On Semi ThermalTrak

[DouglasSelf]

Originally posted by albin
what about Erik Margans use of diodes to control Iq?dare I ask Douglas.I built his test circuit but replaced the bias transistor with two diodes in series.they worked, I thought, but I could be wrong,
Please ignore or not
regards
max albin

If you can remind me when & where the Margan article appeared, I'll take a look.

Douglas

[DouglasSelf]

Originally posted by megajocke
But if we put the diodes in the upper leg of the Vbe multiplier base circuit (like leach) their tempco won't be multiplied. And 160µA sounds like a reasonable current to have flowing there, just use a buffer for the Vbe multiplier transistor and it will have enough beta to be stable with vas current.

Or is your worry that when doing this the tempco of the vbe multiplier transistor part will be too high, decreasing idle current with increased ambient/driver temperature? But this should be pretty stable.

edit:
"But this should be pretty stable." was referring to the ambient temperature which won't vary as much as output transistor case temperature.

Hi

I'm not saying it wouldn't work, though you are right that at least a two-transistor Vbe multiplier would be needed to avoid base current issues.

My main purpose was to see if we agree that the conceptual circuit is what we should be aiming for.

Douglas

[JPV]

Originally posted by DouglasSelf


Hi JPV, sorry I couldn't get back to you yesterday.

My first attempt at applying the ThermalTraks was to assume that the diodes would be used to attempt to directly cancel the transistor Vbe voltages,which means running the diodes at 25 mA (for transistor Ic=100mA) and accepting that the tempco would not match exactly. The compensation should, I think, still be much better than conventional methods but it is certainly not theoretically perfect. This is the approach I used in the attached circuit, which I think of as Plan A. I have never tried to run a VAS at 25 mA,and I fear the linearity might be worse. Certainly TO-92 transistors will get too hot for amplifiers above 100W/8R, which is a problem as the bigger devices tend to have less beta. A VAS buffer may be needed.

I accept what you say about increasing diode tempco's by reducing the current. I have just reread RA Pease's thoughts on it; http://www.national.com/rap/Story/vbe.html and and he certainly quotes - 0.2mV/°C per decade decrease of current. Our diodes have a tempco of 1.7 mV/degC and the transistor Vbe has a tempco of 2.14 mV/degC at 25mA (Bob Cordell's figures) so diode tempco needs to be increased by 0.44 mV/degC. The diode current therefore needs to reduced by 2.2 decades, or 158 times. That gives us a diode current of 158 uA. (not a typo) This is much too low for a VAS operating current so we need a circuit that will replicate the diode voltage at much greater current.
The lower current is clearly not going to give us enough diode voltage to cancel the transistor Vbe's directly. However we cannot use a conventional multiplier circuit as in multiplying the voltage we also multiply the tempco. What is needed is to add a fixed voltage to the diode voltage. If we assume we are using an EF type 2 output, then we also need compensation for the driver Vbe's which can be done with a conventional Vbe multiplier circuit, as used in Plan A.
Another point is that a diode current of 158 uA is quite small, and whatever circuitry we use will have to have its base currents looked at carefully if it is not be inaccurate.
It therefore looks as if we need a system like that shown conceptually in the second diagram. Vdriver compensates the driver Vbe's. The fixed voltage is derived from black box called Vfix, and the diode voltages are replicated by a black box called Vdiodes. I am calling this approach Plan B. It should give much better compensation than Plan A, but it's going to take a bit of designing...

What do people think?

I'm not sure if I can put two pictures in one post, but I'm about to find out.

Douglas

Your plan B is of course one good answer but as you said it requires a bit of design (which should not afraid diy'ers when I see all the troubles they go through with sophisticated front ends and error corrections !)
But as mentionned, the topology used by Leach should work like explained in the attachement.

What do you think?

Jean-Pierre

[JPV]

The file
JPV

[DouglasSelf]

Originally posted by ostripper


I think you have described most HT receivers on the market
today! (thank god they have thermal cutoff in their protection circuits) .

The idea of putting the Vbe on the collector leg is cool
I'll try it.(poor mans way of getting thermaltrak performance)

BTW.. Thanks,Mr. Self, for the BEST book on amplifier design
and for the "blameless design" which with proper layout
and device selection is one killer amp. I look forward to the latest
edition (I only have Ed.3).

Thanks,OS

Thank you for your kind words!

You do realise the current edition is the 4th edition?

[DouglasSelf]

Switching briefly to the problem of practical tests, it would be good if we could agree on a way to do them.

The method I have used in the past to plot output stage bias conditions is to deliberately underbias the amplifier by a modest amount, but enough to make sure it is always underbiased. I have used a bias setting that gave about 0.02% THD with a peak responding measurement mode. The AP System-1 can plot THD against time and this effectively gives a plot of bias conditions against time. (see p379 in the Fourth edition of the Audio Power Amplifier book)

Do people agree that this is a valid method?

Douglas

[megajocke]

What is the dV/dI (maybe called dynamic resistance or something) of the diodes at that current?

If it is big then diverting current with resistors will decrease the temperature coefficient instead of increasing it!

I didn't do the calculations though so I'm not saying it won't work, but have it in mind.

[alfredrofe]

From the article Silcion Chip Sept 08
http://www.siliconchip.com.au/cms/A_...ntArticle.html
the voltage accross the diodes appears to be .5v which is more of the schottky performance. Also,see related performance graphs for a further statrting point in the discussion, from some one who has built a unit and measured the responses as well a having a dose of the big brown smoke, yes the journalists not myself.
alfred

[ostripper]

Hey guys! I thought the thermaltrak devices were designed
to allow for simpler layouts. Post 99 onwards shows
an exercise in more advanced tempco circuit considerations.

The ultra LD amp seems to be a classic "blameless" with
T-trak diodes used as Vbias (simple).

Looking at the board layout would this not add alot of
extra traces/capacitances to the design? (oscillations/puffs
of brown smoke..? drugs or blown op's??).

I,m using 2 pair NJW0281/0302 (which is equivalant to these
t-traks minus the diodes) with short 4cm leads to the Vbe.
(like the aska lifeforce)...rock stable.

The T-trak OP stage using the Leach Vbe seems to be the best
way to go.

BTW.. Mr. self , If I was to give back to the community an amp
based heavily on your "Blameless" design would you approve.?

OS

[JPV]

Originally posted by megajocke
What is the dV/dI (maybe called dynamic resistance or something) of the diodes at that current?

If it is big then diverting current with resistors will decrease the temperature coefficient instead of increasing it!

I didn't do the calculations though so I'm not saying it won't work, but have it in mind.

No
operating the diode at another current will change the Vd of course and more if the dV/dI is high at the operating point. But at a fixed operating current in the diode this will be a fixed Vd for which the Vbe multiplier will be adjusted.
Now, if the temperature changes, the Vd will drift for the tempco at that current and so the Vbias.
Of course the Vd changing with temperature, the current in the diode changes so the tempco but this is a negligible effect because the proportionnal change in current in the diode due to Vd drift is equal to that drift. This is about 30% for the full range of temperature. This is a low change in tempco because the tempco changes logaritmically with the current.

JPV