Optimizing the VBE Multiplier

The VBEM device must be selected to provide low VCEsat as well
as very high beta.

Sanken produce a dedicated transistor for this application with
a 3A rating but still a remarkable beta of more than 500 and
typicaly in excess of 1000.
 

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I think the main issue is the choice of the transistor which leads some to use more complex multipliers which is unnecessary like cfp. Cfp is prone to oscilation, Darlington is not and has about the same gain and same packages as single tranasistor. In fact using just a single super beta transistor is more than enough for 98 percent of designs and has a positive benefit above the others. The higher the hfe the more temperature sensitive the transistor, as is a lower voltage type to a lesser extent.

Good point HM, indeed there s transistors specialy designed for VBEM purpose and as you point it they have high gain but also low VCE sat, high current capability and consequently low re, re = emitter internal resistance.

Other than that i must admit that the VBEM is quite an annoying part in a design, no wonder that i personaly went lateral fets as often as possible.

Edit , i just noticed that i already posted the datasheet...

Same problems same solutions....
 

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You need a solid connection to the sensing transistor. If you have a flaky connector and it opens up (on the base or emitter, say) the VBE multiplier will conduct zero current. Then current at the outputs will rise to several amps. If the outputs don't blow right away, a positive feedback loop occurs as the outputs heat up and the current through them increases over time.

If you're lucky, overcurrent protection kicks in or a fuse blows before damage occurs.

Maybe that goes without saying. Carry on with the discussion about compensation resistors and bypass caps.
 
how about using KSC3503 as its VCE sat is lower value and also smaller form factor.

I have a doubt that can we mount the Vbe transistor directly onto the output stage transistor? use the same screw to mount the output transistor and on top of it fix the vbe mult? I have a large heatsink and I feel the time is taking long to sense the temp.

will I get faster bias settling time this way?
 
I also interest about quasi saturation of Vbe multiplier transistors on recently time.
I usually use BC550C for this duty, but from the results as topic Looking for a small signal NPN with no Quasi Saturation: MEASURED DATA I think better use ZTX8xx and BC337-40 to do same job.
Thanks everyone of that topic who helped me open eyes.
I used 2SC1815GR a long time ago but changed it. It seems a mistake decision. Datasheet of Toshiba shows 2SC1815 is very linear as low Vce voltage.
 
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How works this Vbe multiplier from the circuit in the attachment from Mark Levinson/Madrigal ?
 

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thank you very much for this information - I ask this question because a friend of me is owner of a damaged device with shorted output power transistors of this model from Mark Levinson.
Before ordering new devices I want to put this power amp into operation without the power output stage (i. e. only the front-end with Vbe multiplier drives the speakers - naturally in this kind I have a current control output with some mW power output).
This is the main step in my own troubleshooting procedure wich provide me a good protect to get additional shorted output transistors.

According the circuit description in post #57 follow prepare is necessary for getting this mode after removing the output (triplet follower) buffer:
1) node from R122/R121 together with speaker output terminals are to connect with C, Q25 resp. C, Q38
2) de-solder R49 and R24
If now DC conditions and idle currents in all stages from the front-end conclusive resp. according the showed values in schematic I will check the voltage range (bias) for the idle current in the output stage between C, Q25 and C, Q26 (must be between 3x0V45 and max 3x 0V75 for each half - i. e. between 1V35 and 2V25).
If this values are long time stable I connect speakers for sound check (extremely low level).
Let's see if I can do this and find the reason for the shorted power BjT's in the output buffer stage.