opamp-model

An attempt to model the SID opamp transfer functions

6581 OpAmp

"Op-amp" (self-biased NMOS inverter)

               12V         12V

                T           T
                |           |
                |           |
                |    +------o
                |    |      |
                |    |  ||--+
                |    +--||
                |       ||--+
            ||--+           |
    Vi -----||              o---o----- Vo
            ||--+           |   |
                | Vx    ||--+   |
                o-------||      |
                |       ||--+   |
            ||--+           |   |
         +--||              |   |
         |  ||--+           |   |
         |      |           |   |
         |      |           |   |
         |      V           V   |
         |                      |
         |     GND         GND  |
         |                      |
         +----------------------+

    Vi  - input voltage
    Vo  - output voltage

The schematics above are laid out to show that the "op-amp" logically consists of two building blocks; an enhancement load common source amplifier (on the right hand side of the schematics) with a common drain input stage biased by the output voltage (on the left hand side of the schematics).

W/L values

• M1a (top left) ~ 80/20 ( 3.9/1.0)
• M2a (bottom left) ~ 25/70 ( 1.4/3.1)
• M1b (top right) ~ 40/20 ( 2.2/1.0)
• M2b (bottom right) ~ 650/20 (32.6/1.0)

Reference values, measured on CAP1B/CAP1A on a chip marked MOS 6581R4AR 0687 14

taken from reSID, by Dag Lem

Emulated values

Emulated values with Vdd = 12.18, Vt = 1.31 and n = 1.15:

Emulated values with Vdd = 12.18, Vt = 0.8 and n = 1.15:

Circuit simulation

Assuming uCox = 20uA/V² and Vt = 1.31V

https://www.falstad.com/circuit/circuitjs.html?ctz=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