Winfield Hill <Winfield_member@newsguy.com> wrote: [...]> I see your idea, not bad. It's a nice simplification of this, > incorporating the current-sinking transistor into the opamp. > > +15V >--+--------+--------+----/\/\--+-----> Vout 14.8v > | | | 4.7R | > | R3 | | > | 2.7M | | > } | _| | > | C1 +------| \ |/ > '---||---+ | >------| > 10uF | ,--|__/ |\V > | | | | > R7 '--- |----------+ > TBD | | > 27k | R4 > | | 4.7R > | | | > --+--------+----------+---- > > This scheme is DC regulating as well. The class-A current > is set by R3 and R7, so the dc voltage drop is fixed.Cancellation schemes give a 6dB/octave drop to a notch frequency, then a 6dB/octave rise. The depth of the notch is extremely sensitive to the emitter resistance and probably the temperature of the transistor. Some examples may show large amounts of second harmonic distortion on the output. This does not appear on the frequency analysis plot. In this example, the notch frequency is about 2KHz with a depth of -92dB. Try changing the emitter resistance to get an idea of how critical it is. I don't think you want to rely on this method for any more than a minor amount of cancellation, say 20 dB or thereabouts. Mike Version 4 SHEET 1 1140 1108 WIRE -1072 -432 -1088 -432 WIRE -944 -432 -992 -432 WIRE -896 -432 -944 -432 WIRE -832 -432 -896 -432 WIRE -720 -432 -832 -432 WIRE -656 -432 -720 -432 WIRE -496 -432 -576 -432 WIRE -480 -432 -496 -432 WIRE -832 -416 -832 -432 WIRE -944 -400 -944 -432 WIRE -1088 -352 -1088 -432 WIRE -480 -320 -480 -432 WIRE -944 -304 -944 -336 WIRE -832 -304 -832 -336 WIRE -832 -304 -944 -304 WIRE -720 -304 -720 -432 WIRE -752 -288 -768 -288 WIRE -624 -272 -688 -272 WIRE -544 -272 -624 -272 WIRE -1088 -256 -1088 -272 WIRE -832 -256 -832 -304 WIRE -800 -256 -832 -256 WIRE -752 -256 -800 -256 WIRE -832 -224 -832 -256 WIRE -720 -224 -720 -240 WIRE -768 -176 -768 -288 WIRE -736 -176 -768 -176 WIRE -480 -176 -480 -224 WIRE -480 -176 -736 -176 WIRE -480 -160 -480 -176 WIRE -832 -128 -832 -144 WIRE -480 -64 -480 -80 FLAG -1088 -256 0 FLAG -896 -432 Vin FLAG -496 -432 Vout FLAG -480 -64 0 FLAG -720 -224 0 FLAG -832 -128 0 FLAG -800 -256 U1P FLAG -736 -176 U1N FLAG -624 -272 U1O SYMBOL npn -544 -320 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL voltage -1088 -368 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -976 -432 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 WINDOW 3 -2 123 VRight 0 SYMATTR InstName V2 SYMATTR Value2 AC 1 SYMATTR Value SINE(0 0.1 1200) SYMBOL res -496 -176 R0 SYMATTR InstName R1 SYMATTR Value 4.681 SYMBOL res -672 -448 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 4.7 SYMBOL opamps\\1pole -720 -272 R0 SYMATTR InstName U1 SYMBOL res -848 -432 R0 SYMATTR InstName R3 SYMATTR Value 2.7e6 SYMBOL res -848 -240 R0 SYMATTR InstName R4 SYMATTR Value 27k SYMBOL cap -960 -400 R0 SYMATTR InstName C1 SYMATTR Value 10�f TEXT -824 -528 Left 0 ;'Op Amp Ripple Cancellation TEXT -832 -488 Left 0 !.ac oct 100 0.1 4e6
C-multiplier again
Started by ●May 22, 2010
Reply by ●May 26, 20102010-05-26
Reply by ●May 26, 20102010-05-26
On Wed, 26 May 2010 15:57:09 GMT, Mike <spam@me.not> wrote:>Winfield Hill <Winfield_member@newsguy.com> wrote: > >[...] > >> I see your idea, not bad. It's a nice simplification of this, >> incorporating the current-sinking transistor into the opamp. >> >> +15V >--+--------+--------+----/\/\--+-----> Vout 14.8v >> | | | 4.7R | >> | R3 | | >> | 2.7M | | >> } | _| | >> | C1 +------| \ |/ >> '---||---+ | >------| >> 10uF | ,--|__/ |\V >> | | | | >> R7 '--- |----------+ >> TBD | | >> 27k | R4 >> | | 4.7R >> | | | >> --+--------+----------+---- >> >> This scheme is DC regulating as well. The class-A current >> is set by R3 and R7, so the dc voltage drop is fixed. > >Cancellation schemes give a 6dB/octave drop to a notch frequency, then a >6dB/octave rise. The depth of the notch is extremely sensitive to the >emitter resistance and probably the temperature of the transistor. Some >examples may show large amounts of second harmonic distortion on the >output. This does not appear on the frequency analysis plot. > >In this example, the notch frequency is about 2KHz with a depth of -92dB. >Try changing the emitter resistance to get an idea of how critical it is. > >I don't think you want to rely on this method for any more than a minor >amount of cancellation, say 20 dB or thereabouts.You'd need a trimpot to make up for the tolerance of the 4.7r resistors. And yes, the dynamics are terrible here. And it's a power hog. Feedforward is great when you want a 3:1, or even sometimes 10:1, fix to some problem. Like for temperature compensation or some other situation when negative feedback isn't available. John
Reply by ●May 26, 20102010-05-26
On May 26, 12:07=A0pm, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Wed, 26 May 2010 15:57:09 GMT, Mike <s...@me.not> wrote: > >Winfield Hill =A0<Winfield_mem...@newsguy.com> wrote: > > >[...] > > >> =A0I see your idea, not bad. =A0It's a nice simplification of this, > >> =A0incorporating the current-sinking transistor into the opamp. > > >> =A0+15V >--+--------+--------+----/\/\--+-----> Vout 14.8v > >> =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 =A04.7R =A0=|> >> =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 R3 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 ==A0|> >> =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A02.7M =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0=|> >> =A0 =A0 =A0 =A0 =A0} =A0 =A0 =A0 =A0| =A0 =A0 =A0 _| =A0 =A0 =A0 =A0 ==A0|> >> =A0 =A0 =A0 =A0 =A0| =A0 C1 =A0 +------| =A0\ =A0 =A0 =A0 |/ > >> =A0 =A0 =A0 =A0 =A0'---||---+ =A0 =A0 =A0| =A0 >------| > >> =A0 =A0 =A0 =A0 =A0 =A0 10uF =A0| =A0 ,--|__/ =A0 =A0 =A0 |\V > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 | =A0 =A0| =A0 =A0 =A0 =A0 ==A0|> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0R7 =A0 '--- |----------+ > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 TBD =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 ==A0|> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A027k =A0 =A0 =A0 | =A0 =A0 =A0 =A0 R=4> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0=4.7R> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0==A0|> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 --+--------+----------+---- > > >> =A0This scheme is DC regulating as well. =A0The class-A current > >> =A0is set by R3 and R7, so the dc voltage drop is fixed. > > >Cancellation schemes give a 6dB/octave drop to a notch frequency, then a > >6dB/octave rise. The depth of the notch is extremely sensitive to the > >emitter resistance and probably the temperature of the transistor. Some > >examples may show large amounts of second harmonic distortion on the > >output. This does not appear on the frequency analysis plot. > > >In this example, the notch frequency is about 2KHz with a depth of -92dB=.> >Try changing the emitter resistance to get an idea of how critical it is=.> > >I don't think you want to rely on this method for any more than a minor > >amount of cancellation, say 20 dB or thereabouts. > > You'd need a trimpot to make up for the tolerance of the 4.7r > resistors. And yes, the dynamics are terrible here. And it's a power > hog. > > Feedforward is great when you want a 3:1, or even sometimes 10:1, fix > to some problem. Like for temperature compensation or some other > situation when negative feedback isn't available.Well, apart from THAT, Mrs. Lincoln, how'd you enjoy the play? -- Cheers, James Arthur
Reply by ●May 26, 20102010-05-26
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:> You'd need a trimpot to make up for the tolerance of the 4.7r > resistors. And yes, the dynamics are terrible here. And it's a power > hog. > > Feedforward is great when you want a 3:1, or even sometimes 10:1, fix > to some problem. Like for temperature compensation or some other > situation when negative feedback isn't available. > > JohnI'd stay away from a trimpot. If you try to get exact balance, it may work in the lab. But if you ship it to the customer and there is some noise problem, it may work one day and not work the next. That would be impossible to troubleshoot. Also, it is impossible to do a worst-case tolerance analysis on this method. If you cannot guarantee that you know how it will work under all conditions, give it to Joerg and let him fix it:) Anyway, I have solved your problem. This approach gives -180dB at 1KHz, with a -200dB notch at 1MHz. The IR drop is 18.8 * ImA, so if you have a 10mA drain, you end up with 14.8V. An additional benefit is very good brownout protection. It will keep your lamps lit for a long time! I left the cancellation method in for comparison. Mike Version 4 SHEET 1 1140 1108 WIRE -1072 -432 -1088 -432 WIRE -944 -432 -992 -432 WIRE -896 -432 -944 -432 WIRE -832 -432 -896 -432 WIRE -720 -432 -832 -432 WIRE -656 -432 -720 -432 WIRE -496 -432 -576 -432 WIRE -480 -432 -496 -432 WIRE -320 -432 -336 -432 WIRE -224 -432 -240 -432 WIRE -160 -432 -224 -432 WIRE -48 -432 -80 -432 WIRE -832 -416 -832 -432 WIRE -944 -400 -944 -432 WIRE -224 -400 -224 -432 WIRE -48 -400 -48 -432 WIRE -1088 -352 -1088 -432 WIRE -480 -320 -480 -432 WIRE -224 -320 -224 -336 WIRE -48 -320 -48 -336 WIRE -944 -304 -944 -336 WIRE -832 -304 -832 -336 WIRE -832 -304 -944 -304 WIRE -720 -304 -720 -432 WIRE -752 -288 -768 -288 WIRE -624 -272 -688 -272 WIRE -544 -272 -624 -272 WIRE -1088 -256 -1088 -272 WIRE -832 -256 -832 -304 WIRE -800 -256 -832 -256 WIRE -752 -256 -800 -256 WIRE -320 -240 -336 -240 WIRE -224 -240 -240 -240 WIRE -160 -240 -224 -240 WIRE -48 -240 -80 -240 WIRE 0 -240 -48 -240 WIRE 48 -240 0 -240 WIRE -832 -224 -832 -256 WIRE -720 -224 -720 -240 WIRE -224 -208 -224 -240 WIRE -48 -208 -48 -240 WIRE 48 -208 48 -240 WIRE -768 -176 -768 -288 WIRE -736 -176 -768 -176 WIRE -480 -176 -480 -224 WIRE -480 -176 -736 -176 WIRE -480 -160 -480 -176 WIRE -832 -128 -832 -144 WIRE -224 -128 -224 -144 WIRE -48 -128 -48 -144 WIRE 48 -128 48 -144 WIRE -480 -64 -480 -80 FLAG -1088 -256 0 FLAG -896 -432 Vin FLAG -496 -432 Vout FLAG -480 -64 0 FLAG -720 -224 0 FLAG -832 -128 0 FLAG -800 -256 U1P FLAG -736 -176 U1N FLAG -624 -272 U1O FLAG -336 -432 Vin FLAG -224 -320 0 FLAG -224 -432 Vout2 FLAG -48 -320 0 FLAG -48 -432 Vout3 FLAG -224 -128 0 FLAG -224 -240 Vout4 FLAG -48 -128 0 FLAG 0 -240 Vout5 FLAG -336 -240 Vout3 FLAG 48 -128 0 SYMBOL npn -544 -320 R0 SYMATTR InstName Q1 SYMATTR Value 2N4401 SYMBOL voltage -1088 -368 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -976 -432 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 WINDOW 3 -2 123 VRight 0 SYMATTR InstName V2 SYMATTR Value2 AC 1 SYMATTR Value SINE(0 0.1 2.111e3) SYMBOL res -496 -176 R0 SYMATTR InstName R1 SYMATTR Value 4.681 SYMBOL res -672 -448 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 4.7 SYMBOL opamps\\1pole -720 -272 R0 SYMATTR InstName U1 SYMBOL res -848 -432 R0 SYMATTR InstName R3 SYMATTR Value 2.7e6 SYMBOL res -848 -240 R0 SYMATTR InstName R4 SYMATTR Value 27k SYMBOL cap -960 -400 R0 SYMATTR InstName C1 SYMATTR Value 10�f SYMBOL cap -240 -400 R0 SYMATTR InstName C2 SYMATTR Value 10000�f SYMATTR SpiceLine Rser=20m Lser=2.5nh SYMBOL res -336 -448 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 4.7 SYMBOL cap -64 -400 R0 SYMATTR InstName C3 SYMATTR Value 10000�f SYMATTR SpiceLine Rser=20m Lser=2.5nh SYMBOL res -176 -448 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 4.7 SYMBOL cap -240 -208 R0 SYMATTR InstName C4 SYMATTR Value 10000�f SYMATTR SpiceLine Rser=20m Lser=2.5nh SYMBOL res -336 -256 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R7 SYMATTR Value 4.7 SYMBOL cap -64 -208 R0 SYMATTR InstName C5 SYMATTR Value 10000�f SYMATTR SpiceLine Rser=20m Lser=2.5nh SYMBOL res -176 -256 M90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 4.7 SYMBOL cap 32 -208 R0 SYMATTR InstName C6 SYMATTR Value 10�f SYMATTR SpiceLine Rser=2m Lser=2.5nh TEXT -824 -528 Left 0 ;'Op Amp Ripple Cancellation TEXT -832 -488 Left 0 !.ac oct 100 0.1 4e6
Reply by ●May 26, 20102010-05-26
On Wed, 26 May 2010 17:53:56 GMT, Mike <spam@me.not> wrote:>John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > > >> You'd need a trimpot to make up for the tolerance of the 4.7r >> resistors. And yes, the dynamics are terrible here. And it's a power >> hog. >> >> Feedforward is great when you want a 3:1, or even sometimes 10:1, fix >> to some problem. Like for temperature compensation or some other >> situation when negative feedback isn't available. >> >> John > >I'd stay away from a trimpot. If you try to get exact balance, it may >work in the lab. But if you ship it to the customer and there is some >noise problem, it may work one day and not work the next. That would be >impossible to troubleshoot. > >Also, it is impossible to do a worst-case tolerance analysis on this >method. If you cannot guarantee that you know how it will work under all >conditions, give it to Joerg and let him fix it:) > >Anyway, I have solved your problem. This approach gives -180dB at 1KHz, >with a -200dB notch at 1MHz. The IR drop is 18.8 * ImA, so if you have a >10mA drain, you end up with 14.8V. An additional benefit is very good >brownout protection. It will keep your lamps lit for a long time! > >I left the cancellation method in for comparison. > >MikeIt's a shame that supercaps have such high ESRs. John
Reply by ●May 26, 20102010-05-26
On May 26, 1:39=A0pm, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Wed, 26 May 2010 17:53:56 GMT, Mike <s...@me.not> wrote: > >John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > > >> You'd need a trimpot to make up for the tolerance of the 4.7r > >> resistors. And yes, the dynamics are terrible here. And it's a power > >> hog. > > >> Feedforward is great when you want a 3:1, or even sometimes 10:1, fix > >> to some problem. Like for temperature compensation or some other > >> situation when negative feedback isn't available. > > >> John > > >I'd stay away from a trimpot. If you try to get exact balance, it may > >work in the lab. But if you ship it to the customer and there is some > >noise problem, it may work one day and not work the next. That would be > >impossible to troubleshoot. > > >Also, it is impossible to do a worst-case tolerance analysis on this > >method. If you cannot guarantee that you know how it will work under all > >conditions, give it to Joerg and let him fix it:) > > >Anyway, I have solved your problem. This approach gives -180dB at 1KHz, > >with a -200dB notch at 1MHz. The IR drop is 18.8 * ImA, so if you have a > >10mA drain, you end up with 14.8V. An additional benefit is very good > >brownout protection. It will keep your lamps lit for a long time! > > >I left the cancellation method in for comparison. > > >Mike > > It's a shame that supercaps have such high ESRs. > > JohnI was thinking about that. Maxwell Technology makes unit with milliohm ESRs, but I wasn't sure there wasn't some funky noise problem, like electrolyte convection or who knows what. Oh, and they're a few cubic inches--not surface mountable. But as for leakage, I've seen a *really* clever dodge around that. Walt Jung, I think, in a low-noise reference IIRC. -- Cheers, James Arthur
Reply by ●May 26, 20102010-05-26
On May 26, 10:57=A0am, Mike <s...@me.not> wrote:> Winfield Hill =A0<Winfield_mem...@newsguy.com> wrote: > > [...] > > > > > =A0I see your idea, not bad. =A0It's a nice simplification of this, > > =A0incorporating the current-sinking transistor into the opamp. > > > =A0+15V >--+--------+--------+----/\/\--+-----> Vout 14.8v > > =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0| =A0 =A04.7R =A0| > > =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 R3 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 ==A0|> > =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A02.7M =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0| > > =A0 =A0 =A0 =A0 =A0} =A0 =A0 =A0 =A0| =A0 =A0 =A0 _| =A0 =A0 =A0 =A0 ==A0|> > =A0 =A0 =A0 =A0 =A0| =A0 C1 =A0 +------| =A0\ =A0 =A0 =A0 |/ > > =A0 =A0 =A0 =A0 =A0'---||---+ =A0 =A0 =A0| =A0 >------| > > =A0 =A0 =A0 =A0 =A0 =A0 10uF =A0| =A0 ,--|__/ =A0 =A0 =A0 |\V > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 | =A0 =A0| =A0 =A0 =A0 =A0 ==A0|> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0R7 =A0 '--- |----------+ > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 TBD =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 ==A0|> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A027k =A0 =A0 =A0 | =A0 =A0 =A0 =A0 R4 > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =4.7R> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 ==A0|> > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 --+--------+----------+---- > > > =A0This scheme is DC regulating as well. =A0The class-A current > > =A0is set by R3 and R7, so the dc voltage drop is fixed. > > Cancellation schemes give a 6dB/octave drop to a notch frequency, then a > 6dB/octave rise. The depth of the notch is extremely sensitive to the > emitter resistance and probably the temperature of the transistor. Some > examples may show large amounts of second harmonic distortion on the > output. This does not appear on the frequency analysis plot. > > In this example, the notch frequency is about 2KHz with a depth of -92dB. > Try changing the emitter resistance to get an idea of how critical it is. > > I don't think you want to rely on this method for any more than a minor > amount of cancellation, say 20 dB or thereabouts. > > Mike<snip LTSpice model> 20dB sounds about right. The advantages of this approach are low drop- out voltage and superior low-frequency noise cancellation (compared to practical passive equivalents). A big part of the dynamic limitation is the f.f. network rolling off. If you change C1 to 100uF, and tack 100uF on the output to cover the high-end, overall performance is much improved--nearly as good as a passive version using 10,000uF caps, and a lot smaller. For super massive attenuation of input noise and ripple, other approaches are better. If John could knock down that 50mV switcher ripple with an LC at the input, that's a bonus. But he won't--The Brat would kill him. -- Cheers, James Arthur
Reply by ●May 26, 20102010-05-26
On May 24, 3:57=A0pm, David Eather <eat...@tpg.com.au> wrote:> On 24/05/2010 12:11 PM, dagmargoodb...@yahoo.com wrote: > > > > > On May 23, 6:56 pm, David Eather<eat...@tpg.com.au> =A0wrote: > >> On 24/05/2010 8:45 AM, John Larkin wrote: > > >>> On Mon, 24 May 2010 08:28:03 +1000, David Eather<eat...@tpg.com.au> > >>> wrote: > > >>>> On 24/05/2010 8:07 AM, John Larkin wrote: > >>>>> On Sun, 23 May 2010 13:26:26 -0700 (PDT), dagmargoodb...@yahoo.com > >>>>> wrote: > > >>>>>> On May 23, 11:29 am, John Larkin > >>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> =A0 =A0 wrote: > >>>>>>> On 23 May 2010 04:28:01 -0700, Winfield Hill > > >>>>>>> <Winfield_mem...@newsguy.com> =A0 =A0 wrote: > >>>>>>>> John Larkin wrote... > > >>>>>>>>> I need a super-low noise power supply. I have a 15 volt switchi=ng> >>>>>>>>> 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 transist=or, an> >>>>>>>>> R-C lowpass and an emitter follower, with a slow opamp loop wra=pped> >>>>>>>>> around it for DC regulation. It looks fine on paper, simple loo=p 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 s=uch, I> >>>>>>>>> have... =A0ftp://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 equ=ivalent> >>>>>>>>> seems to look like the expected dynamic Re of about 2 ohms, wit=h a C-E> >>>>>>>>> resistor of around 6.6K. Reducing Vb (and Vout) doesn't help mu=ch.> > >>>>>>>> You're complaining about a 70dB improvement? =A0There is a simpl=e> >>>>>>>> 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). =A0The 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 t=he> >>>>>>> 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 > > >>>>>> Win's idea looks pretty decent to me, IIUIC: > > >>>>>> FIG. 1 =A0 =A0 (View in fixed font) > >>>>>> =3D=3D=3D=3D=3D=3D > > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0Q1 > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 2n3906 > >>>>>> Vin>--+----. =A0 =A0.-------+---+------+--> =A0 =A0 +13.3v > >>>>>> =A0 =A0 =A0 =A0 =A0| =A0 =A0 V =A0/ =A0 =A0 =A0 =A0| =A0 | =A0 =A0==A0|> >>>>>> =A0 =A0 =A0 =A0 R1 =A0 =A0------ =A0 =A0 =A0 | =A0 R2 =A0 =A0--- C=1> >>>>>> =A0 =A0 =A0 =A0 470 =A0 =A0 | =A0 =A0 Q2 =A0 | =A0 1k =A0 =A0--- 1=5uF> >>>>>> =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0| =A0 2n3904 | =A0 | =A0 =A0 =A0| > >>>>>> =A0 =A0 =A0 =A0 =A0'------+---. =A0 =A0 / =A0 =3D=3D=3D =A0 =A0=3D==3D=3D> >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0\ =A0 ^ > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0----- > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0R3 > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A033 > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 +14v>---' > > >>>>>> LT Spice says 31uV of the 50mV 1KHz ripple gets through (32dBv), > >>>>>> and the load step is 340uV. =A0That's a lot stiffer than the origi=nal,> >>>>>> which > >>>>>> had a 4.5mV load step (d(i) =3D 2mA for both). > > >>>>>> The Sziklai version has the same ripple; I don't quite understand > >>>>>> how Early explains that--Early should wreck the load step response > >>>>>> too, shouldn't it? > > >>>>>> FIG 1's load step is only 60uV if you replace R1 with a 5mA curren=t> >>>>>> source, > >>>>>> the 1KHz ripple stays the same. > > >>>>>> This shunt filter only needs 200mV headroom: > > >>>>>> FIG. 2 > >>>>>> =3D=3D=3D=3D=3D=3D > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0R1> >>>>>> +15V>--+------------------/\/\/\--------+--> =A0 =A0 Vout =3D 14.8=v> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A05 =A0==A0 =A0 =A0 =A0 |> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0.-------+------+--------+ > >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 | =A0 =A0 =A0| ==A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 | =A0 =A0 R6 ==A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 | =A0 =A0 1k ==A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 R3 =A0 =A0 =A0R5 =A0 =A0 =A0| ==A0 =A0 =A0|<' Q3> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A02.7M =A0 =A0 10K =A0 =A0 +------|==A0 2n3906> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 | =A0 =A0 =A0| ==A0 =A0 =A0|\> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 | =A0 =A0|/ Q2 ==A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 +----| =A02n390=4 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0| =A0 =A0 =A0 | =A0 =A0|>. ==A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 | =A0 C1 =A0 | =A0 =A0|<' =A0 =A0 =A0 | =A0 ==A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 '---||---+----| =A0Q1 =A0 =A0 '--------+ > >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A010uF =A0| =A0 =A0|\ 2n3906 =A0 =A0 =A0 ==A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 R2 =A0 =A0 =A0| =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0R4> >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 5k =A0 =A0 =A0| =A0 =A0 =A0 ==A0 =A0 =A0 =A0 4.7R> >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0| =A0 =A0 =A0 ==A0 =A0 =A0 =A0 =A0|> >>>>>> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =3D=3D=3D =A0 =A0=3D=3D=3D =A0==A0 =A0 =A0 =A0 =A0 =A0=3D=3D=3D> > >>>>>> LT Spice says 20dBV rejection @ 1KHz, zero @ d.c., natch. > > >>>>> Only 100 dB to go! But I don't understand Q1s biasing. > > >>>> Improved ripple response (but I think a little defective - it only w=orks> >>>> when Vin drops). > > >>>> When Vin drops Q1 turns on via base current drawn out through C1. Q1 > >>>> robs base current from Q2 turning it off, which in turn turns off Q3=and> >>>> reduces the current flow and hence voltage loss through R1. > > >>> So what's the quiescent current of Q1? Of Q3? > > >>> John > > >> =A0 =A0R2 is missing - from the base of Q1 to GND - I suggest a value =of 18k> >> but it is a weird circuit I think a ripple reduction of no more than 4=6db> > > If by 46dB you mean power, i.e. 20log(Vin/Vout) =3D 46, yes, that's > > easily possible--that implies 0.5% gain accuracy. > > > If you mean 46dBv, i.e. 10log(Vin/Vout) =3D 46, i.e. Vout / Vin =3D 25p=pm,> > no, that ain't happening, not unless you use op amps and some mighty > > fine resistors. > > Um, I thought power was 10*log(what ever / what ever else) and voltage > was 20*log(what ever / what ever else) e.g. a reduction in ripple of 46 > db =3D a factor of 200, no?My description was crap--it was a question of notation. I'm not sure of the dB convention as applied to these circuit--some R.F. work a ways back ruined me. In R.F., 'dB' in our shop meant a ratio of powers unless otherwise specified. So, if you said '40dB' I'd assume you meant a ratio of 10,000 in power, or, equivalently--power being V^2/R--that you meant a factor of merely 100, voltage. I vaguely recall, but I don't remember clearly, that we used to used the symbol 'dBv' to designate when a ratio of voltages was being reported. To convert that to power comparison, we'd have to multiply those dB by 2. Anyway, the other posts have cleared it all up. Thanks. -- Cheers, James Arthur
Reply by ●May 26, 20102010-05-26
On Wed, 26 May 2010 13:03:33 -0700 (PDT), dagmargoodboat@yahoo.com wrote:>On May 26, 10:57�am, Mike <s...@me.not> wrote: >> Winfield Hill �<Winfield_mem...@newsguy.com> wrote: >> >> [...] >> >> >> >> > �I see your idea, not bad. �It's a nice simplification of this, >> > �incorporating the current-sinking transistor into the opamp. >> >> > �+15V >--+--------+--------+----/\/\--+-----> Vout 14.8v >> > � � � � �| � � � �| � � � �| � �4.7R �| >> > � � � � �| � � � R3 � � � �| � � � � �| >> > � � � � �| � � �2.7M � � � | � � � � �| >> > � � � � �} � � � �| � � � _| � � � � �| >> > � � � � �| � C1 � +------| �\ � � � |/ >> > � � � � �'---||---+ � � �| � >------| >> > � � � � � � 10uF �| � ,--|__/ � � � |\V >> > � � � � � � � � � | � | � �| � � � � �| >> > � � � � � � � � �R7 � '--- |----------+ >> > � � � � � � � � TBD � � � �| � � � � �| >> > � � � � � � � � �27k � � � | � � � � R4 >> > � � � � � � � � � | � � � �| � � � � 4.7R >> > � � � � � � � � � | � � � �| � � � � �| >> > � � � � � � � � --+--------+----------+---- >> >> > �This scheme is DC regulating as well. �The class-A current >> > �is set by R3 and R7, so the dc voltage drop is fixed. >> >> Cancellation schemes give a 6dB/octave drop to a notch frequency, then a >> 6dB/octave rise. The depth of the notch is extremely sensitive to the >> emitter resistance and probably the temperature of the transistor. Some >> examples may show large amounts of second harmonic distortion on the >> output. This does not appear on the frequency analysis plot. >> >> In this example, the notch frequency is about 2KHz with a depth of -92dB. >> Try changing the emitter resistance to get an idea of how critical it is. >> >> I don't think you want to rely on this method for any more than a minor >> amount of cancellation, say 20 dB or thereabouts. >> >> Mike > ><snip LTSpice model> > >20dB sounds about right. The advantages of this approach are low drop- >out voltage and superior low-frequency noise cancellation (compared to >practical passive equivalents). > >A big part of the dynamic limitation is the f.f. network rolling off. >If you change C1 to 100uF, and tack 100uF on the output to cover the >high-end, overall performance is much improved--nearly as good as a >passive version using 10,000uF caps, and a lot smaller. > >For super massive attenuation of input noise and ripple, other >approaches are better. > >If John could knock down that 50mV switcher ripple with an LC at the >input, that's a bonus. But he won't--The Brat would kill him.No, I survived. The Gerbered board had... Wall wart connector Polyfuse Transzorb 10 uF ceramic 47 uH inductor two 10 uF ceramics and one 120 uF polymer aluminum to make "+15 volts." That's 12 dB/octave starting at about 2 KHz. Then the LM8261 low-noise LDO reg, which has its own 15 ohms + 2x10uF + 120uF at its output. I also use two Hobbsonian c-multipliers in other supplies that don't need LDO or regulation. Paranoia, groveling for nanovolts. But I really need to measure some actual c-multiplier circuits to see what the Early slopes are like. Could be that LT Spice is grossly pessimistic. I note here that everyone, including myself, would rather sit in a swivel chair and simulate and theorize, than get up and solder and measure. John
Reply by ●May 26, 20102010-05-26
On Wed, 26 May 2010 13:53:55 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>On Wed, 26 May 2010 13:03:33 -0700 (PDT), dagmargoodboat@yahoo.com >wrote: > >>On May 26, 10:57�am, Mike <s...@me.not> wrote: >>> Winfield Hill �<Winfield_mem...@newsguy.com> wrote: >>> >>> [...] >>> >>> >>> >>> > �I see your idea, not bad. �It's a nice simplification of this, >>> > �incorporating the current-sinking transistor into the opamp. >>> >>> > �+15V >--+--------+--------+----/\/\--+-----> Vout 14.8v >>> > � � � � �| � � � �| � � � �| � �4.7R �| >>> > � � � � �| � � � R3 � � � �| � � � � �| >>> > � � � � �| � � �2.7M � � � | � � � � �| >>> > � � � � �} � � � �| � � � _| � � � � �| >>> > � � � � �| � C1 � +------| �\ � � � |/ >>> > � � � � �'---||---+ � � �| � >------| >>> > � � � � � � 10uF �| � ,--|__/ � � � |\V >>> > � � � � � � � � � | � | � �| � � � � �| >>> > � � � � � � � � �R7 � '--- |----------+ >>> > � � � � � � � � TBD � � � �| � � � � �| >>> > � � � � � � � � �27k � � � | � � � � R4 >>> > � � � � � � � � � | � � � �| � � � � 4.7R >>> > � � � � � � � � � | � � � �| � � � � �| >>> > � � � � � � � � --+--------+----------+---- >>> >>> > �This scheme is DC regulating as well. �The class-A current >>> > �is set by R3 and R7, so the dc voltage drop is fixed. >>> >>> Cancellation schemes give a 6dB/octave drop to a notch frequency, then a >>> 6dB/octave rise. The depth of the notch is extremely sensitive to the >>> emitter resistance and probably the temperature of the transistor. Some >>> examples may show large amounts of second harmonic distortion on the >>> output. This does not appear on the frequency analysis plot. >>> >>> In this example, the notch frequency is about 2KHz with a depth of -92dB. >>> Try changing the emitter resistance to get an idea of how critical it is. >>> >>> I don't think you want to rely on this method for any more than a minor >>> amount of cancellation, say 20 dB or thereabouts. >>> >>> Mike >> >><snip LTSpice model> >> >>20dB sounds about right. The advantages of this approach are low drop- >>out voltage and superior low-frequency noise cancellation (compared to >>practical passive equivalents). >> >>A big part of the dynamic limitation is the f.f. network rolling off. >>If you change C1 to 100uF, and tack 100uF on the output to cover the >>high-end, overall performance is much improved--nearly as good as a >>passive version using 10,000uF caps, and a lot smaller. >> >>For super massive attenuation of input noise and ripple, other >>approaches are better. >> >>If John could knock down that 50mV switcher ripple with an LC at the >>input, that's a bonus. But he won't--The Brat would kill him. > >No, I survived. The Gerbered board had... > >Wall wart connector > >Polyfuse > >Transzorb > >10 uF ceramic > >47 uH inductor > >two 10 uF ceramics and one 120 uF polymer aluminum to make "+15 >volts." That's 12 dB/octave starting at about 2 KHz. > >Then the LM8261 low-noise LDO reg, which has its own 15 ohms + 2x10uF >+ 120uF at its output.Using an LM8261 op-amp to make an LDO?
