Maintaining a Vbe Multiplier's bias value
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From: Tim Williams on 9 Feb 2010 19:10 "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 -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Tim Williams on 9 Feb 2010 19:19 "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:9kk3n5tjcntp1b1itrlbldjgta7st0oai8(a)4ax.com... >>The trend now is to stabilize everything with feedback. It works, >>and it works well. Unless you are a purist and have some religious reason >>to >>avoid this technique, there is no sense in reinventing the wheel. > > It's not a religious reason, unless _learning_ is a religion, > I suppose. I don't mind being told that "one day when you > are ready, you will use global NFB to take care of this." There are valid reasons for doing so. High bandwidth is one: when you need high GBW, you don't have time to wait for the signal to propagate through the 5 or 10 or 100 transistors you have[1]. Those kinds of systems are usually built with a lot of low-gain, high bandwidth stages -- the feedback is local, so the phase shift per stage is small, meanwhile the overall phase shift (from input to output) can be arbitrarily large. The disadvantage is there's nothing global to account for distortion or DC offset. However, you can add low frequency servos to stabilize it, getting the best of both worlds. [1] Unless you happen to be Linear Technology. For instance, their LT1016 stupid-fast comparator claims an internal 60GHz GBW which is unity-gain stable (you can use it as an op-amp). Better not have anything near that feedback node. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Jon Kirwan on 9 Feb 2010 19:23 On Tue, 9 Feb 2010 17:48:43 -0600, "Tim Williams" wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:sgr3n5tb6hamiocr5fjb37e5lj5t9asu0l(a)4ax.com... >> I'm also familiar with matrix solutions and have developed my >> own programs for solving them a little easier than my TI >> calculator allows for and with better accuracy in difficult >> cases. I can also do Laplace, but frankly I have NOT yet >> learned the shortcuts often used. So I wind up with pages of >> partial fractions in the end, converting back to time domain >> with tables, and seeing how things look there. > >Find "residues" in your TI, it's exactly what you need. That's something I know I have yet to study and so I have NOT looked them up in the TI manual. I'll need to study and understand them much better than now, before trying that. >Also, polynomial >factorization, if you aren't using it already. I use it sometimes when I have the TI nearby. I don't carry it, everywhere, though. And I do think about problems lots more places than where I have a TI nearby. Besides, it's nice to have it in my head and to keep on working on that part with practice. I remember some of the conversion table and can actually _do_ some derivations the hard way when I am lacking both tables _and_ a calculator. I do like keeping up a personal skill, so that I'm not overly frustrated when the mood strikes and the rare tools are not handy at the moment. I always have a finger, my brain, and some dirt no matter where I am, unless I wind up chained to some prison wall. Then I'll have to work on my memory, too. ;) With your recommendation, it's now off to find a good book on complex analysis. Jon
From: Jon Kirwan on 9 Feb 2010 19:36 On Tue, 09 Feb 2010 17:01:50 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Tue, 09 Feb 2010 15:35:14 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Tue, 09 Feb 2010 16:24:43 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >>>On Tue, 09 Feb 2010 14:30:54 -0800, Jon Kirwan >>><jonk(a)infinitefactors.org> wrote: >>> >>>>On Tue, 9 Feb 2010 15:48:42 -0600, "Tim Williams" >>>><tmoranwms(a)charter.net> wrote: >>>> >>>>>"Bob Monsen" <rcmonsen(a)gmail.com> wrote in message >>>>>news:1265746051.657782(a)sj-nntpcache-3.cisco.com... >>>>>> On a related note, there was an article in a recent EDN about a self >>>>>> biasing preamp which was kinda cool. Instead of trying to track the >>>>>> difference using diodes or a multiplier, it used a couple of transistors >>>>>> and an opamp to set the correct values at the bases of the pass >>>>>> transistors. It was so novel (at least to me) that I typed it into >>>>>> LTSpice. Here it is: >>>>> >>>>>Could you take a screenshot of the schematic? >>>> >>>>I'll include an ASCII version here: >>>> >>>>>: R2 >>>>>: +V = 12V ,------/\/\---------------------, >>>>>: | 1k | >>>>>: | +V | >>>>>: | | | >>>>>: | \ | >>>>>: | / R1 | >>>>>: | \ 1k5 | >>>>>: | / | >>>>>: | | +V | >>>>>: C2 | ,----+ | | >>>>>: || 10uF R4 | | | 2N3904| | >>>>>: ,------||------/\/\---------+ | | | | >>>>>: I| || 100 | | Q4 e>| |/c Q3 | >>>>>: N| | | |-------| | >>>>>: | | | c/| |>e | >>>>>: | | C1 --- | | | >>>>>: --- | 10uF--- |2N3906 | | >>>>>: - V2 | | | | | >>>>>: --- SINE(0 .2 1k) | | | | | C3 >>>>>: - | C5 | | | | || 470uF >>>>>: | | || 10p| | +----+-||----,O >>>>>: | +V +---||----+ | | || |U >>>>>: | | | || | | | |T >>>>>: gnd | | | | | \ >>>>>: | | | |2N3904 | / R5 >>>>>: \ ,-------, | | | | \ 8 >>>>>: / R3 | | | +V | Q1 c\| |<e Q5 / >>>>>: \ 1k | +V | | | 2N| |-------| 2N3906 | >>>>>: / | | | | |\| | e<| |\c | >>>>>: | | |\| | '-|-\ | | | | >>>>>: | '--|-\ | | >----+----' | gnd >>>>>: | | >-+-----|+/ | >>>>>: +-------|+/ |/| LT6234 | >>>>>: ,-----+ |/| LT6234 | gnd >>>>>: | | | gnd >>>>>: --- C4 \ gnd >>>>>: --- 1uF/ R9 >>>>>: | \ 1k >>>>>: | / >>>>>: | | >>>>>: gnd gnd >>>> >>>>(This was auto-generated from my LTspice to ASCII program.) >>>> >>>>Jon >>> >>>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. >>> >>> ...Jim Thompson >> >>I just double-checked. It's just that way in Bob's posted >>LTspice schematic. And when I simulate the thing, it >>produces a 2V p-p output into R5 from a .2V p-p input. Takes >>a few cycles to settle on a DC center level, though. >> >>Jon > >A demonstration of why not to trust simulators. Among many other such demonstrations -- and a good part about why _I_ need to understand these things, for myself. >What does LTspice >show for the voltages at Q1:c and Q1:e? Q4:c and Q4:e? > > ...Jim Thompson I know your own preference to include diagrams, rather than just words. So here is a link: >: http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1 e+c.gif Forgive the coloring. I didn't mess with it to make it prettier. Jon
From: Jon Kirwan on 9 Feb 2010 19:39
On Tue, 09 Feb 2010 16:36:40 -0800, I wrote: >I know your own preference to include diagrams, rather than >just words. So here is a link: > >>: http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1 e+c.gif Sorry. I missed a %20 in there, so it may or may not work in your browser. Try: http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1%20e+c.gif Jon |