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Old 6th January 2007, 02:37 PM   #1
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Default What is this diode for?

From Onsemi's thermaltrak audio transistor's app note (AND8196/D) there is this schematic.

What is the purpose of D4-D5? Should it be connected to supply rails (for not-exceeding rail voltages), not to base of Q9-Q10?

If it is connected like that, it will bypass the VBE drop of Q7-Q8 plus drop on R18-R19?
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Old 6th January 2007, 02:52 PM   #2
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Looks like a soft clipping circuit, so BE on Q7 and Q8 are shortend when Base voltages on Q9 and Q10 rises above the +/-45V
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Old 6th January 2007, 02:58 PM   #3
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Which voltage is bigger, the VBE drop of MPSA06 or drop of 1N4148?
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Old 6th January 2007, 03:07 PM   #4
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Looking only at D4, it will shortcircuit BE + R18, when voltage drop rises in R18 due to voltage higher than 45V (or larger current throug R18 to Base of Q9)
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Old 6th January 2007, 03:08 PM   #5
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I was pondering a constant current source maybe? " Soft clipping"Hmm ,sweet a bjt amp with the "tube sound" cool!
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Old 6th January 2007, 03:25 PM   #6
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This doesn't look right to me. Why don't you ask the author of the app note?
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Old 6th January 2007, 03:37 PM   #7
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Onsemi has EE Phd's, I'm not EE at all
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Old 6th January 2007, 03:41 PM   #8
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Quote:
Soft clipping"Hmm ,sweet a bjt amp with the "tube sound" cool!
Tube Sound from Solid State
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Old 6th January 2007, 04:12 PM   #9
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It looks like a very basic current limit. The diode (D4, D5) will conduct current from the base of the pre-driver transistor (Q7, Q8) when the current through the driver transistor’s series base resistor (R18, R19) gets too large. I’ve seen this done before, but with two diodes connected in series, as a single diode drop is too close to the base-emitter drop of the pre-driver transistor.
D4 will start seriously stealing base current from Q7 at only a couple of mA through R18, but when that happens, VAS transistor Q4 will just turn on harder to compensate (due to the action of nfb) and provide direct drive to the base of driver transistor Q9 through D4!
It looks to me like the designer of this circuit may have been aware of this deficiency, as the VAS transistors (Q4, Q6) are biased with hardly any quiescent current, thus limiting their drive capability.
R8 sets the LTP tail current to approximately 1.4mA, giving ~700uA through each LTP collector. That gives a drop of 0.7V across R2 - only just enough to keep VAS transistor Q4 turned on! Q3 mirrors the collector current of Q2 to give the same balanced drive to the other VAS transistor, Q6. R11 and R12 therefore limit the collector current of each VAS transistor to approximately 7mA.

This really is a horrendous circuit – you only need to look at the THD plots for confirmation. To call it a “standard industry design” is a bit of a joke. The basic topology may be common, but the way it has been mutilated for this implementation definitely isn’t.
It’s also a bit ridiculous that, in an application note outlining an alternative, non-adjustable biasing scheme for a bipolar power amplifier, no mention at all is made of the quiescent current of the output transistors. Just look at the enormously high THD for low signal levels, where a class AB amplifier should be operating in class A.
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Old 6th January 2007, 04:22 PM   #10
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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?
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