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Fri Dec 17 00:11:32 EST 2010

Using CMOS inverters as linear amplifiers

With proper biasing, the s-curve inverter amplitude transfer function
can be exploited for smooth distortion.  I was thinking about moving
this one step further and implement SVFs.

TAOE's section on the CMOS inverter mentions the use of CMOS inverters
as cheap open loop amplifiers when the waveforms aren't important.  It
mentions a biasing network with a shunting capacitor that's open loop
for AC.  This also appears in [3] ("treble boost").

The app note[2] mentions linear (feedback) applications.


Questions:

  - How much current does a 1/2 biased inverter draw?

  - Can we feed the inverter with a current source to control gain?

    How are g_m, gain, current and voltage->current related?  One
    strange thing about [2] is that DC gain _drops_ when voltage
    rises.

  - How to bias a-symmetrically?


Bottom line:

  * A CMOS invertor behaves mostly like a low-gain opamp with
    non-inverting input tied to +- 1/2 Vcc.

  * For distortion, we're interested in the open-loop gain.


[1] md5://4547016f823d5ad574cf77c44608409d
[2] http://www.qsl.net/l/lu7did//docs/QRPp/TTL_CMOS%20As%20Linear%20Amplifier_AN-88.pdf
[3] http://www.aronnelson.com/gallery/main.php/v/WGTP/Red+Rooster.GIF.html?g2_imageViewsIndex=1




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