THERMAL COMPENSATION OF Vbe MULTIPLIER

[fotios]

A subject that causes to me questions also is the Vbe multiplier. This except the idle current that offers to output transistors, it ensures also thermal compensation to keep the bias in a safe margin unless the output devices will be in danger of destruction due to overheating. Those from you who have read the D.Self book, you have seen that he puts in this place the MJE340. From my experiments I have realised that MJE 340 is a lot slow in the recovery of the preset idle current after the pause of amplification procedure during that the heatsink it is heated significantly. I have tried other devices of common type such as BD137 and BD139 and I have realized that these are by far much faster from MJE340 in thermal compensation and thus in the recovery of the original idle current after heating. Concretely, I realized that as much higher the Vce of transistor as much slower its thermal reaction. In an amplifier with a supply of +/-70Vdc (thus 140Vpp) the peak of signal across the C-E contacts of transistor it touches many times upon the 140Vpp. Thus the use of lower Vce transistors such as BD137 (60V) or BD139 (80V) it is a risk. From the other hand I fear the use of MJE340 (Vce=300V) because his slowness (a thermal runaway watch for in output transistors? I am not sure)
Also and this time I wait for your answers with your thoughts, your experience and your opinion on this subject.
Thanks in advance for your help
Fotios

[janneman]

Fotios, are you aware there is a whole thread extensively dealing with this issue?

Somehwere around here:

http://www.diyaudio.com/forums/showt...43#post1277243

Jan Didden

[fotios]

Originally posted by janneman
Fotios, are you aware there is a whole thread extensively dealing with this issue?

Somehwere around here:

http://www.diyaudio.com/forums/showt...43#post1277243

Jan Didden

Hi Jan
No i never have seen this thread at whole (it is verry long and my time it is allways limited due to work, many times i spend my sleep to read or write posts) and i had not idea that in this thread there is also a discussion around the idle current (Vbias if you prefer) generated by the Vbe multiplier. I taked a look in the post that you addressed me, but i have not find anything with clear and practical documentation to give an answeer in my bothering. Sorry, but the formulation of my thoughts it is clearly of a practical implementation of Vbe multiplier with 2 or 3 concrete types of transistors (MJE340 vs. BD137 and BD139 in speed of thermal recovery of preset Ic and the Vce sustain). In this thread of B.Cordell "BJTs vs. FET" the only that i found are some exchanges of long discussions and equations and not a practical conclusion (as much as i have read). I will frame with other words my bothering. From a first view i suppose that the use of a MJE340 it is reasonable because his Vce breakdown it is 300V thus it is in position to sustain with safe large excursions of signal of 140Vpp. But his sloweness to recover his preset Ic - or idle current - after a significant heating of his case (which increase his Ic significatly) it is enough in time to cause a thermal runaway in output transistors? This is my query exactly. Also i excluded the use of diodes in this place because i know VERRY WELL that they needed at least two doudle diodes "special selected" in series with two simple diodes to achieved a voltage of +1,75V to -1,75V at the base of the two opposite driver transistors for a class AB bias level of output devices. Also i excluded the use of the curious feedback schemes with two transistors because they have proved in D.Self book only in amplifiers of small supply. Also i prefer the traditional methods of managing of idle current. I need direct answeers in this, and not long theories and equations. For the last we have enough time after 2 or 3 months to pass our winter with amusement. To the present, i request you not spoil my thread.
With all respects
Fotios

[Nordic]

Ignore my ignorance, but I though the VBE transistor will only see the voltage it is regulating... say 3V or so...

[MikeB]

Yes, that's 100% correct...

[fotios]

Originally posted by Nordic
Ignore my ignorance, but I though the VBE transistor will only see the voltage it is regulating... say 3V or so...

Originally posted by MikeB
Yes, that's 100% correct...

Thanks craftsmen for your advices. Also me i supposed the same as you. A confirmation it is a very good thing. I use a BD139 as Vbe multiplier in my monster "Dirty Harry" (i don't remember where it is found in the forum because have passed 5 months from his publication in diyforum but you can view it in my web page by clicking on the button down left) of +/-82V supply. In the plan presented, the MJE340 appears was substituted with a BD139 and the trimer with 5K.
Hey, MikeB BTW i taked a look in your web page in your symasym amplifier and i discovered that our thougts around a circuit architecture are concverging at big part. Nice work man!
My best regards also to my friend Nordic.
You are colleagues to me.
Regards from Greece
Fotios

[fotios]

To be sure, just now i made an experiment with the aid of "Dirty Harry". First i connected accross the C-E pins of BD139 the test leads of a RMS voltmeter. Then i supplied the amplifier with +/-80Vdc and i injected in his input a sinus wave of 1KHz. By increasing the drive signal untill i see clipping in the scope connected in the output, the maximum value that i saw in voltmeter was 26Vrms thus: 26 X 1,414 = 36,77Vp so the maximum voltage swing was 36,77 X 2 = 73,54Vpp. I thing this extremelly condition voltage can sustained from a BD139 of Vce=80Vdc. The conclusion it is that, the Vbe multiplying transistor sees only the half of the full voltage swing allways across his C-E contacts. Thus in my case of the amplifier with supply +/-70Vdc which translated in: 70Vp : 1,414 = 49,5Vrms as much and thus 49,5 : 2 = 24,75Vrms which is translated in 24,75 X 1,414 = 35Vp X 2 = 70Vpp (the voltage level of one of the supply rails). The use of a MJE340 then it is useless. Attention! we speak for voltage peaks and not for power dissipation, thus the same is in effect for a signal of any frequency.
Fotios

[lumanauw]

In my experience, high Vce transistor (like MJE340-350) seems not suitable for VBE multiplier. It cannot track the thermal compensation correctly. The smaller Vce transistor (BD139-140 or others with smaller Vce) works better for thermal tracking.

[fotios]

Originally posted by lumanauw
In my experience, high Vce transistor (like MJE340-350) seems not suitable for VBE multiplier. It cannot track the thermal compensation correctly. The smaller Vce transistor (BD139-140 or others with smaller Vce) works better for thermal tracking.

I am sure for this. As i said, from experiments it proved that the smaller Vce BD137-BD139 are by far much faster in thermal reaction from the higher Vce MJE340. In Vbe multiplier the task of transistor it is to keep constant his Ic under all thermal conditions as we know; as the junction temperature it is increased the Ic must be going decreased and vice versa. The only problem it is the Vce of the transistor used must be a little bigger than the voltage of one of the two supply rails. For me, i have changed already the dull MJE340 with a nice BD139 (Vce=80Vdc). So, dear lumanauw, i think we must to point out this topic in mr. D.Self. Maybe he can draw for us a graph with the time delay of Ic decrease vs. junction temperature for different types of transistor and another one with the types of these transistor vs. the danger of output devices thermal runaway or anythig else bad result. We may don't forget that a idle current of 20mA per output device it is translated in a voltage (respect to gnd) of 1,74V at the base of each of the two driver transistors, and consequently in a voltage of 0,6V approximatelly (respect to gnd) in the base of output transistors. As you can understand during operation each half of amplified signal from VAS vary between +1,74V and the positive supply level and between -1,74V and the negative supply level at the base of drivers under normal conditions. If the Ic of Vbe multiplier goes increased then also the voltage across his C-E junction goes increased and so on in the base of drivers and what does means that for the preset voltage at the bases of output transistors.
Fotios

[janneman]

Originally posted by fotios
To be sure, just now i made an experiment with the aid of "Dirty Harry". First i connected accross the C-E pins of BD139 the test leads of a RMS voltmeter. Then i supplied the amplifier with +/-80Vdc and i injected in his input a sinus wave of 1KHz. By increasing the drive signal untill i see clipping in the scope connected in the output, the maximum value that i saw in voltmeter was 26Vrms thus: 26 X 1,414 = 36,77Vp so the maximum voltage swing was 36,77 X 2 = 73,54Vpp. I thing this extremelly condition voltage can sustained from a BD139 of Vce=80Vdc. The conclusion it is that, the Vbe multiplying transistor sees only the half of the full voltage swing allways across his C-E contacts. Thus in my case of the amplifier with supply +/-70Vdc which translated in: 70Vp : 1,414 = 49,5Vrms as much and thus 49,5 : 2 = 24,75Vrms which is translated in 24,75 X 1,414 = 35Vp X 2 = 70Vpp (the voltage level of one of the supply rails). The use of a MJE340 then it is useless. Attention! we speak for voltage peaks and not for power dissipation, thus the same is in effect for a signal of any frequency.
Fotios

Fotios,

No worry, I am not out to spoil anybody's thread, of course not.
But there is something wrong here. If you really measure across the Vbe multiplier C and E, you cannot see 26VRMS! You can see only a DC voltage of a few volts. If this would be more your output stage would explode!

What *may* have happened is that you had your multimeter on AC instead of DC? (It should be on DC). Also, sometimes what happend to me is that I connect the meter as you say but I forgot that the meter itself is also grounded and that the input ground is not floating, so I really was mesuring C to ground. In that case, you would indeed have the 26VAC or something.

Whatever it is, your measurements are incorrect.

Jan Didden