7 Easy Pieces

[Nelson Pass]

Originally posted by Stocker
Isn't #6 the kissing cousin of my massively parallel small-signal amplifier idea?

I believe they could kiss, but I wouldn't want to see their
offspring.

[Tazzy]

The GC Zen Aleph is mighty sexy

[Steven]

Originally posted by Nelson Pass

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.

Funny, I posted the same idea some months ago.
http://www.diyaudio.com/forums/showt...476#post325476

Steven

[lumanauw]

I cannot understand how GC-ZEN-ALEPH-1a.pdf and GC-BOOTSTRAP-1a.pdf works. What is the concept and how does those circuit works?

How does a GC chip can be made Aleph current source?
How does GC current bootstrap works?

[azira]

Originally posted by lumanauw
How does GC current bootstrap works?

The bootstrap circuit is a lot easier to analyze if you simplify it a little. Pretend you drive it from a single ended source (so GND the -in). Then the first opamp looks just like a non-inverting opamp. The 2nd opamp is a little harder to analyze. Firstly there's the output resistor from the first opamp going into the load, that's probably a current sharing resistor so think something low value. 2ndly you see that the RF2 for the bootstrap opamp ties into the same thing. I suspect that RF1+RF2 of the 2nd opamp dictate the current sharing with Rout of the first opamp. You'd want their values to be the same yet small. Next you see that there's a positve feedback mechanism on the 2nd opamp but it's through a resistor so it sort of "defers" the + input voltage to the first opamp. Both of these facts lead it to the notion that the 2nd opamp is more like a comparator, all it wants to do is drive HARD towards one rail or the other. Then if you think about what happens if the 2nd opamp tries to drive too hard, the NFB mechanism of the first opamp sees this and tries to correct for it which will affect the way the 2nd opamp behaves.
So in short, what you have is that the 2nd opamp is a "current buffer" for the first opamp but is rigged to be slightly out of control so that the first opamps NFB loop is the dominant factor in the action.

Here's a simplified version... take this and add some PFB around the buffer and change the driver to a diff input and you'll see a similar result. Look at the OPA541 datasheet and you should see a similar circuit (although the figures are mislabeled in it).
--
Danny

[azira]

One more thing.

The reason you can't do what I've shown with GCs is that GCs aren't unity gain stable. So then the real trick to this design is to use the 2nd opamp as unity gain while having it actually have gain. Hence having it connected to the load through the feedback loop as an attenuator.

[Nelson Pass]

Originally posted by Steven
Funny, I posted the same idea some months ago

So you did. (I don't read the chip amp forum, so I missed it).

Of course the idea wasn't original, as I've pointed out elsewhere,
the first I saw something like it was from Jim Bongiorno, many,
many years ago.

[lumanauw]

Hi, Azira,

Here's a simplified version... take this and add some PFB around the buffer and change the driver to a diff input and you'll see a similar result. Look at the OPA541 datasheet and you should see a similar circuit (although the figures are mislabeled in it).

I can understand your example. It is like giving more current, so the load is handled by 2 GC in your example. But you pointed the important thing. GC cannot operate in buffer mode like in this example.

The bootstrap circuit is a lot easier to analyze if you simplify it a little. Pretend you drive it from a single ended source (so GND the -in). Then the first opamp looks just like a non-inverting opamp. The 2nd opamp is a little harder to analyze. Firstly there's the output resistor from the first opamp going into the load, that's probably a current sharing resistor so think something low value. 2ndly you see that the RF2 for the bootstrap opamp ties into the same thing. I suspect that RF1+RF2 of the 2nd opamp dictate the current sharing with Rout of the first opamp. You'd want their values to be the same yet small. Next you see that there's a positve feedback mechanism on the 2nd opamp but it's through a resistor so it sort of "defers" the + input voltage to the first opamp. Both of these facts lead it to the notion that the 2nd opamp is more like a comparator, all it wants to do is drive HARD towards one rail or the other. Then if you think about what happens if the 2nd opamp tries to drive too hard, the NFB mechanism of the first opamp sees this and tries to correct for it which will affect the way the 2nd opamp behaves.

I still dont understand the 2 resistors from the output from the 2nd GC to its +input and -input. The one to -input understandable as gain factor resistor combined with resistor from -input to output. But the one to +input gives positive feedback? Is the 2nd GC like you said works like comparator, giving output only +rail or -rail? Like classD? How come it works in analog output swing?

[metalman]

This circuit is a work in progress towards building a Bootstrapped SuSy GainClone using LM3875 op-amps, based on Nelson's circuit suggestion earlier in this thread. I have simulated it in Spice but I have not built a protoype yet. The sumulations look good, but as we all know, simulation is not the real world. Before I move into building a test circuit, does anyone have any comments on improvements?

Cautionary Note: Although I have given this circuit a good deal of thought, I am not an EE. Use with caution and at your own risk!

Terry

[metalman]

Forgot to mention. The sum total of my diagnostic equipment is a Fluke multimeter. Anyone around in the Vancouver area with a decent scope that would be interested in doing a little testing?

Terry