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TTL as linear devices

Discussion in 'Electronic Basics' started by [email protected], May 18, 2005.

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  1. Guest

    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.

  2. Chris

    Chris Guest

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

    These app notes in particular would be worth a look:
    CMOS Linear Applications
    CMOS Oscillators
    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
  3. 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.
  4. tlbs

    tlbs Guest

    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
  5. John Larkin

    John Larkin Guest

    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.

  6. Bob Masta

    Bob Masta Guest

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

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
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