Vi limiter design

[Manowar1985]

Hi again all.Could anyone please help me to understand how to calculate the values of the resistors of the limiter to get the specific single slope line drawn in Safe operating area of the transistor.Thanks a lot

[nickb]

I think a schematic is called for. It's not possible to answer the question without one.

[Helmholtz]

^^^^ that and/or a drawing of the SOAR you are referring to.

[Manowar1985]

Hi all again.I tried to build an amp with vi limiter that looks exactly like the picture that I have attached to my comment. (I got the picture from this website:https://studfile.net/preview/5239985/page:59/
I used the same values of resistors.
I have 2 questions .
1)
How the resistor values correspond to the specific soar .How are they calculated so I can calculate my own
2)
Sometimes when I switch off the amplifier the output transistors get burned even without being connected to a load.Why does this happen with vi limiter on.

I use bd911 and bd912 as output devices .
2n5551 and 2n5401 as protecting transistors
The diodes are 1n4148.

I will be grateful for your help.Thank you all

[Leo_Gnardo]

Sometimes when I switch off the amplifier the output transistors get burned even without being connected to a load.Why does this happen with vi limiter on.

Some SS amps do not take kindly to running no load. Some of the big Sansui hi fi amps of the late '70s early 80s were infamous for shorting output transistors with no load. Also some of the earliest SS amps in the 60's particularly those with germanium transistors.

What if you connect a load resistor that doesn't draw very much current compared to the expected load, say 100R more or less? It might stabilize your amp during shutdown and would take a mere pinch away from power going to the speakers. Although it's a rough analogy, some Dynaco tube amps used this trick. IIRC a 2W 68R resistor was standard issue in Mark III, Stereo 70 and similar amps. Cheap experiment, worth a try.

[J M Fahey]

In no particular order:

1) that is a *complex* design protection circuit.

It´s beyond a "5 minute" Forum answer explanation, notice that even a *book* dedicated to Power Amp design does no give you the full design process, just mentions a couple features.

2) it´s transistor specific, that one is meant for a robust MJ1502x (they say so in the text), it will not protect much weaker BD911/12.

3) I used those long ago, then gave up, too fussy, rail voltage dependent (which in theory is their main feature) and does not *really* protect that much, short circuit current is not low enough, if you want to make it more sensitive it clamps transistors hard before they put out needed current, practically nobody uses them any more.
Acoustic used a simplified (3 resistor) form made out of R17/19/26 (and R18/20/27) ; so far I can design it; the 4 resistor one in your example is more complex and the 5 resistor + Zener diode one is way beyond me:


Again: there´s a reason nobody uses them any more, except, maybe, in a high power rack mounted PA power amp.
And even so ....

4) for transistors to explode on turn OFF (or on transients) even unloaded they MUST be passing current, both at the same time, so they straight short the power supply (and die trying)

Maybe they are unstable and briefly oscillate, maybe the complex short circuit protection tries to clamp both at the same time and they btry to fight back.

In principle build and test amp with no VI limiting, does it work properly?

[Manowar1985]

Thanks again people for your time and your answers.
I should give you some clues about my circuit.
I use simple differential amp with simple current mirror.I have used emitter degeneration resistors on both of them.I have used constant current source the complementary feedback pair circuit and the extention current source for the voltage amplifier stage.The current setting for the input stage is 6mA and gor the voltage amplifier stage is 15mA.
The voltage amplifier stage is a darlington stage with a current limiter.The output stage is a triple darlington stage with 3 pairs in parallel.That means 2 transistors for the predrivers 2 for the drivers and 6 for output.I have put the vi limiter on sensing all 3 legs of output stages.
The input resistor is 22kohm,the feedback is the same,the ground resistor is 1kohm with a capacitor of 1000uf.The miller cap is 100pf.


The other problems are


1)When placing the speed up capacitor on the output stage the ops blows with or without the vi limiter.

2)When I remove the vi limiter and the speedup capacitor it blows again

3)When I remove the speed up capacitor but not the vi limiter most of the times it doesn't blow

4)Without the vi limiter the ops is more resistant to blow up when i use simple darlington or CFP or even direct ops without drivers


I have to say the most important thing

The Vbe multiplier was at 0.6 v bias that means that NO CURRENT was flowing through the ops.So why did it blow up with no load. Will the addition of soft start circuit help in this case

How am I supposed to protect the ops because I have burnt so many if I can't use the corrent vi limiter



Thanks a lot.
Have a nice day

[nickb]

As JMF has pointed out they are not perfect, but not without value either. I can testify that such a circuit saved my a$$ just a few weeks ago, The key to success is to be conservative with the output transistors.

Anyway, to answer your question, if you make the simplifying assumption that the transconductance of the transistor is infinite, then you can write the following :

Vbe = (Vce + Io*Re)*a + Io*b
= Vce*a + Io*Re*a + Io*b
= Vce*a + Io*(Re*a + b)

=> Io = (Vbe - Vce*a)/(Re*a + b) = Vbe/(Re*a + b) - Vce*a/(Re*a + b)
This is an equation of a straight line with slope "- Vce*a/(Re*a + b)" and Y axis intersection of "Vbe/(Re*a + b)"

Where
a = (R1 // R2)/(R1 // R2 + R3)
b = Re1*(R3 // R2)/(R3 // R2 + R1)
Vbe = TR1 base -emitter voltage
Vce = TR3 collector to emitter voltage
Io = output current
"//" means "in parallel with"

For the values you show above you get this plot of Vce vs Io:



Attached is an openCalc spreadsheet with the formulas. Hope that helps.

[nickb]

I was writing my reply to your original question while you posted. You have an instability problem, not a SOAR issue. Still need the schematic of your amp as requested in post #2.

[Manowar1985]

nick b I am posting the amp schematic and audio frequency generator schematic that I have built.
So the thing is that I have 2 issues
1)The instability problem
2)ops burning

Or do you think that instability causes the burning.

Thank you very much

[Manowar1985]

I must say that I used the bd911 912 were the bd139 140 shojld the used because I run out of those but I don't think they cause a great problem

[nickb]

You have massive current gain in the output stage. For peak output into 4 ohms the base current of Q5 is just 6uA. The current through the voltage amplifier stage is 33mA and uses slow BD911 power transistors. There is also large gain before the voltage amplifier. My gut reaction is slow transistors plus massive gain is going to give you stability issues.

It looks like you have the design in a simulator. Do an ac sweep from 10Hz to 100MHz and see if you get any resonant peaks. Then start taking out some of the gain to try to tame it.

You can use your PC or phone as a signal generator. I think you may well need a scope to make good progress on this. That would be the first step - put the scope of the output and look for oscillation..

[Manowar1985]

does the current/Voltage open loop gain have to do with the oscillations or the closed loop and why.How about the burning.Is the open loop gain responsible for this too.And by the way thank you for your spreadsheet with the formulas .It was very helpful.

[nickb]

It's a big subject. Here is a start https://www.allaboutcircuits.com/tec...-to-stability/

[Manowar1985]

Thanks a lot.I appreciate for sharing all this valuable information