Maintaining a Vbe Multiplier's bias value
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From: Jim Thompson on 12 Feb 2010 20:47 On Fri, 12 Feb 2010 17:35:48 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Fri, 12 Feb 2010 18:14:37 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>On Fri, 12 Feb 2010 17:08:57 -0800, Jon Kirwan >><jonk(a)infinitefactors.org> wrote: >> >>>On Fri, 12 Feb 2010 14:38:05 -0700, Jim Thompson >>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>> >>>>On Fri, 12 Feb 2010 14:25:06 -0700, "bg" <bg(a)nospam.com> wrote: >>>> >>>>> >>>>>Jim Thompson wrote in message ... >>>>>>On Fri, 12 Feb 2010 13:43:37 -0700, "bg" <bg(a)nospam.com> wrote: >>>>>> >>>>>>>big snip - >>>>>>> >>>>>> >>>>>>Son of a gun. Yesterday you couldn't even spell "engineer", now you >>>>>>are one (;-0 >>>>> >>>>>I finally did something right? Wow!!! >>>>> >>>> >>>>Actually you found the "art" part needed to rough-in a design. Now go >>>>back and fix the flaws ;-) >>>> >>>> ...Jim Thompson >>> >>>One improvement is to boostrap from emitter to base divider >>>and use a resistor from there to the BJT base, with the >>>signal tying in directly to the base via the cap. >>> >>>Jon >> >>Don't try to get too exotic too fast... you'll get peaking, or worse, >>oscillation, if you don't know what you're doing. >> >>But that's a good thing... try it and learn from it ;-) >> >> ...Jim Thompson > >Okay. Slow down. > >So how about a simple (non-Wilson, for now) current mirror in >the emitter? It's easy to select a resistor for the other >side of it to set the current. Then an R+C leg can be used >to set the AC gain! > >Jon Go for it. You're doing fine, you're well on your way to learning! Report back... including those forays where you get burned (figuratively and actually :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jim Thompson on 12 Feb 2010 21:28 On Fri, 12 Feb 2010 18:47:46 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Fri, 12 Feb 2010 17:35:48 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Fri, 12 Feb 2010 18:14:37 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >>>On Fri, 12 Feb 2010 17:08:57 -0800, Jon Kirwan >>><jonk(a)infinitefactors.org> wrote: >>> >>>>On Fri, 12 Feb 2010 14:38:05 -0700, Jim Thompson >>>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>>> >>>>>On Fri, 12 Feb 2010 14:25:06 -0700, "bg" <bg(a)nospam.com> wrote: >>>>> >>>>>> >>>>>>Jim Thompson wrote in message ... >>>>>>>On Fri, 12 Feb 2010 13:43:37 -0700, "bg" <bg(a)nospam.com> wrote: >>>>>>> >>>>>>>>big snip - >>>>>>>> >>>>>>> >>>>>>>Son of a gun. Yesterday you couldn't even spell "engineer", now you >>>>>>>are one (;-0 >>>>>> >>>>>>I finally did something right? Wow!!! >>>>>> >>>>> >>>>>Actually you found the "art" part needed to rough-in a design. Now go >>>>>back and fix the flaws ;-) >>>>> >>>>> ...Jim Thompson >>>> >>>>One improvement is to boostrap from emitter to base divider >>>>and use a resistor from there to the BJT base, with the >>>>signal tying in directly to the base via the cap. >>>> >>>>Jon >>> >>>Don't try to get too exotic too fast... you'll get peaking, or worse, >>>oscillation, if you don't know what you're doing. >>> >>>But that's a good thing... try it and learn from it ;-) >>> >>> ...Jim Thompson >> >>Okay. Slow down. >> >>So how about a simple (non-Wilson, for now) current mirror in >>the emitter? It's easy to select a resistor for the other >>side of it to set the current. Then an R+C leg can be used >>to set the AC gain! >> >>Jon > >Go for it. You're doing fine, you're well on your way to learning! > >Report back... including those forays where you get burned >(figuratively and actually :-) > > ...Jim Thompson And do the math as you go... so you understand what you're seeing. I was fortunate, grew up in a radio and TV repair shop, and I was already math-heavy when I trotted off to MIT... had a Russian battle axe of a teacher named Evelyn Truchovesky for my first round of Algebra... she pounded me so good I adopted her way of writing "E" in my signature... to this day ;-) Washed dishes first year. Second year onward, tech'd in Woodson, Jackson, Melcher MHD lab (in Building 20)... invaluable experience! Learn Laplace short-hand, it'll be invaluable! ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jon Kirwan on 12 Feb 2010 22:05 On Fri, 12 Feb 2010 19:28:51 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Fri, 12 Feb 2010 18:47:46 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>On Fri, 12 Feb 2010 17:35:48 -0800, Jon Kirwan >><jonk(a)infinitefactors.org> wrote: >> >>>On Fri, 12 Feb 2010 18:14:37 -0700, Jim Thompson >>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>> >>>>On Fri, 12 Feb 2010 17:08:57 -0800, Jon Kirwan >>>><jonk(a)infinitefactors.org> wrote: >>>> >>>>>On Fri, 12 Feb 2010 14:38:05 -0700, Jim Thompson >>>>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>>>> >>>>>>On Fri, 12 Feb 2010 14:25:06 -0700, "bg" <bg(a)nospam.com> wrote: >>>>>> >>>>>>> >>>>>>>Jim Thompson wrote in message ... >>>>>>>>On Fri, 12 Feb 2010 13:43:37 -0700, "bg" <bg(a)nospam.com> wrote: >>>>>>>> >>>>>>>>>big snip - >>>>>>>>> >>>>>>>> >>>>>>>>Son of a gun. Yesterday you couldn't even spell "engineer", now you >>>>>>>>are one (;-0 >>>>>>> >>>>>>>I finally did something right? Wow!!! >>>>>>> >>>>>> >>>>>>Actually you found the "art" part needed to rough-in a design. Now go >>>>>>back and fix the flaws ;-) >>>>>> >>>>>> ...Jim Thompson >>>>> >>>>>One improvement is to boostrap from emitter to base divider >>>>>and use a resistor from there to the BJT base, with the >>>>>signal tying in directly to the base via the cap. >>>>> >>>>>Jon >>>> >>>>Don't try to get too exotic too fast... you'll get peaking, or worse, >>>>oscillation, if you don't know what you're doing. >>>> >>>>But that's a good thing... try it and learn from it ;-) >>>> >>>> ...Jim Thompson >>> >>>Okay. Slow down. >>> >>>So how about a simple (non-Wilson, for now) current mirror in >>>the emitter? It's easy to select a resistor for the other >>>side of it to set the current. Then an R+C leg can be used >>>to set the AC gain! >>> >>>Jon >> >>Go for it. You're doing fine, you're well on your way to learning! >> >>Report back... including those forays where you get burned >>(figuratively and actually :-) >> >> ...Jim Thompson > >And do the math as you go... so you understand what you're seeing. Don't worry, that I will do. It's what drives me. I enjoy developing different questions and doing the differentials to see where that takes me. Gives me an excuse. Anything in particular to include in the math? (I may choose to simplify, so it wouldn't hurt to have a clue where not to do that.) I assume I should be looking at instantaneous effects due to T_ambient, but what about differential T in various parts? Anything important here? (The current mirror does help a little here, I'd guess, without doing the math yet.) >I was fortunate, grew up in a radio and TV repair shop, and I was >already math-heavy when I trotted off to MIT... had a Russian battle >axe of a teacher named Evelyn Truchovesky for my first round of >Algebra... she pounded me so good I adopted her way of writing "E" in >my signature... to this day ;-) I entered college already knowing _some_ calc. Enough to score a perfect 800 on my SAT math part before entering. Didn't help me pay the bills, though, or deal with people-created paper work barriers. >Washed dishes first year. I worked a burger joint, washing dishes and cleaning; and worked also as a janitor doing commercial steam-cleaning of carpets at restaurants while taking more than a full load. Couldn't handle two jobs _and_ school _and_ the rest, at one time, at 18. Almost, maybe. But unlike in horseshoes and a-bombs, _almost_ doesn't cut it. Dad was dead, mom wasn't able to help out, and eventually decided to become self- employed. Which is how I've been, almost all the years since then. >Second year onward, tech'd in Woodson, >Jackson, Melcher MHD lab (in Building 20)... invaluable experience! > >Learn Laplace short-hand, it'll be invaluable! > > ...Jim Thompson I'm gradually getting more comfortable with Laplace, as I continue to work more problems. It is an especially nifty way to solve some differential equations, which is what it was designed to do, I think. Jon
From: JosephKK on 12 Feb 2010 23:04 On Tue, 09 Feb 2010 12:25:27 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Tue, 09 Feb 2010 06:49:11 -0500, Bitrex ><bitrex(a)de.lete.earthlink.net> wrote: > >>Jon Kirwan wrote: >>> On Mon, 08 Feb 2010 22:17:31 -0800, John Larkin >>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> On Mon, 08 Feb 2010 22:11:51 -0800, Jon Kirwan >>>> <jonk(a)infinitefactors.org> wrote: >>>> >>>>> On Mon, 08 Feb 2010 20:49:24 -0800, John Larkin >>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>> >>>>>> On Mon, 08 Feb 2010 20:43:03 -0800, Jon Kirwan >>>>>> <jonk(a)infinitefactors.org> wrote: >>>>>> >>>>>>> On Mon, 08 Feb 2010 17:54:13 -0800, John Larkin >>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>>> >>>>>>>> Hang a big capacitor across it. >>>>>>> Nice try. >>>>>>> >>>>>>> Jon >>>>>> No, seriously, that solves a bunch of problems. >>>>>> >>>>>> John >>>>> Which problems does a slew-dependent, C*dV/dt bypass current >>>>> solve? >>>>> >>>>> Jon >>>> A big cap across the biasing gadget keeps the voltage drop across it >>>> fairly constant, of course. That nukes some of the problems you >>>> referred to. More peak current is available to the output bases, for >>>> example. >>> >>> What size cap would help with power supply ripple? Seems the >>> dV/dt is so small that a fair sized cap would be required to >>> make any difference. Similarly for low frequency amplified >>> signal out of the VAS. When you say "big," maybe you mean >>> it. >>> >>> Ban is suggesting global NFB from output back to input. >>> You've said as much when you say to apply "lots of NFB." I >>> don't doubt the sincerity of either of you and I'm certain it >>> will do a lot. But right now I'm interested in seeing what >>> can be done right on this local subcircuit and at LF as well >>> as higher frequencies. Unless someone wants to walk me >>> through the thinking towards the larger concepts. I'm good >>> either way, as it's the learning that takes place I'm looking >>> for. But without such guidance, I need to move along at the >>> pace I can handle while guiding myself. >>> >>> Jon >> >>Hey Jon, I found a derivation of the input impedance of the two-resistor >>/transistor Vbe multiplier you might be interested in looking at: >> >>http://paginas.fe.up.pt/~fff/eBook/MDA/Mult_Vbe.html > >That one takes an approach that I'm not familiar with and >didn't take. I'll have to consider the approach more. >However, I did take a look at the end of it. It says: > > R = (R1+(R2||re)) / (1+(1/R1+gm)*(R2||re)) > >If I understand the value gm, and I may not, it's just 1/re >or else re=1/gm. Basically, just the (kT/q)/Ic I'd mentioned >when I wrote. If that is the case, I used these to see how >that page predicts: > > ic=.005 > vt=k*300/q > gm=ic/vt > re=1/gm > r1=1000 > r2=1000 > r2p=r2*re/(r2+re) > >and then computed: > > (r1+r2p)/(1+(1/r1+gm)*r2p) > >and got: > > 502.5719049 Ohms. > >This is so far from my own calculations of about 15.4 Ohms >that I just _had_ to put this into LTspice and test it. To >do that, I simply set up the basic circuit with the two >resistors and BJT and then hooked up a variable current >source to the topside. I set it up as an AC source of 5mA >with peaks of 500uA, and then ran a .TRAN on it and plotted >the upper rail of the structure's voltage. I used a 2N2222 >BJT, as well. Convenient, and I have them laying about. > >Anyway, so I ran the sims and got 17.44mV, peak to peak. >Divided by the peak to peak current variation of 1mA gives an >apparent R of 17.44 Ohms. My calculations arrived at 15.4 >Ohms, or so. > >All this could be operator error. I may be operating the web >page you suggested incorrectly, so that the 503 Ohms I get is >because I didn't know what I was plugging in and where. I >may be operating LTspice incorrectly, so that it's results >aren't usable and it's just luck that the numbers worked out >in my favor. > >But there it is. > >Here is the LTspice file: > >Version 4 >SHEET 1 880 680 >WIRE 128 0 16 0 >WIRE 224 0 128 0 >WIRE 288 0 224 0 >WIRE 128 32 128 0 >WIRE 16 112 16 0 >WIRE 224 112 224 0 >WIRE 128 160 128 112 >WIRE 160 160 128 160 >WIRE 128 208 128 160 >WIRE 16 224 16 192 >WIRE 128 320 128 288 >WIRE 224 320 224 208 >WIRE 224 320 128 320 >WIRE 128 336 128 320 >FLAG 128 336 0 >FLAG 288 0 V_rail >FLAG 16 224 0 >SYMBOL npn2 160 112 R0 >SYMATTR InstName Q1 >SYMATTR Value 2N2222 >SYMBOL res 112 192 R0 >SYMATTR InstName R1 >SYMATTR Value 1k >SYMBOL res 112 16 R0 >SYMATTR InstName R2 >SYMATTR Value 1k >SYMBOL current 16 192 R180 >WINDOW 123 0 0 Left 0 >WINDOW 39 0 0 Left 0 >SYMATTR InstName I1 >SYMATTR Value SINE(5m 500u 50) >TEXT -76 296 Left 0 !.tran 1 > >>For bypassing purposes the rule of thumb I've always heard is to make >>the impedance of the capacitor 1/10th the value of the impedance looking >>in to the circuit at the lowest audio frequency. > >Well, let's assume that I got lucky and LTspice and I agree >on the figure of about 16 Ohms. With a signal at 20Hz, we >are talking: > > C = 1/(2 PI f (R_ac/10)) = 5000uF > >Yikes. John L. wasn't kidding when he wrote "big." Luckily, >in steady state it could be a low voltage cap! > >Jon That is substantially larger than what i have seen in commercial audio amplifiers (about 5X to 25X), but not particularly surprising.
From: Jon Kirwan on 13 Feb 2010 00:23
On Fri, 12 Feb 2010 20:04:36 -0800, "JosephKK"<quiettechblue(a)yahoo.com> wrote: ><snip of discussion about a 5000uF cap across a Vbe multiplier> >That is substantially larger than what i have seen in commercial >audio amplifiers (about 5X to 25X), but not particularly surprising. I like the idea of first making the function blocks themselves better behaved. Jon |