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Old 9th December 2009, 03:55 PM   #1
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Default Yet another discrete op-amp

By request, here’s where I would start on a discrete op-amp using my floating current mirror error correction. I tried to keep the device count minimum so someone could actually fit it on a small PC board/adaptor. The basic signal path is folded cascode, floating mirror, and triple EF output. Bias is a single FET current maker, the reuse of as much of this current as possible makes virtually the entire bias current noise common mode. Almost all resistors are simply scaled from 500 Ohms and series parallel connections of 500 Ohms so a strip would work very well for matching. Offset and gain can be trimmed with input source and drain resistors, so I would not obsessively match devices.

This is a single gain stage amplifier compensated with a single pole at node VA. Using generic LTSPICE 2N4401/2N4403 models and 2SK140’s the Aol was 300K and Vos 800uV untrimmed. The floating current mirror acts as a unity gain current conveyor to the voltage error of the OS across whatever impedance is between VB and the Output, and as stated before this gives a “positive feedback” of one with first order cancellation of the OS Vbe voltage modulation.

Experimenters could play with “VAS killing” by placing a resistor and compensation capacitor to ground at VA and the same network from VB to Output.

With the same generic models I saw ppm distortions at 10kHz 20V p-p into 100 Ohms. This is a work in progress and I hope to take a spare differential VAS board that I made and put this together over the holiday break. There are lots of possibilities for component selection, input stage gm and GBW for different applications.

As usual nothing proprietary here (patent is expired), free to use/not use/abuse. Probably a <5$ BOM here on an all SOT version. All constructive comments welcome.
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Old 9th December 2009, 04:18 PM   #2
EUVL is offline EUVL  Europe
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I would have an impossible task to fit this in a 10x10mm footprint (DIP8 compatible).

Thanks for sharing,
Patrick
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Old 9th December 2009, 04:18 PM   #3
Mooly is offline Mooly  United Kingdom
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Does this one have "Bass"
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Old 9th December 2009, 04:37 PM   #4
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Originally Posted by Mooly View Post
Does this one have "Bass"

Large or small mouth? My avatar would like to think it has trout instead.
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Old 9th December 2009, 04:48 PM   #5
Salas is online now Salas  Greece
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Can that circuit fish for more detail, and will it have moist mids?
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Old 9th December 2009, 04:48 PM   #6
jez is offline jez  United Kingdom
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Trout, salmon, seabass... all delicious! I have salmon in the oven as I type this
Another interesting design Scott. Thanks for sharing it! I look forward to simulating this one
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Old 9th December 2009, 04:50 PM   #7
Mooly is offline Mooly  United Kingdom
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lol Trout will do nicely... fresh caught (by my dad) today

And the design looks great ... wonder if our friends will give it a try.
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Old 9th December 2009, 06:16 PM   #8
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Can that circuit fish for more detail, and will it have moist mids?
It even lends itself to low/non-GFB. Might even try MAT-02's with the inductor bypassed emitter resistors ala the JE990.
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Old 9th December 2009, 06:37 PM   #9
Salas is online now Salas  Greece
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What about some Rohm types?
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Old 9th December 2009, 06:40 PM   #10
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What about some Rohm types?
I have to pay for those But yes they are a fine choice. Not always mentioned the RHOM trannies are some of the best ever in and near saturation so they are superlative in those Leach type MC head amps running on a couple of cells.
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