TTL as linear devices

Hi All,

I was looking at a '70 era circuit book and found that devices like the
7400 nand gate could be used as a linear amplifier of small signals.

You could use one of the inputs as a control signal to turn on/off the
output and the other would be bias with a resistor-divider and the ac
signal injected via a cap. Gain was like 15db.

Can other device families such as the HCT or HCTLS or LS, S, be used
and can I get a higher frequency than standard TTL?

How about ECL, can ECL be used in the linear small-signal mode?

I look forward to your input.

pdrunen.

 

[Chris]

pdrunen@aol.com wrote:

Hi All,

I was looking at a '70 era circuit book and found that devices like
the
7400 nand gate could be used as a linear amplifier of small signals.

You could use one of the inputs as a control signal to turn on/off
the
output and the other would be bias with a resistor-divider and the ac
signal injected via a cap. Gain was like 15db.

Can other device families such as the HCT or HCTLS or LS, S, be used
and can I get a higher frequency than standard TTL?

How about ECL, can ECL be used in the linear small-signal mode?

I look forward to your input.

pdrunen.

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

http://www.fairchildsemi.com/apnotes/an_bydoc_d5.html

These app notes in particular would be worth a look:

http://www.fairchildsemi.com/an/AN/AN-88.pdf
CMOS Linear Applications

http://www.fairchildsemi.com/an/AN/AN-118.pdf
CMOS Oscillators

http://www.fairchildsemi.com/an/AN/AN-140.pdf
CMOS Schmitt Trigger - A Uniquely Versatile Design


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

 

[tlbs]

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.