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Old 23rd June 2004, 07:05 PM   #1
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Default 7 Easy Pieces

Just when you thought it was safe to go back to the work bench....

Here's 7 easy pieces for chip amps.


www.passlabs.com/np/GC-VAR-INV-FDBK-1a.pdf

You might find this a trivial circuit, but for those who
don't know it, it can be very important. Chip amplifiers
have enormous amounts of open loop gain (120 dB = 1,000,000)
which can create stability problems when the circuit is set
for low gain, say only 10 or 20 dB. This circuit places a
resistor to ground at the negative input, which throws away
some of the open loop gain, resulting in less feedback. In
fact you can put a variable resistor here, and tune the
amount of feedback to taste.



www.passlabs.com/np/GC-VAR-BAL-FDBK-1a.pdf

Does the same thing for a balanced input circuit. Generally
you want the pots at equal value, but for the most perfect
common mode input rejection, separate pots allow trimming.



www.passlabs.com/np/GC-OUTPUT-BIAS-1a.pdf

You say your chip amp needs more output bias? Here's an old
trick.



www.passlabs.com/np/GC-OUTPUT-BIAS-2a.pdf

Here we see the same thing as GC-SS-3a, but without the
SuperSymmetry jazz, just a pair of chip amps. The voltage
sources will be low values, as will the output resistors,
and additional output stage is provided by the DC difference
in the output voltages across those output resistors. The
load output sees the split value, so there's no DC seen there.
You also get the advantage of delivering about twice the
current as a single chip amp.



www.passlabs.com/np/GC-BOOTSTRAP-1a.pdf

What? You want your chip amp to behave as if it's seeing a
higher impedance and also deliver more current? You can
parallel chip amps equally, or you can set one up as a
current bootstrap, relieving the load on the first amp,
but also leaving it in control of the signal. Sort of
like power steering. Watch how you set the current gain
of the bootstrap. For this circuit, 50% is a good number.



www.passlabs.com/np/GC-BOOTSTRAP-2a.pdf

And of course you can go crazy and use a lot of them. In
this case, the current gain would probably want to be set
to divide the current equally between all the amps, including
the first one. In the case of 4 bootstrap amps, each would
probably want to be set at delivering 20% of the output
current.



www.passlabs.com/np/GC-ZEN-ALEPH-1a.pdf

How can we resist not making a Zen amp with such a current
source? Here we see that the Aleph current source is easily
duplicated by a chip amp.

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Old 23rd June 2004, 09:36 PM   #2
uli is offline uli  Austria
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Hihihi, dont shock people late in the evening!



Uli

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Old 23rd June 2004, 09:38 PM   #3
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Very nice examples. These will keep me busy for quite some time.
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Old 23rd June 2004, 09:41 PM   #4
karma is offline karma  Canada
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same here gc zen hmm
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Old 23rd June 2004, 09:46 PM   #5
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"You want me to hold the chip amp where?"
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Old 23rd June 2004, 09:54 PM   #6
joensd is offline joensd  Germany
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Anybody got a spice model for an appropriate opamp to simulate with?
Think I need a cup of tea.

Jens - already thinking a pot might be better
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Old 23rd June 2004, 10:27 PM   #7
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An homage to Feyneman...

Nelson, you're one cool dude
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Old 24th June 2004, 12:05 AM   #8
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Quote:
Jens - already thinking a pot might be better
Metalman - Already thinking pot might be better!

In the last 24 hours my head has chipped exploded spun inverted balanced , bootstrapped and in true Pass fashion finished in a state of Zen

I'm sure I hurt something in there but what a ride

Thanks Nelson!
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Old 24th June 2004, 12:29 AM   #9
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My pleasure.
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Old 24th June 2004, 12:46 AM   #10
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Ok Nelson, i have to ask this, exactly how much time did you spend on those schematics? I knowing you, probably didn't take you long, but I'm curious later

-Matthew K. Olson
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