Maintaining a Vbe Multiplier's bias value

Egads. Loads of PDF files. Now I have to create a directory, download them one by one, and then call them up with my slow machine to look. Any particular page or file where you saw it? (No, that isn't where I saw the comment.)

But thanks for the link. I'll add it to those I read, also.

Jon

Jon Kirwan wrote in message ...

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. 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. Sometimes you will see those bypassed with a large cap if the impedance causes to much signal loss. Diodes can also offer temperature compensation. In any case, an output stage will have way more current flowing in that bias chain than is actually needed as base bias current. 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. And remember, that " Fix it with feedback " applies here too. 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. That is why temperature compensation is used too. 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. 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.

The link. Do the math, it's a hoax, good only at one current and temperature pair... besides being Beta sensitive. ...Jim Thompson

I'm still in "discrete" mode. For example, I am _less_ interested in opamp topologies and design techniques than I am in _how_ to design opamps. There is nothing like knowing the details about how they are designed inside to understand the gotchas that aren't readily accessible to someone using them.

A comparison here might be like "using a handgun" vs "understanding how handguns are designed and built." A gunsmith requires a very detailed knowledge and while this level of detailed knowledge may not make them a better shooter, that knowledge still informs them about the handgun in ways that most shooters have little idea about. And I think it prepares them for certain unusual circumstances a little better.

I'm at the gunsmith level, right now. I am NOT wanting to go shooting, just yet.

Yes.

No ICs. I might like to thoroughly _understand_ the internal design of the TLV431, first. Then I'm willing to use it.

Well, I'm interested in focusing on the crafted design of Vbe multipliers, right now. I can _always_ slap a cap on whatever that winds up being, later on. So set that aside.

What also bugs me is how that darned thing is going to interact with the larger system, eventually. I don't like ignorantly littering a schematic with poles and zeros and phase delays where right now I have very little idea right what then happens when I close the outer NFB loop. I'm still "in the trenches" and trying to understand each piece in detail and think at that level. The capacitor is at the next level above and is outside my "view."

Besides, it doesn't do much for LF. The Z is too high and in parallel, ignorable.

Okay. I'm going to save it, too. I'm not ready to assimilate it, of course. But I definitely want it around when I _am_ ready for it.

Thanks, Jon

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

"Tim Williams"

More gobbledegook presented as fact.

Gotta love that on usenet.

.... Phil

Could you take a screenshot of the schematic?

Tim

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.

That makes sense.

I had thought of that, as well. Though, of course, I hadn't put values to it.

Their Eg and N would seem to suggest some difficulties with curve matching, but I agree broadly.

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.

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.

Yes, the mantra is slowly deepening within me.

Good point for me to remember!! Thanks.

I begin to see, better. Thanks, again.

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!

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

I'll include an ASCII version here:

(This was auto-generated from my LTspice to ASCII program.)

Jon

"Jon Kirwan" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...

BS, a couple of good answers have come.

Jon, you should read this book, bit torrentwise Audio Power Amplifier Design Handbook, 4th Ed. - (Malestrom) by Doug Self, one of the deeper going but still very practical publications, you'll love it. Ban

What in the world ?:-) ...Jim Thompson

View in fixed-spaced font. And it's a rendition of the schematic that Bob Monson had posted, earlier, from EDN. He wrote, "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."

I merely re-arranged it in LTspice to be a little more to my taste and then passed it through a program that generates ASCII from that.

Jon

Agreed. We are past that question.

I just received a copy of the 5th edition, today. I'll start, though the author says that it assumes a certain level of prior training. And skimming through, I agree.

Jon

Okay!!! It has a great section on Vbe multipliers under Chapter 15 on Thermal Compensation!! This is helpful. And it includes a discussion on that collector resistor there and in Chapter 7, where a chart is presented with various values for my R3 shown and the curves over current. Nice!! It also appears, on first glance, to confirm my impressions!! This is very good.

Jon

Burr-Brown was famous for using bias compensation like that in the front ends of some of their operational amplifiers, but I doubt its efficacy in power output stages.

The Burr-Brown scheme is similar to a discussion here a few (seven :-) years ago...

I am still struggling to understand it. There are very obvious parts that I completely understand. For example, the divider for a "center" voltage followed by a unity gain buffer for drive current compliance. I would guess that the gain is determined by the NFB resistor divided by the input impedance, which is mostly R4 in this case... so 10. I see an opamp looking like an integrator, but I'm frankly unfamiliar with the 4-BJT arrangement structure and need to think about that one.

I'll download it now and look when I get a moment to engage a little thought.

Thanks, Jon

I do "see" the emitter followers of Q3/Q5 on the schematic, of course. It's the R1/Q1/Q4/C1 parts that I'm assuming is the bias compensation and is the part I don't follow. The C5 looks like a very lightly applied integrator cap, which I take is needed to avoid oscillation. And that's about where I'm stuck.

Jon

Study up on writing loop and nodal equations and either solving by simultaneous equations or matrix manipulation. ...Jim Thompson

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