TTL as linear devices

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A lot of the the old National Semiconductor App Notes are now at Fairchild:

These app notes in particular would be worth a look:

A word of advice -- just because you _can_ do something, doesn't mean you necessarily should. Many of these circuits (particularly with the 'C14 schmitt trigger and oscillators) are really worthwhile to have in your bag o' tricks. Others are pretty lame. Gumball op amps and comparators are widely sourced and inexpensive, and will run circles around a lot of these circuits with the same number of discrete components and a lot less power.

Good luck Chris

I had never heard of that for standard TTL. The bi-polar logic series (standard, LS, ALS, F, etc.) do not have very linear output switching characteristics compared to CMOS.

On the other hand, It is fairly well known that CMOS inverters (4000 series, C, HC, AC, AHC) can be used as amplifiers, in a similar way to what you describe. Most newer technologies are faster than standard TTL, so you could achieve higher bandwidths.

Since ECL is designed so the devices do not go into hard saturation or hard cut-off, I should think they would be more readily useable as linear devices. ECL circuits are similar to the internal first stage of an Op-Amp. The devices are always biased "on", similar to a class-A amplifier.

Multistage logic, like the HC/HCT/4000B stuff, tends to oscillate if used like this; too many poles, too much gain. The unbuffered "UB" CMOS types generally work. Most real TTL (bipolar) parts will work.

ECL usually works fine as a fast linear amp, especially the simple gates. The nuclear folks like to use ecl in things like wire chambers that need many channels of fast gain cheap.

John

Almost any logic function that inverts, can be used as a linear amplifier, with the addition of a bias (DC feedback) arrangement, and possibly a phase compensation network, (to make sure the total loop gain falls below 1 before the loop phase shift increases more than about 145 degrees). Even multistage CMOS inverters, like the buffered HC family can be stabilized as linear amplifiers. You may have to be concerned about power consumption for some types, though.

Don Lancaster has some amp circuits in his classic "CMOS Cookbook".

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis