Thermal Stability of Lateral FET Driven Bipolar Follower. - diyAudio
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Old 23rd January 2013, 03:59 PM   #1
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Default Thermal Stability of Lateral FET Driven Bipolar Follower.

Does anyone have an off-hand idea how far off from stable this circuit would be? The actual application will probably use more like ten pairs of bipolar outputs for a 2.66 ohm nominal load with 65 volt rails. With only 15mA emitter current (basically class B), I wonder if it would run away if it got hot..
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Last edited by Andrew Eckhardt; 23rd January 2013 at 04:02 PM.
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Old 23rd January 2013, 04:23 PM   #2
Mooly is offline Mooly  United Kingdom
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My instincts say not very stable because you would need around what ? 2 volts or less across the 47 ohm which is 40 milliamps or less. The FET's wouldn't hold that steady enough to be used for biasing... I don't think so anyway. The bias would be prone to wander quite a bit.

No proof, just a feeling
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Old 23rd January 2013, 05:12 PM   #3
Bensen is offline Bensen  Belgium
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I have the same opinion as Mooly.
I would change the R100 to a vbe multiplier.

See for example the Classé amps, these are proven designs.

Greetz
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Old 23rd January 2013, 05:17 PM   #4
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Great help.Thanks guys..
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Old 23rd January 2013, 06:49 PM   #5
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you may also wana take a look at Zeck amplifier schematics ...Long time ago but they have been doing something similar i think
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Old 23rd January 2013, 11:15 PM   #6
AKSA is offline AKSA  Australia
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The bias voltage for maybe 100mA quiescent might find there is almost 2.4 volts bias between the lat fets gates. You have the lat fets biased above 27mA, but this is a bit low and this would need changing the 47R to 12R to deliver around 100mA. At this higher lat fet bias, you are around zero tempco, but negative tempco at 400mA and not very efficient. The compensation would be quite severe at the output quiescent, and they would probably might into thermal overload if you only use a resistor to set the bias voltage.

Yes, a Vbe multiplier is a good idea, using a single bipolar like a BD139. Collector to base is 1K5 and base to emitter around 330+200R trimpot in series for adjustment.

Hugh

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Old 24th January 2013, 02:03 AM   #7
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Thanks Hugh. That about dials it in. I can see that fiddling around without the Vbe multiplier is not going to go..
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Old 24th January 2013, 09:34 AM   #8
effebi is offline effebi  Italy
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Quote:
Originally Posted by Andrew Eckhardt View Post
Does anyone have an off-hand idea how far off from stable this circuit would be? The actual application will probably use more like ten pairs of bipolar outputs for a 2.66 ohm nominal load with 65 volt rails. With only 15mA emitter current (basically class B), I wonder if it would run away if it got hot..
Just a guess , but isn't .2 ohm a little small for ensure proper Ic balancing of 10 pairs? I would go for 0.47 at least. I have a similar design (10 pairs To3) on 2 Ohm that works happily with 1 Ohm emitter resistors.
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Old 24th January 2013, 10:09 AM   #9
effebi is offline effebi  Italy
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In any case, if you are going with a long heat sink and forced ventilation place your bias spreader on the output side of the air flow. That side is going to be hotter and where your transistors will most probably fail if ever.
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