Pooge 5.51 positive regulator

ALW

Member
2001-06-12 10:15 am
UK
Quick mod

Add a cap across R861, with an impedance that's low relative to it's value.

It reduces noise significantly at higher frequencies, and lowers output impedance an order of magnitude.

In this form it's a VERY good sounding regulator.

You could try a TL431 as a reference, set to it's min o/p (2.5V), or just a zener of suitable value.

Andy.
 
pooge 5.51

What value of cap across R861 do you recommend?:confused: I take it that the voltage in should be ~4.3 vdc or so to get 5 vdc out? For 15 vdc,would I come off the V out leg of a 317 into this circuit for the replacement on my circuit board or does this circuit replace a 317?

Mike
 
Re: Quick mod

ALW said:
Add a cap across R861, with an impedance that's low relative to it's value.

It reduces noise significantly at higher frequencies, and lowers output impedance an order of magnitude.

In this form it's a VERY good sounding regulator.

You could try a TL431 as a reference, set to it's min o/p (2.5V), or just a zener of suitable value.

Andy.

Hi Andy, seems I beat you to this thread by about 15 mins...?:D
Your advice is right on the dot, as usual.

BTW, how's the supplies coming along?

Jan Didden
 
Pooge etc

I don't think you can give a hard freq value. Basically, the feedback divider (the two resistors that divide down the output voltage to the base of the comparator transistor) also divide down the error voltage with the same ratio. That decreases the accuracy of the loop with the same ratio. Now, for DC, that is not critical. I mean, you probably don't worry about an output voltage of 15.2 or 14.88 instead of 15. But the ac errors (noise, ripple) you really want to be zero. So when you put a cap over the top resistor, the AC feedback ratio can be made 1, that is, ALL ac errors are fully fed back to the base of the comparator transistor. But of course that only happens when the cap impedance is zero, which for a practical cap cannot be realised.
At the same time, realise that the performance of these circuits deteriorates with rising frequency. At low freq, it is probably good enough to do without the effect of the cap, but above 1 kHz or so, the cap can start making a contribution.
Taking into account these factors, I use rule of thumb to make the cap such a value that it starts to work at 1kHz, so at 1kHz Xc should be the value of the upper resistor. But there is not much stopping you taking a (much) larger cap.
Hope this helps,

Jan Didden