Maintaining a Vbe Multiplier's bias value
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From: John Larkin on 11 Feb 2010 23:18 On Thu, 11 Feb 2010 19:04:07 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Feb 10, 10:34�pm, "Phil Allison" <phi...(a)tpg.com.au> wrote: >> "George Herold" >> �"Phil Allison" >> >> >> >> >> >> >> >> > ** Push-pull class A is the MOST COMMON method use in tube and transistor >> > audio power amplifiers. >> >> > With transistor amps, it is only necessary to set to standing bias current >> > to a high value like an amp or two. >> >> > > You can, but it stops being class A for large signal swings or low load >> > > impedances. >> >> > ** No need for either thing to happen. >> >> > One picks the load to suit the amplifier and the peak ( class A) current >> > is >> > double the bias setting current. >> >> > Eg: >> >> > With a bias of 2 amps, peak load current is 4amps which allows +/-32 volts >> > into 8 ohms. >> >> > Class A power is then 64 watts rms. >> >> > The DC rails need to be about +/- 35 volts and the supply current a steady >> > 2 >> > amps. >> >> Do you just use a current source as bias as in the Amps from Pass labs? >> >> ** No �- �doing that is utterly STUPID. >> >> The only difference between a class A and class B push-pull amp �IS �the >> bias setting. >> >> One just turns the bias trim pot to get the desired standing current or with >> tubes adjust the grid bias to get the same result. >> >> > Tubes can't do push-pull because "it's hard to get positrons from the >> > filament" �to parapharse what I read. >> >> ** How pathetic. >> >> .... � �Phil- Hide quoted text - >> >> - Show quoted text - > >Excellent, Thanks Phil. Crank up the bias. > >I think almost everything I build runs class A. I set a DC bias and >then modulate on top of it. I run an opamp into a pass element, apply >feedback and then make the load 'look' as resistive as possible. >(Then cross your fingers and see if that works.) > > >So how do you do push pull with tubes, or say with only Jon's npn >transistors? > >George H. Usually two "NPN" tubes whose plates drive a center-tapped output transformer. John
From: George Herold on 11 Feb 2010 23:24 On Feb 11, 11:08 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote: > "George Herold > > So how do you do push pull with tubes, or say with only Jon's npn > transistors? > > ** There are literally *millions* of push-pull tube amps in use - the vast > majority of tube hifi and guitar amps are push pull designs. > > NPN output transistor amps are called "quasi-complementary push pull " - > many millions of them made and sold too. > > Use Google to find the schems. > > Idiot. > > .... Phil Thanks Phil I'll try google. Why are there still so many tube amps? You'd think someone could make a solid state amp that sounded 'right'. George H.
From: Ban on 11 Feb 2010 23:33 "George Herold" <ggherold(a)gmail.com> schrieb im Newsbeitrag news:31c905ec-8918-4164-87e8-b524d6b9a156(a)i39g2000yqm.googlegroups.com... On Feb 11, 5:27 pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: > On Thu, 11 Feb 2010 11:50:23 -0700, Jim Thompson > > > > > > > I'd still like to _learn_ about FAB processes, geometries, > mask steps, subtrates (and if any BJTs include a bond to such > things), and differences between them. For example, I've > heard you talk about processes that include gold as a step > (or more?) I'd like to know what does what. I can (and have > attempted) a few 2D spatial integrals aka Hauser's analysis > of crowding on r_b many years ago, and I'm vaguely aware of > the fact that he neglected to account for lateral base > diffusion which happens when the crowding and some local base > widening takes place. I actually _did_ take measurements of > real Hamamatsu diodes, years ago, and reversed out from the > measurements what the dopant concentrations had to have been > so that I could better model the behavior over a wide range > of temperature operations (Hamamatsu flatly refused to give > me any such information.) The resulting model I created > _did_ model that photodiode at -40C to 55C better than I'd > expected it to do and much better than the gross models I had > at the time were able. So at some point, I'd like to study > these things to get a better feel... but I'd like to know who > has what FABs and what the processes are capable of and > produce. > > I hope it's not as difficult as pulling dopant numbers out of > Hamamatsu was! I'm not wanting to know specific recipes or > anything -- just process capabilities. Hopefully, FAB and > process capabilities and locations are something that is > known about and published. I can hope. > > Jon- Hide quoted text - > > - Show quoted text - A very comprehensive book, bible lets say is The Crcuits and Filters Handbook by Chen don't get fooled by "Handbook", it's the biggest book I got. :-)) ciao Ban
From: Ban on 11 Feb 2010 23:38 "George Herold" <ggherold(a)gmail.com> schrieb im Newsbeitrag news:f7a4838e-716b-40a8-9b19-7c65cf86dff9(a)o3g2000yqb.googlegroups.com... On Feb 11, 11:08 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote: > "George Herold > > So how do you do push pull with tubes, or say with only Jon's npn > transistors? > > ** There are literally *millions* of push-pull tube amps in use - the vast > majority of tube hifi and guitar amps are push pull designs. > > NPN output transistor amps are called "quasi-complementary push pull " - > many millions of them made and sold too. > > Use Google to find the schems. > > Idiot. > > .... Phil >Thanks Phil I'll try google. Why are there still so many tube amps? >You'd think someone could make a solid state amp that sounded >'right'. A lot of transistor amps sound right, but some people prefer to add a bit of 2nd harmonics, or they want to enjoy the gloom. ciao Ban
From: Ban on 11 Feb 2010 23:41
"Phil Hobbs" <pcdhSpamMeSenseless(a)electrooptical.net> schrieb im Newsbeitrag news:U9-dnabS2fr6SOnWnZ2dnUVZ_uti4p2d(a)supernews.com... > On 2/11/2010 10:35 PM, George Herold wrote: >> On Feb 11, 1:14 pm, John Larkin >>> >>> If you drive both adjust pins with the signal input, the 317 output is >>> Vin+1.25 and the 337 output is Vin-1.25. Connect them to the output >>> through a couple of resistors, valued to set the idle current. Where's >>> the deadband? >>> >>> Or you can take the output from the 317 output pin, with the 337 now >>> acting like a constant-current sink to the 317. >>> >>> I like to use LM1117s as power emitter followers, inside the loop of >>> an opamp. That makes a cheap, well protected power driver, for load >>> cell excitation and such. I did a bunch of tests to see whether >>> flailing the adj pin can damage the regulator, and never managed to >>> break one. >>> >>> John >> >> Cool! I think I got it... though if I try it in the future and let >> the smoke out of something... then I might have questions. >> >> George H. > > IIRC the LM395 is basically an LM309 with the voltage reference removed. > Emitter-follower regulators are nearly bulletproof unless you discharge a > cap into the output. > or the reference input. But some power diode antiparallel will take care of that. ciao Ban |