C-multiplier again
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From: John Larkin on 22 May 2010 22:57 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
From: David Eather on 23 May 2010 06:50 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)?
From: Winfield Hill on 23 May 2010 07:28 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
From: Paul Keinanen on 23 May 2010 08:09 On Sat, 22 May 2010 19:57:54 -0700, John Larkin <jjlarkin(a)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.
From: John Larkin on 23 May 2010 10:22
On Sun, 23 May 2010 15:09:00 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >On Sat, 22 May 2010 19:57:54 -0700, John Larkin ><jjlarkin(a)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 |