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What is this diode for?
What is this diode for?
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Old 6th January 2007, 04:37 PM   #11
GK is offline GK  Australia
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Quote:
Originally posted by lumanauw
Hi, Glen,

Like you said, if it is 2 diodes in series, it makes sense to be a limiter. But it is 1 diode here. What is it?

It's a current limiter alright, but with just one diode. From memory, the last time I measured one, an 1N4148 dropped about 1V at 5-10mA or so.
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Old 6th January 2007, 04:51 PM   #12
Amp_Nut is offline Amp_Nut  India
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IMHO, D-4 conducts and bypasses the base drive of Q-7, shutting off the drive of Q-7 when the voltage swing exceeds the +45 Vol;t rail of the power transistors.

The driver and CVoltage gain stages are powered via a higher ... 55 V Volt rail, and you want to drive the Power Transistor base more positive than its collector.

D-4 does that ... Clamps the Drive Voltage to approx the 45V rail.
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Old 6th January 2007, 04:53 PM   #13
forr is offline forr  France
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As the associated transistor Vbe is higher than at least 0.5 V, the diode would certainly be conducting all the time. As said by Lumanauw, maybe an error in the drawing.
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Old 6th January 2007, 05:01 PM   #14
darkfenriz is offline darkfenriz  Poland
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For soft clipping the diodes should be connected to collectors (not bases) of drivers.
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Old 6th January 2007, 05:29 PM   #15
GK is offline GK  Australia
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Quote:
Originally posted by forr
As the associated transistor Vbe is higher than at least 0.5 V, the diode would certainly be conducting all the time. As said by Lumanauw, maybe an error in the drawing.

I agree that the diode will be conducting, but I don't think there is an error in the drawing. Looking at the rest of the circuit, I just think the designer didn't know what he was doing. Why on earth would anyone bias the VAS transistors so low?

The pre-driver transistors (Q7, Q8) would essentially do nothing, with the under biased VAS transistors (Q4, Q6) providing drive current to the driver transistors (Q9, Q10) directly through the diodes (D4, D5).
This somewhat less than ideal scenario would go a long way in explaining the amplifiers hopeless THD performance.
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Old 6th January 2007, 06:14 PM   #16
lumanauw is offline lumanauw  Indonesia
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Quote:
but I don't think there is an error in the drawing. Looking at the rest of the circuit, I just think the designer didn't know what he was doing.
Anyone knows Mark Busier (the one who prepares this app note)?
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