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Old 20th December 2005, 02:22 AM   #51
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Hi John-china,

No not quite. You need to replace the resistive feed from the power supply with the CCS. The second cascode BJT is included as well and may or may not have a load R.

The zener here contributes to a very stable operating point without the noise getting through due to the NFB feedback pair. But the output Z of the CCs is orders of magnitude higher than the original R feed.

Much of the simplicity is lost though.

pic -
Cheers,
Greg
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Old 20th December 2005, 03:57 AM   #52
AJT is offline AJT  Philippines
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Excessive GNFB is no problem as long as the feedback network and amp signal path is FAST........ This means at least 150MHz input device, 100MHz voltage amp, and 30MHz drivers and output devices.
exactly as prof leach proposed in his 1975 article published in the AUDIO magazine article about slew rate problems and how to vaoid them.

the japanese have been at this for a long time, that is why they come up with high Ft's power transistors for use in their amps.

that is why Mr. AKSA prefers them for his amps too.
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Old 20th December 2005, 05:23 AM   #53
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Default Hi Greg, lots of options, some "simpler" ones:

losing the feedback Q and moving the zener bypass C to the ccs Q base in your circuit will be quieter - it is really "inefficient" to try to filter Zener noise with a parallel cap due to the low dynamic Rz - using RC filtered Zener V as ccs ref lets you use a much larger ccs R, total noise should be smaller than 2 Q ccs shown, if not quite as high in Zout and psrr it is still very good

with the original Leach bias adding a bootstrap resistor to each tail from the amplifier output can hugely increase effective tail impedance if the existing bias R and the added bootstrap R ratio equals the feedback network ratio - won't work all the way to DC with feedback network AC grounded but should give ~50x (1% R's) tail impedance boost over most of the audio range really cheap

I like to see trim pots in schematics like this, manual trimming of diff pair Ic balance is required with single ended outputs - may require vas degen trim for complete circuit balance
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Old 20th December 2005, 05:39 AM   #54
AKSA is offline AKSA  Australia
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JCX,

Another method of trimming the current balance in a diff pair is to either trim the load on the single ended output (normally a current mirror, so merely alter the CM degeneration), OR trim the base bias resistor on the input (with matched LTP of course). This avoids unbalancing the gain of the LTP outputs; an advantage for distortion cancellation even if the output is in SE.

cheers,

Hugh
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Old 22nd December 2005, 05:26 AM   #55
andy_c is offline andy_c  United States
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As a former student of Dr.Leach from the mid '70s to '80, I'd like to weigh in on this discussion by just posting this link
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Old 22nd December 2005, 05:46 AM   #56
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Quote:
Originally posted by andy_c
As a former student of Dr.Leach from the mid '70s to '80, I'd like to weigh in on this discussion by just posting this link
Dr. Leach doesnt sing too bad
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Old 22nd December 2005, 05:50 AM   #57
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Thanks Andy,

Sort of paints a different picture in my head of Dr Leach.
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