Discrete Opamp Open Design

[bcarso]

Originally Posted by scott wurcer

BTW took two strips of 100 4401/4403 from ebay and find most matched better than 1-2mV Vbe.

Sounds like we better get Purchasing to qualify eBay as an approved vendor

[bear]

dug these out...

[kgrlee]

Originally Posted by scott wurcer

I'll help you out this weekend, and get you going (I have not had these problems). I found an old laptop to put LTSPICE on so I won't irritate our lawyers. The OS resistors depend on absolute Vbe a little I'll show you how to compute a close value (you only need the diode function on your handheld VOM).

BTW took two strips of 100 4401/4403 from ebay and find most matched better than 1-2mV Vbe.

Hopefully Lord Shiva will look favrably on our efforts.

We grovel at your feet Oh Lord Shiva. Our brother Wurcer is restored to the Light

Was that between 4401s & 4403s or within a strip of 4401s, within a strip of 4403s? ie did the 4401s match the 4403s?

"OS resistors" Was is das? asks dis bum hus unnerstening of 3 or 2LA is wonky

[aparatusonitus]

Originally Posted by scott wurcer

BTW took two strips of 100 4401/4403 from ebay and find most matched better than 1-2mV Vbe.

Could you please point out that particular ebay seller, thx!

[kgrlee]

Originally Posted by scott wurcer

I'll help you out this weekend, and get you going (I have not had these problems). I found an old laptop to put LTSPICE on so I won't irritate our lawyers. The OS resistors depend on absolute Vbe a little I'll show you how to compute a close value (you only need the diode function on your handheld VOM).

I uploaded ricardo.txt cos the transistor models should give the same results in other SPICES unless LTspice is doing something other SPICEs aren't.

But what you say implies that using R3, one of the BF862 ltp source degenerating resistors, to adjust offset is suboptimal and there is a better tweak point.

On the FET990, I use the input current mirror resistors which is similar to how its done on many IC OPAs.

[scott wurcer]

Originally Posted by aparatusonitus

Could you please point out that particular ebay seller, thx!

Unfortunately, I bought them 2 or 3 yr ago and bagged them up. I meant the matching of 4401 to 4401 etc. Obviously buying strips probably gets devices off of the same wafer near eachother.


EDIT - I don't see any on eBay right now from the same manu, but there are some pretty cheap and of course the pictures don't match the items. One guy picturing strips of 2N2907's.

[scott wurcer]

Originally Posted by kgrlee

We grovel at your feet Oh Lord Shiva. Our brother Wurcer is restored to the Light

Was that between 4401s & 4403s or within a strip of 4401s, within a strip of 4403s? ie did the 4401s match the 4403s?

"OS resistors" Was is das? asks dis bum hus unnerstening of 3 or 2LA is wonky

The 20 Ohm ones, they simply depend on the vbe differences, assuming reasonable thermal connection. This is not hard to do. I have lots of those little TO-92 heatsinks left from the group buy, but just putting the right devices "face to face" and wraping them with a piece of copper tape should be more than enough.

[grhughes]

Scott: Are you going to look at cascoding sinks and sources to improve CMRR as R. Marsh suggests? Thanks Ray

[aparatusonitus]

Originally Posted by scott wurcer

Unfortunately, I bought them 2 or 3 yr ago and bagged them up. I meant the matching of 4401 to 4401 etc. Obviously buying strips probably gets devices off of the same wafer near eachother.


EDIT - I don't see any on eBay right now from the same manu, but there are some pretty cheap and of course the pictures don't match the items. One guy picturing strips of 2N2907's.

Could you name a manu of yours devices?

[kgrlee]

Originally Posted by scott wurcer

The 20 Ohm ones, they simply depend on the vbe differences, assuming reasonable thermal connection.

BINGO! The 20Rs on the Output Stage biasing.

Do we tweak the 20Rs so that with 'no' load (incl. F/B bits so no Iq is lost), there is no voltage between the bases of the 'bias' devices and the output?

But that means another tweak pot for i/p stage offset. Got a recommendation for which one to use?

[edit]Just realised the Output Iq isn't just what's going through the 4R7s but a substantial amount goes through the 'bias' transistors & the 20Rs too. When you say "Adj for 8mA", is that just through the 4R7s or the total through the outputs? [/edit]

I have lots of those little TO-92 heatsinks left from the group buy, but just putting the right devices "face to face" and wraping them with a piece of copper tape should be more than enough.

.. from another project, I recommend a drop of cyano-acrylate in addition to the Cu strap. Substantial improvement in thermal matching.