Maintaining a Vbe Multiplier's bias value
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From: Martin Montonion on 13 Feb 2010 08:25 JosephKK wrote: > 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. And there are very good reasons for limiting the size of capacitors in commercial designs. Even if the increased volume and cost is not an issue, reliability will go down for a bigger cap. -- Cool links for aspiring physicists, from a Nobel laureate: http://www.phys.uu.nl/~thooft/theorist.html
From: Jim Thompson on 13 Feb 2010 11:09 On Fri, 12 Feb 2010 19:05:46 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Fri, 12 Feb 2010 19:28:51 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > [snip] >> >>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 Actually, shorthand Laplace was created by none other than Oliver Heaviside: http://en.wikipedia.org/wiki/Oliver_Heaviside ...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 13 Feb 2010 14:40 On Sat, 13 Feb 2010 09:09:39 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Fri, 12 Feb 2010 19:05:46 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Fri, 12 Feb 2010 19:28:51 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >[snip] >>> >>>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 > >Actually, shorthand Laplace was created by none other than Oliver >Heaviside: http://en.wikipedia.org/wiki/Oliver_Heaviside > > ...Jim Thompson Now that's an impressive man. Thanks! I'll dig in! Jon
From: JosephKK on 13 Feb 2010 19:48 On Tue, 9 Feb 2010 18:58:36 -0800, "Bob Monsen" <rcmonsen(a)gmail.com> wrote: >"Jim Thompson" <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote in >message news:4604n5lppq5jocclt3si1gdjveeu345den(a)4ax.com... >> >> Something smelly there. I'll try it tomorrow... the Moo Goo Gai Pan >> I love to cook with wine. Sometimes I even put it in the food. > >The original is from EDN, Oct 22, 2009, page 45. "Class B amplifier has >automatic bias". > >Regards, > Bob Monsen > I found the original on the 'net http://www.edn.com/article/CA6702272.html?text=%22Class+B+amplifier+has++automatic+bias%22 Something fishy indeed.
From: JosephKK on 13 Feb 2010 19:49
On Tue, 9 Feb 2010 18:10:10 -0600, "Tim Williams" <tmoranwms(a)charter.net> wrote: >"Jim Thompson" <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote in >message news:mhr3n5pv3mnp4dlfo1pmvib066vjjovj44(a)4ax.com... >> Did something get lost in the ASCII? Otherwise collector-to-collector >> as in Q1-Q4 is a no-no... one of those devices will saturate. > >It's a current mirror, based on hFE instead of Vbe (yuck!). When Q1 or Q4 >saturates, bias current (or op-amp current) is diverted to the output >transistors, driving the load. > >Tim No, i don't think it is even that good. |