 will do ya fine.
That said, I just did a power supply that needed ±15 V, +5 V and +3.3 V, and I used LT1963A for the positive rails and LT1175 for the negative rail. And yes, the 1175 is frightfully expensive.
--- End quote ---
I'm limited by my input voltage that is going to be 9V dc.
Then the LM317L / LM337L would drop around 2.5V.
So that brings its down to max 6.5V at their outputs. Lets keep it 6 for giving it a little safety margin.
So now I would be powering the op-amps with +6/-6V.
The opamp is a non rail to rail type so its output would swing to +/- 2V of the supply rails.
So 6 -2 gives me 4V as the max output swing.
And I have to handle some line level inputs, which I though would be 1.2Vrms but I realised that one of the inputs would be around 4Vrms max.
Having said that I'm still game for the 317 / 337 but I say that the LP2951 was at a similar price range so wondered it would be better to go in for that, if I get a suitable negative voltage alternative to the 337.
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