Maintaining a Vbe Multiplier's bias value
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From: Jon Kirwan on 9 Feb 2010 16:20 On Tue, 09 Feb 2010 14:07:27 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Tue, 09 Feb 2010 12:59:01 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Tue, 09 Feb 2010 08:33:45 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >>><snip> >>>Useless nonsense.... >> >>Are you talking about my comments, those of others here, or >>the link you posted below? >> >>>http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf >> >>I'll take a look, today. >> >>Thanks, >>Jon > >The link. Do the math, it's a hoax, good only at one current and >temperature pair... besides being Beta sensitive. > > ...Jim Thompson Ah! Thanks! I can use lessons like this, too! If I can see what you see there, then that means something. Very good way to teach. Will keep your points in mind as I read it. Jon
From: Phil Allison on 9 Feb 2010 16:46 "Tim Williams" > > The general idea is to put the Vbe transistor on the same heatsink as the > outputs, if not glued to a transistor directly. > > Unfortunately, for widely mismatched current densities, this doesn't work. ** Huh ?? More gobbledegook presented as fact. > http://webpages.charter.net/dawill/Images/Ampere.gif > In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough > tempco to compensate the far beefier (= lower current density??) output > darlingtons. ** Proof by assertion and an isolated example known only to the poster. Gotta love that on usenet. ..... Phil
From: Tim Williams on 9 Feb 2010 16:48 "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? Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Jon Kirwan on 9 Feb 2010 17:08 On Tue, 9 Feb 2010 14:05:32 -0700, "bg" <bg(a)nospam.com> wrote: ><snip> >Your circuit is an example of collector feedback. Collector feedback does >not work well with large signal swings and it lowers the input impedance. A >lower input impedance means that the drivers will also need a lower output >impedance. The bottom line is that you will not see this bias method used in >a power output stages. Another option is to use emitter feedback but for the >emitter resistors to be effective, they will drop alot of signal output and >waste power, which explains why those resistors are usually very low values, >They have very little effect unless the emitter current is large. At the DC >bias current level, they don't do a thing. >Local feedback (emitter feedback or collector feedback) both create more >problems then they solve for stabilizing the power output stage bias point. >To keep the output stage bias point stabilized, the use of overall feedback >is the standard practise. A simple typical amplifier might have a >differential input stage, followerd by a voltage amplifier, followed by the >power output stage. The output from the power stage is fed back to the >differential input stage. High open loop gain with large feedback is the key >to better stabilization of the operating points. Fix it with feedback is a >term to remember. This point is made, again and again, so it must be true! And if it weren't true, why else would opamps have been such a successful building block?? So I completely buy the idea. I am still trying to study each part, though. At some point, I will raise my head a bit above that level and take a larger look. But I'm not yet prepared for it, as the pieces themselves are still too fuzzily understood. I want to quantify those, in detail, before expanding my view. In doing so, I hope to have a somewhat better understanding of opamps, themselves, too. Not just from a large scale view, but also in understanding problems within them and how to choose among various approaches when struggling with a specific application in mind. I take your point. But it doesn't change, at all, my interest in seeing what can be reasonably done with at teh Vbe multiplier level to accomodate variations in current. That remains interesting in and of its own right. >The typical bias chain using diodes can be made with resistors as well, but >diodes have the advantage of dropping the bias voltage while having a lower >impedance to the signal. That makes sense. >Sometimes you will see those bypassed with a large >cap if the impedance causes to much signal loss. I had thought of that, as well. Though, of course, I hadn't put values to it. >Diodes can also offer temperature compensation. Their Eg and N would seem to suggest some difficulties with curve matching, but I agree broadly. >In any case, an output stage will have way more >current flowing in that bias chain than is actually needed as base bias >current. This seems to argue with something I read John L. saying, but I didn't accept it (or reject it), yet. I will need to get there in due course. But I'm still taking pieces one at a time. >The voltage drops developed in the bias chain will not be greatly >affected by changes in the base emitter junction because the base bias >current is small compared to the current in the bias chain. I read this sentence a few times to try and make sure I followed it well. If I do, and I may not, I think I addressed this when I tried to calculate the R_ac figure. The numbers I come up with for a 5mA "bias chain" current with 1mA in the base bias current and 4mA in the collector, come out as around 15 Ohms, or so. If I'm right about that, it seems almost certain that there is _some_ response to even modest variations in current through it. A 500uA change yields a 7.5mV change. When I LTspice it, I get a simulation that matches what I calculate, too. >And remember, >that " Fix it with feedback " applies here too. Yes, the mantra is slowly deepening within me. >So variations in the power >supply have a very reduced effect on the bias point. The feedback signal is >a voltage, and enough feedback will compensate to keep the output voltage >offset at zero. It will not compensate for for excessve collector currents >or power dissaption if the offset voltage remains low. Good point for me to remember!! Thanks. >That is why temperature compensation is used too. I begin to see, better. Thanks, again. >In the early years of transistors, it was common to see transistor stages >using many of the techniques used with vacuum tubes. Dc coupled amplifiers >were rare, because any bias shift was amplified in further stages. Feedback >was applied locally, and overall feedback had no effect on the DC operating >points. I seem to recall that vacuum tube amplifiers even let the consumer modify the global NFB. But, as you say, since it wasn't so critical to the design that was probably why it was allowed in the first place. With BJTs, it seems now to me that global NFB is _so_ important that such things cannot be left as "tweeks" by some consumer playing with a knob! >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." I can gather and accept it, of course. But I also cannot believe an amplifier can be designed with bags of random bolts tossed together and "fixed with global NFB" in the end. There are parts in there and they need to perform some intended function to some reasonable approximation. And I am still working on understanding each piece as well as some thoughts about various approaches at that level to improve the ideas. For example, it's important to understand not just vaguely, but quantitatively on various scores, how a diff-amp behaves and why I may want to have a current mirror on the tails. I don't want to just hear "put a current mirror there" and learn nothing then about why. Later on, when I'm looking globally at an amplifier, I can look backwards and say, "Hmm. That Wilson mirror is great, but I really don't need it. The bog standard 2-BJT mirror is fine enough." But I want to say that from _understanding_ the details, not from others merely assuring me about it. See the difference? Meanwhile, I'm still interested in seeing if my quantitative analysis was correct (or wrong) and if there are some other topologies for it, other than the two I mentioned, that may be interesting to look at. Thanks, Jon
From: Jon Kirwan on 9 Feb 2010 17:30
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 |