Maintaining a Vbe Multiplier's bias value

I haven't. It's not rocket science. But it is interesting at my level.

I'll look, but the title appears more on the contruction side. I am using this to educate myself a little better.

I recall reading of a recommendation suggesting that all electronic devices use less than 1W when on, but not performing their intended application. This would also seem to require a little added effort to achieve, as well. But I take your point.

I'm still learning about BJTs. In fact, that's what this is about for me. MOS later. ICs later. BJTs now.

Jon

I've not. Could you be more specific?

Jon

I think your perspective is more comprehensive than mine, obviously. And I am still building up bits, piecewise. It's how I have to approach this.

That said, and I may yet be getting this wrong, but it seems to me that I've seen some serious attention in amplifier schematics; not only looking at global NFB to solve such problems -- though that seems central, of course.

But I need to take things one at a time, right now. This is education for me, after all. Not constructing an amplifier to solve some problem I have. I've no problem focusing upon this, a bit, until I subsume it, and then not wind up using all the options I looked at. Learning doesn't only come from taking all the right steps, but also from taking many others that aren't entirely in the right direction. I could only hope to be so perfect as to never choose wrongly. And if so, I probably wouldn't be learning.

Anyway, the understanding I wrote can certainly be wrong. However, your later comment seems to say it is wrong because it's "better" to do it using the NFB from output to the diff amp. Yet I still wonder if doing some of this locally is appropriate. In any case, it seems certain that temp comp is one of its functions. Unless I've somehow completely missed things altogether.

If I set it "wide enough" it seems to operate that way. Perhaps I've got that wrong, too?

I think it is useful for me to learn by studying the small building blocks, right now, and considering some thoughts (but not necessarily all the right ones) about larger issues these may need to cope with. I'm in no way ready for the "larger view" you are talking about. Not yet. And only a rare few can start there. I'm not such. For me, it goes from small to large, then back to small, then back out again, and so on until it "gels."

I think I will take this structure just a little further in thinking... perhaps a 3rd BJT, I'm thinking. But not more than that. Diminishing returns. I was just wondering if there was more I hadn't come across. Perhaps not.

Thanks, Jon

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

Well, George... No, I've not abandoned it. Actually, it's my hope to wind up building the amplifier and then operating it (by hopefully choosing a design where that is possible) in different modes for the learning experience of it. I hope that is in the cards. I really do.

But to make a sharp point on it, although it's probably just an extreme case, I remember reading about a 10W amplifier, single channel, dissipating 120W! Creeps me out. So I definitely _want_ to consider other classes of operation. And cripes, I want to learn, anyway. So why not keep my options open?

Jon

ASCII is what I'll post. It's the only way to get them archived or properly posted to a text newsgroup. I no longer have access to the binary for schematics, sadly. If I lose some people because they cannot manage fixed-spaced fonts, I guess I lose them. I could place links up on my domain, I suppose. But in this case, the schematics are really very basic and not overly burdensome in ASCII. Besides, Win Hill posted some really nice examples here, before. Folks seemed to live with that. Not sure why you are picking on me, here.

Okay. I'll do that if folks here aren't interested at all in talking about it.

Jon

Which makes sense to me. I think I already understood this, generally, if not in intimate detail. One of the reasons I included in the opening salvo, talking about Eg matching.

I need to think about this, more. As you write above, several alternatives appear in my mind and if you wouldn't mind including a short example, I'd appreciate it.

I can't agree, yet. Slow changes, without a crazy-sized cap there, will have the same effective R_ac I'd mentioned before. The cap's Z just won't change it. And I'm not ready to chalk everything up and pile it all onto the global NFB, either -- not because I disagree with you or John, because I can't... I just don't know enough either way. But because this whole post is about _learning_ something.

In particular, I was very specific about what I'd like to study right now. Vbe multipliers and various incarnations that may help to deal with current ripple (from 20Hz to

20kHz, I suppose.) I'm wanting to make sure my analysis so far isn't grossly wrongly made, accepting corrections as they arrive, and I'd like to consider interesting ideas, too.

Hopefully, my question here on this narrow subject won't be taken as "Well, what does he know about the field of audio amplifier design?" If that's the question to be asked, the answer is easy. "Not much."

Cripes, I just started looking at the whole idea about two weeks ago. If I knew enough to ask all the right questions on this topic in barely more than 10 days, I'd likely be headed into being the next Bobby Fischer of audio amps!

I'm just a hobbyist, for gosh sake. I found a few circuits on the web that included a collector resistor in the Vbe multiplier and, at first, had no idea why it was there. I asked in .basics and no one else seemed firm about knowing, either. I grew more curious about it and sat down and lo, and behold, the scales fell from my eyes and I could _see_! I could actually see why it was there. Not only why, but how to estimate quantitative values for it and what to expect as a result. It's that sense of discovery that sometimes pushes one further.

So I want to understand a little better how one might do even more about compensating vs current variations? In this focus, I don't even need to care about amplifier design, at all. It is purely about the Vbe multiplier right now and nothing else. Sure, audio amplifier design questions caused me to look more closely at this structure. That was my inspiration to set down this short path, right now.

But is it wrong to want to explore this area a little more before moving on?

I think I see that, though I'll see it a lot better later on. Hopefully where I'll be able to put quantities to it.

I think I gather that much. It's got very low impedance when the dV/dt is there.

Yes, and yes. John L. mentioned this, too, a week ago and more. No question I've got the point, there. The CCS's aren't perfect and where they don't do so well, it gets all nicely lumped into the global NFB and left for it to deal with. I don't mind, though, investigating things just a little more. And I will very soon start taking on the CCS's themselves. I know a few and I know there are a lot more than I'm not even remotely aware of, too. So that is going to be fun. But I'm not one to just borrow and run. I need to _understand_ the mathematics and try my hand at deriving certain features in quantitative ways, not just qualitative ones. I expect to analyze at least four or five different CCS structures before I move on, in what quantitative detail I can manage at the time.

I've seen that done time and time again in ICs. I can remember tracing my fingers from one to another to another as I spent time understanding them better.

Last July, in fact, here in this very group, I posted this about the LM334:

No one here _did_ fully answer my question, Tim. There it is, and I did try to follow it.

I'm aware of the frequent practice, at least.

Intermodulation distortion?? I never saw the term IMD before, but that seems as though it must be what you just said. Fits, anyway. Which brings me back to the MC1495, again. And yes, I think I see why you bring it up.

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:

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.

Miso is all blabber and no content... best if ignored ;-) ...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.

(1) Split R1, bypass that junction to ground

(2) Make R5 and R6 into mirrors, resistor feed from VDD, but split and bypassed.

(3) As you said, replace R4:R3 with a mirror, I don't think a compound device mirror, such as Wilson, is necessary. Study this if you want more info....

(4) Since you're on a learning curve, just replace D1/D2 with 1.5*Vbe, losing about 1/5 of the Q3 quiescent current in the resistors. Bypassing base-to-base (of Q4-Q5) will help at all but very low frequencies.

(5) Long haul as you "oomph" the power: Q3 goes to Darlington, as do Q4 and Q5; D1/D2 becomes more complicated (Darlington extension of Vbe multiplier).

Start simple, then grow it, that way you learn before you flame it ;-) ...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.

A trek of 23,000 miles starts with but the first step in the wrong direction.

John

Can you explain "mushy" in any more technical terms?

John

Useless nonsense....

...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.

Not while I'm drinking my coffee, please... :)

Regards, Bob Monsen

u

" I remember reading about a 10W amplifier,

It might have been here,

I got to reading about amplifiers on the above site... Do in part to your interest.

George H.

"Jon Kirwan" wrote in message news: snipped-for-privacy@4ax.com...

Sorry, I didn't read the entire message...

However, if you want a stiff multiplier, use a TLV431 instead of a BJT. Somewhat more expensive, but it'll be VERY stiff.

However, you don't really want to hold that value constant. You want the voltage to compensate for the temperature of the output transistors. You might be able to use a diode to track the temperature change, and then use that in the feedback loop to compensate the TLV431.

A honking big capacitor, one that has very low impedance at your frequencies of interest, is probably the best idea I've seen on the thread.

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:

Version 4 SHEET 1 948 680 WIRE -288 -304 -608 -304 WIRE -16 -304 -288 -304 WIRE 144 -304 -16 -304 WIRE 320 -304 144 -304 WIRE 512 -304 320 -304 WIRE -288 -272 -288 -304 WIRE -608 -240 -608 -304 WIRE 320 -240 320 -304 WIRE 320 -128 320 -160 WIRE 320 -128 240 -128 WIRE 512 -128 512 -304 WIRE 448 -80 384 -80 WIRE 240 -64 240 -128 WIRE -608 -16 -608 -160 WIRE -160 -16 -608 -16 WIRE -128 32 -464 32 WIRE 48 32 -48 32 WIRE 112 32 48 32 WIRE 512 32 512 -32 WIRE 512 32 192 32 WIRE 560 32 512 32 WIRE 656 32 624 32 WIRE 672 32 656 32 WIRE -464 96 -464 32 WIRE 48 96 48 32 WIRE 128 96 48 96 WIRE 240 96 240 0 WIRE 240 96 192 96 WIRE 320 96 320 -32 WIRE 512 96 512 32 WIRE 672 128 672 32 WIRE -608 144 -608 -16 WIRE -608 144 -704 144 WIRE 32 144 -96 144 WIRE 448 144 384 144 WIRE 144 160 144 -304 WIRE -464 176 -464 160 WIRE -16 176 -16 -304 WIRE 48 176 48 96 WIRE 112 176 48 176 WIRE -96 192 -96 144 WIRE -48 192 -96 192 WIRE 240 192 240 96 WIRE 240 192 176 192 WIRE 320 192 240 192 WIRE -464 208 -464 176 WIRE 32 208 32 144 WIRE 32 208 16 208 WIRE 112 208 32 208 WIRE -160 224 -160 -16 WIRE -48 224 -160 224 WIRE -608 240 -608 144 WIRE -704 256 -704 144 WIRE -704 352 -704 320 WIRE -608 352 -608 320 WIRE -608 352 -704 352 WIRE -464 352 -464 288 WIRE -464 352 -608 352 WIRE -288 352 -288 -192 WIRE -288 352 -464 352 WIRE -272 352 -288 352 WIRE -16 352 -16 240 WIRE -16 352 -272 352 WIRE 144 352 144 224 WIRE 144 352 -16 352 WIRE 512 352 512 192 WIRE 512 352 144 352 WIRE 672 352 672 208 WIRE 672 352 512 352 FLAG -272 352 0 FLAG 656 32 out FLAG -464 176 in SYMBOL npn 384 96 M0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL npn 448 -128 R0 SYMATTR InstName Q3 SYMATTR Value 2N3904 SYMBOL pnp 384 -32 R180 SYMATTR InstName Q4 SYMATTR Value 2N3906 SYMBOL pnp 448 192 M180 SYMATTR InstName Q5 SYMATTR Value 2N3906 SYMBOL voltage -288 -288 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL cap 224 -64 R0 SYMATTR InstName C1 SYMATTR Value 10µF SYMBOL res 208 16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res -32 16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 100 SYMBOL voltage -464 192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 .2 1k) SYMBOL cap -480 96 R0 SYMATTR InstName C2 SYMATTR Value 10µF SYMBOL cap 624 16 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 470µ SYMBOL res 656 112 R0 SYMATTR InstName R5 SYMATTR Value 8 SYMBOL res 304 -256 R0 SYMATTR InstName R1 SYMATTR Value 1k5 SYMBOL Opamps\\LT6234 144 192 R0 SYMATTR InstName U1 SYMBOL res -624 224 R0 SYMATTR InstName R9 SYMATTR Value 1k SYMBOL cap 192 80 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C5 SYMATTR Value 10pF SYMBOL res -624 -256 R0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL Opamps\\LT6234 -16 208 R0 SYMATTR InstName U2 SYMBOL cap -720 256 R0 SYMATTR InstName C4 SYMATTR Value 1µF TEXT 552 -296 Left 0 !.tran 0 .1 0 1u TEXT 552 -256 Left 0 !.four 1k 10 v(out) TEXT 552 -216 Left 0 ;.noise V(out) V2 oct 1001 1 100k

Regards, Bob Monsen

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

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

There is no wrong direction at the start. It's all good.

Jon

Understood.

I had been thinking more like this structure:

However, I take your point.

Understood. Although I'm not able to make my own decisions on this, yet, I've read repeatedly that the distortions to deal with are not at the input stage. The input stage can be made better, the improvements are small in comparison to what remains in the rest of a well-designed system. Point taken.

Thanks, I will!

Thanks for taking a moment to confirm the "1/5th" division. I'd already figured that was commonly done and had some ideas of my own about why that makes sense. (I could talk about that, but I'm sure you already know and I think I know, too.)

Okay. That's how I see it, too.

This is what I'd like to explore, right now. Extensions. It's because it is where my mind is at, right now. And I want to explore this more fully before walking away from it and moving on.

hehe. Good advice, of course. As I'm still struggling to make sure I understand each piece, right now, I'm just not yet ready to put it all together -- not even in a low power system. I might be able to vaguely grasp what I would be doing, but I prefer taking each part and thoroughly looking at its function before moving on. Then, when I look once again at the whole, I can better "read" what I see and that helps a lot in terms of gaining a global view. I'm still in the trenches, right now, and not allowing myself to raise my head much above that until I get some of the details nailed down.

Speaking of that, can you confirm (or correct) the equation I developed for the simple Vbe multiplier's small signal R? Or the relative scale and _sign_ of the Early effect correction to it, which I peg near -1 part per thousand in the case I cited?

All this is good for me to go through.

Thanks, Jon