Forums

C-multiplier again

Started by John Larkin May 22, 2010


I need a super-low noise power supply. I have a 15 volt switching
wall-wart input and want as close to 15 volts, regulated, as I can
get; 14 would be nice, 13.5 is OK.

The LDOs that I can find are all pretty noisy and have mediocre PSRR.

So I thought about using a Phil Hobbs-ian c-multiplier transistor, an
R-C lowpass and an emitter follower, with a slow opamp loop wrapped
around it for DC regulation. It looks fine on paper, simple loop to
stabilize, but I figured I may as well Spice it and be sure.

What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I
have...

ftp://jjlarkin.lmi.net/C-multiplier.gif

which has psrr of about 70 dB at low frequencies, improving as the
output cap finally kicks in at around 5 KHz. The transistor equivalent
seems to look like the expected dynamic Re of about 2 ohms, with a C-E
resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much.

I'm using the LT Spice 2N3904 model, which I take to be a sort of
generic small-signal NPN. The 33r base resistor value doesn't seem to
matter.

There must be a better way, ideally one that doesn't throw away 0.7
perfectly good volts.

John

On 23/05/2010 12:57 PM, John Larkin wrote:
> > > > I need a super-low noise power supply. I have a 15 volt switching > wall-wart input and want as close to 15 volts, regulated, as I can > get; 14 would be nice, 13.5 is OK. > > The LDOs that I can find are all pretty noisy and have mediocre PSRR. > > So I thought about using a Phil Hobbs-ian c-multiplier transistor, an > R-C lowpass and an emitter follower, with a slow opamp loop wrapped > around it for DC regulation. It looks fine on paper, simple loop to > stabilize, but I figured I may as well Spice it and be sure. > > What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I > have... > > ftp://jjlarkin.lmi.net/C-multiplier.gif > > which has psrr of about 70 dB at low frequencies, improving as the > output cap finally kicks in at around 5 KHz. The transistor equivalent > seems to look like the expected dynamic Re of about 2 ohms, with a C-E > resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much. > > I'm using the LT Spice 2N3904 model, which I take to be a sort of > generic small-signal NPN. The 33r base resistor value doesn't seem to > matter. > > There must be a better way, ideally one that doesn't throw away 0.7 > perfectly good volts. > > John >
A SM boost regulator followed by a LM317 (up to 80db PSRR)?
John Larkin wrote...
> > I need a super-low noise power supply. I have a 15 volt switching > wall-wart input and want as close to 15 volts, regulated, as I can > get; 14 would be nice, 13.5 is OK. > > The LDOs that I can find are all pretty noisy and have mediocre PSRR. > > So I thought about using a Phil Hobbs-ian c-multiplier transistor, an > R-C lowpass and an emitter follower, with a slow opamp loop wrapped > around it for DC regulation. It looks fine on paper, simple loop to > stabilize, but I figured I may as well Spice it and be sure. > > What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I > have... ftp://jjlarkin.lmi.net/C-multiplier.gif > which has psrr of about 70 dB at low frequencies, improving as the > output cap finally kicks in at around 5 KHz. The transistor equivalent > seems to look like the expected dynamic Re of about 2 ohms, with a C-E > resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much.
You're complaining about a 70dB improvement? There is a simple way to use your 0.7 volts, well maybe 0.8 volts, to get even more rejection: change your simple NPN follower into a Sziklai connection (AoE page 95). The base resistor across the added PNP creates a relatively-fixed collector current for your NPN, which means a fixed Vbe, for improved AC ripple rejection. -- Thanks, - Win
On Sat, 22 May 2010 19:57:54 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>I need a super-low noise power supply. I have a 15 volt switching >wall-wart input and want as close to 15 volts, regulated, as I can >get; 14 would be nice, 13.5 is OK.
Ask yourself, does it really make sense to use a unipolar power supply? With 70 dB PSRR and 1 V ripple would be is 0,3 uV. You would have to be very careful with the ground wire topology, in order to not destroy the PSRR. A bipolar power supply would be more appropriate and not suffer from ground loop currents. Alternatively an RC/RC voltage divider could be used to create the virtual ground reference for the op-amps.
On Sun, 23 May 2010 15:09:00 +0300, Paul Keinanen <keinanen@sci.fi>
wrote:

>On Sat, 22 May 2010 19:57:54 -0700, John Larkin ><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > >>I need a super-low noise power supply. I have a 15 volt switching >>wall-wart input and want as close to 15 volts, regulated, as I can >>get; 14 would be nice, 13.5 is OK. > > >Ask yourself, does it really make sense to use a unipolar power >supply? With 70 dB PSRR and 1 V ripple would be is 0,3 uV.
Ask yourself, did I do that math right?
> >You would have to be very careful with the ground wire topology, in >order to not destroy the PSRR.
It's a 4-layer board with a solid ground plane.
> >A bipolar power supply would be more appropriate and not suffer from >ground loop currents.
I have bipolar power supplies. I need very low noise ones.
> >Alternatively an RC/RC voltage divider could be used to create the >virtual ground reference for the op-amps.
The circuit I'm doing has a lot of discretes, and every nanovolt of noise hurts. John
On May 22, 7:57=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> I need a super-low noise power supply. I have a 15 volt switching > wall-wart input and want as close to 15 volts, regulated, as I can > get; 14 would be nice, 13.5 is OK. > > The LDOs that I can find are all pretty noisy and have mediocre PSRR. > > So I thought about using a Phil Hobbs-ian c-multiplier transistor, an > R-C lowpass and an emitter follower, with a slow opamp loop wrapped > around it for DC regulation. It looks fine on paper, simple loop to > stabilize, but I figured I may as well Spice it and be sure. > > What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I > have... > > ftp://jjlarkin.lmi.net/C-multiplier.gif > > which has psrr of about 70 dB at low frequencies, improving as the > output cap finally kicks in at around 5 KHz. The transistor equivalent > seems to look like the expected dynamic Re of about 2 ohms, with a C-E > resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much. > > I'm using the LT Spice 2N3904 model, which I take to be a sort of > generic small-signal NPN. The 33r base resistor value doesn't seem to > matter. > > There must be a better way, ideally one that doesn't throw away 0.7 > perfectly good volts.
How much current do you need to produce? At low currents, a fast rail-rail op-amp can make a good clean power supply. They work with as little as 0.3V of head room. If you get the ones that are stable into a capacitive load, like the LT1498, you can bypass the output.
On Sun, 23 May 2010 07:39:01 -0700 (PDT), MooseFET
<kensmith@rahul.net> wrote:

>On May 22, 7:57&#2013266080;pm, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> I need a super-low noise power supply. I have a 15 volt switching >> wall-wart input and want as close to 15 volts, regulated, as I can >> get; 14 would be nice, 13.5 is OK. >> >> The LDOs that I can find are all pretty noisy and have mediocre PSRR. >> >> So I thought about using a Phil Hobbs-ian c-multiplier transistor, an >> R-C lowpass and an emitter follower, with a slow opamp loop wrapped >> around it for DC regulation. It looks fine on paper, simple loop to >> stabilize, but I figured I may as well Spice it and be sure. >> >> What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I >> have... >> >> ftp://jjlarkin.lmi.net/C-multiplier.gif >> >> which has psrr of about 70 dB at low frequencies, improving as the >> output cap finally kicks in at around 5 KHz. The transistor equivalent >> seems to look like the expected dynamic Re of about 2 ohms, with a C-E >> resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much. >> >> I'm using the LT Spice 2N3904 model, which I take to be a sort of >> generic small-signal NPN. The 33r base resistor value doesn't seem to >> matter. >> >> There must be a better way, ideally one that doesn't throw away 0.7 >> perfectly good volts. > >How much current do you need to produce?
15 mA maybe, fairly steady. I'm running photodiodes and discrete jfets and such.
> >At low currents, a fast rail-rail op-amp can make a good clean >power supply. They work with as little as 0.3V of head room. >If you get the ones that are stable into a capacitive load, like >the LT1498, you can bypass the output.
Right, I've been considering that. I have the LM8261 in stock, a rrio C-load amp that has low frequency psrr of about 100 dB. Noise is a little high, 10 nv per, but that's already 15x better than your typical voltage regulator. And I can get a better opamp by applying money. I'm thinking about an R-C after the opamp, 10 or 20 ohms and a 120 uF polymer aluminum cap. That only costs 150-300 mV and has a corner frequency in the 100 Hz ballpark, so fixes the opamp's PSRR falloff at high frequencies and rolls off the wideband noise. The DC feedback can still be from the output, so regulation stays good. This is practically my existing circuit, without the transistor! John

John Larkin wrote:

> > > I need a super-low noise power supply. I have a 15 volt switching > wall-wart input and want as close to 15 volts, regulated, as I can > get; 14 would be nice, 13.5 is OK. > > The LDOs that I can find are all pretty noisy and have mediocre PSRR.
http://www.abvolt.com/misc/psrr.jpg The topology like this is stable and provides for ~100dB of PSRR. You can also simulate huge LC filter with gyrators. Oh, and the trivial solution would be cascading regulators one after the other. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
On 23 May 2010 04:28:01 -0700, Winfield Hill
<Winfield_member@newsguy.com> wrote:

>John Larkin wrote... >> >> I need a super-low noise power supply. I have a 15 volt switching >> wall-wart input and want as close to 15 volts, regulated, as I can >> get; 14 would be nice, 13.5 is OK. >> >> The LDOs that I can find are all pretty noisy and have mediocre PSRR. >> >> So I thought about using a Phil Hobbs-ian c-multiplier transistor, an >> R-C lowpass and an emitter follower, with a slow opamp loop wrapped >> around it for DC regulation. It looks fine on paper, simple loop to >> stabilize, but I figured I may as well Spice it and be sure. >> >> What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I >> have... ftp://jjlarkin.lmi.net/C-multiplier.gif >> which has psrr of about 70 dB at low frequencies, improving as the >> output cap finally kicks in at around 5 KHz. The transistor equivalent >> seems to look like the expected dynamic Re of about 2 ohms, with a C-E >> resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much. > > You're complaining about a 70dB improvement? There is a simple > way to use your 0.7 volts, well maybe 0.8 volts, to get even > more rejection: change your simple NPN follower into a Sziklai > connection (AoE page 95). The base resistor across the added > PNP creates a relatively-fixed collector current for your NPN, > which means a fixed Vbe, for improved AC ripple rejection.
Since the problem is the Early effect, namely the effective C-E resistance bleeding ripple through, it didn't seem to me like the Sziklai thing would help. The PNP doesn't insulate the NPN from the ripple. So I spiced it. If the LT Spice transistor models are to be trusted, it's actually worse. The optimum value for the PNP's b-e resistor is zero. John
Winfield Hill  <Winfield_member@newsguy.com> wrote:

[...] 

> You're complaining about a 70dB improvement?
Phil promised 140dB: http://groups.google.com/group/sci.electronics.design/msg/143f77519fed66e8