[Coco] CoCo 1 Serial IO OP-AMP

[Gene Heskett]

On Monday 18 May 2015 00:45:33 Josh Harper via Coco wrote:
> hi guys
> may I ask what reasons to change this chip

The 741 is almost the first operational amplifer, and it very slow, with 
an open loop gain north of 100,000, which is nice, but its bandwidth is 
such that this phenominal gain is down by 3db, or to half as the signal 
frequencies rise, such that at the positively snails pace of 10Hz, too 
low for you to hear.  The LF356-357 family has a bit less gain, but its 
gain/bandwidth product is 10,000 higher than the 741.  The net effect is 
that when trying to run drivewire at speeds of 57,600 baud and up, its 
ability to slew the output voltage at such a rate severely limits its 
output.  Figuratively speaking, the LF356 can do, at megahertz rates, 
what the 741 gets done at only 2400 baud & slower.

This is a case where the good stuff was available, and its possible the 
coco's original designers specced a better part, but some bean counter, 
not knowing what we would be doing with it 30 years later, overrode the 
engineers bill of goods because he could get the 741's 14 cents cheaper.

FWIW, nothing to our beloved CoCo's, op-amps with 100 gigabyte 
gain/bandwidth products are available today for less that a fiver.  The 
tech never stops getting both faster and lower power used.

> I don't know much about this chip
> thanks guys
>        From: Gene Heskett <gheskett at wdtv.com>
>  To: coco at maltedmedia.com
>  Sent: Sunday, May 17, 2015 8:47 PM
>  Subject: Re: [Coco] CoCo 1 Serial IO OP-AMP
>
> On Sunday 17 May 2015 20:27:36 Melanie and John Mark Mobley wrote:
> > What is a good replacement chip to use in the CoCo 1 Serial IO
> > MC1741 OP-AMP?
> > I was thinking a TI LF356 from Digi-Key.
> > We are having trouble getting DriveWire to run on a CoCo 1.
> >
> > John Mark Mobley
>
> That should work if the circuit layout is good.  Its about 100x faster
> than the 741.
>
> Cheers, Gene Heskett
> --
> "There are four boxes to be used in defense of liberty:
>  soap, ballot, jury, and ammo. Please use in that order."
> -Ed Howdershelt (Author)
> Genes Web page <http://geneslinuxbox.net:6309/gene>
>
>
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Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>