[time-nuts] Low noise voltage regulators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Feb 26 09:00:34 UTC 2010

Magnus Danielson wrote:
> Bruce Griffiths wrote:
>> Garry Thorp wrote:
>>> With the 723, you can make the reference noise as low as you want, by
>>> heavy RC filtering. This applies whether you use its own reference or a
>>> better external reference.
>>> The 723 also seems to work quite happily with a feedback capacitor from
>>> the output to the inverting input, reducing the AC gain to unity. The
>>> output noise will then just be the buffer amplifier's input noise
>>> voltage, ~5-6nV/rtHz. This is about 20dB better than most modern
>>> 'low-noise' LDOs will do. (I can't remember what the amplifier's noise
>>> was like at very low frequencies  -  it was many years ago at a
>>> different company, and I don't have the results any more.)
>>> Garry
>> Using a capacitor from the output to the inverting input of the error 
>> amplifier to reduce the high frequency noise gain of the regulator is 
>> often used in high performance series regulators.
> It also reduces the loop bandwidth of the loop and then it becomes 
> weaker for higher frequency variations, so keen detail in decoupling 
> is needed. To a certain limit it should work well for a particular 
> design.
Not directly, its analogous to using an opamp configured as a follower 
with gain, bypassing the feedback resistor with a capacitor reduces the 
high frequency closed loop gain to unity which would increase the 
regulator closed loop bandwidth if it weren't for the fact that an 
increased value of compensation capacitance may be required to ensure 
There's little point in amplifying the reference and regulator amplifier 
input noise at high frequencies. A dc and low frequency gain sufficient 
to produce the required dc output voltage coupled with unity gain at 
high frequencies (within the closed loop bandwidth) is better than 
having a relatively flat regulator gain vs frequency.
> The great thing about the 723 is that it has sufficient support for a 
> number of things in one package, that you very easily can build a 
> complete PSU with it. None of the features is very unique, but the 
> combination allows for quick and sound designs for most peoples use.
> Cheers,
> Magnus

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