Author Topic: Negative version of LP2951 LDO  (Read 4175 times)

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Offline ZeroResistanceTopic starter

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Re: Negative version of LP2951 LDO
« Reply #25 on: September 14, 2019, 06:14:17 am »

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

By biasing the op-amp to mid supply level Vcc/2, won't the input signals see the mid Vcc supply rather that pure 0V.
And if the input signals see that would there be a chance of crosstalk between them?
 

Offline MagicSmoker

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Re: Negative version of LP2951 LDO
« Reply #26 on: September 14, 2019, 12:00:20 pm »
...
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

By biasing the op-amp to mid supply level Vcc/2, won't the input signals see the mid Vcc supply rather that pure 0V.
And if the input signals see that would there be a chance of crosstalk between them?

Yes, the (-) input will be sitting at Vcc/2 with no signal present if the (+) input is biased to Vcc/2 - ensuring both inputs are at the same voltage is the main function of an op-amp, after all - but why would that change the amount of crosstalk between mixer inputs compared to biasing the (+) input at 0V? The answer is it that it doesn't.

When the input resistors are the same value as the feedback resistor in an inverting mixer biased for single-supply operation the output of the op-amp will be sitting at Vcc/2 with no signal present, right? If you apply a 2Vpp sine wave to one of the mixer inputs then the output will go up to a peak value of (Vcc/2)+1V and a trough value of (Vcc/2)-1V; still a 2Vpp signal, except centered around Vcc/2 instead of 0V, and inverted, of course. Remember, the (+) input in a single-supply inverting amplifier is biased at a fixed voltage, so the output of the op-amp has to generate a mirror image of whatever signal is applied to the (-) input to cancel out that signal, leaving just the fixed bias voltage; this cancellation of the input signal by the op-amp output is why this configuration is said to act like a virtual earth, and it is also why the input impedance to the (-) input in this configuration is that of the series input resistor.

Since the junction of all the input resistors and the single feedback resistor is essentially earthed, every input is effectively isolated from the rest because no signal can make it past the (virtual) earth, regardless of whether that junction is at 0V or Vcc/2 or whatever.

This is kind of a tricky concept to understand, but once you get it it will make perfect sense.
 
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Offline Mp3

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Re: Negative version of LP2951 LDO
« Reply #27 on: September 14, 2019, 10:48:09 pm »
Hi, i know this might not be exactly what you want (i dont know exactly what you want) but i recently used this with a headphone amplifier which needed a bipolar power supply. It was cheap and does the job. I attached a female DC barrel plug extender to the Vin / GND and use a plain center positive 12v dc supply. Negative and positive voltage could be a little better matched (getting something like -14.75 and +14.9) but for the price it does the job.

https://www.amazon.com/gp/product/B0752CK8DK/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

I did not feel comfortable making a rail divider on this amp so i got that while i await delivery of a pcb to build a bipolar psu.
High school graduate
 
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Offline dmills

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Re: Negative version of LP2951 LDO
« Reply #28 on: September 14, 2019, 11:09:27 pm »
I'm limited by my input voltage that is going to be 9V dc.
So why are you bothering with a voltage regulator at all?

9V (Or even +-9V, so 18V is well within the ratings of most sane audio opamps (+-15 to 18V is what most pro gear goes for), so adding a regulator will just cost you precious supply voltage and headroom, run off the raw rail (well decoupled!).

Regards, Dan.
 

Offline schmitt trigger

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Re: Negative version of LP2951 LDO
« Reply #29 on: September 15, 2019, 12:01:49 am »
I have to ask,9 volts, from a battery?

If so, a linear voltage regulator is very wasteful.

And one of the primary reasons to use a regulator, ripple reduction, is not required with batteries.
 

Offline exe

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Re: Negative version of LP2951 LDO
« Reply #30 on: September 15, 2019, 09:12:34 am »
My friends, did we already established that regulation is actually required? A good opamp has quite good PSRR, so regulation might not be even needed.

Probably, regulation itself is not even needed. What needed is something to remove ripple. So, consider a capacitor multiplier as a regulator: https://en.wikipedia.org/wiki/Capacitance_multiplier . Or may be just a simple RC-filter (just don't make R too big).
 
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Offline MagicSmoker

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Re: Negative version of LP2951 LDO
« Reply #31 on: September 15, 2019, 09:40:56 am »
My friends, did we already established that regulation is actually required? A good opamp has quite good PSRR, so regulation might not be even needed.

Probably, regulation itself is not even needed. What needed is something to remove ripple. ...

Yep, see my post on the previous page...

...the regulation of the supplies doesn't need to be particularly good; just try to keep the ripple to a minimum.

...saying the exact same thing as you.  8)
 

Offline ZeroResistanceTopic starter

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Re: Negative version of LP2951 LDO
« Reply #32 on: September 18, 2019, 02:54:41 pm »
Hi, i know this might not be exactly what you want (i dont know exactly what you want) but i recently used this with a headphone amplifier which needed a bipolar power supply. It was cheap and does the job. I attached a female DC barrel plug extender to the Vin / GND and use a plain center positive 12v dc supply. Negative and positive voltage could be a little better matched (getting something like -14.75 and +14.9) but for the price it does the job.

https://www.amazon.com/gp/product/B0752CK8DK/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1

I did not feel comfortable making a rail divider on this amp so i got that while i await delivery of a pcb to build a bipolar psu.

Nice find! I see that its a switched inductor topology.  How did you filter out the noise that thing is putting out?
Currently I've opted for a switched capacitor charge pump  for generating the -ve supply rail.
 

Offline ZeroResistanceTopic starter

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Re: Negative version of LP2951 LDO
« Reply #33 on: September 19, 2019, 06:25:21 am »
I'm limited by my input voltage that is going to be 9V dc.
So why are you bothering with a voltage regulator at all?

9V (Or even +-9V, so 18V is well within the ratings of most sane audio opamps (+-15 to 18V is what most pro gear goes for), so adding a regulator will just cost you precious supply voltage and headroom, run off the raw rail (well decoupled!).

Regards, Dan.

The reason for me going for regulators, what the adaptor which I was getting the supply from is a switching type, and my thinking was the regulators would attenuate the switching noise, and provide a cleaner source of power for the audio analog circuitry.
 

Offline ZeroResistanceTopic starter

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Re: Negative version of LP2951 LDO
« Reply #34 on: September 19, 2019, 06:26:36 am »
I have to ask,9 volts, from a battery?

If so, a linear voltage regulator is very wasteful.

And one of the primary reasons to use a regulator, ripple reduction, is not required with batteries.
No its not a battery, its a 9V adapter.
 

Offline schmitt trigger

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Re: Negative version of LP2951 LDO
« Reply #35 on: September 19, 2019, 09:08:24 am »
If you would like high frequency ripple reduction, LDOs are not the best option. At powerline ripple frequencies they may have easily 45 or 60 db of ripple rejection.
But at higher ripple frequencies, it drops to essentially zero. Some LDO datasheets clearly show this.

In my humble opinion, it would be far better to have a transistor capacitance multiplier for HF ripple reduction.
 
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