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As part of my assignment, I need to design a Cs amplifier with certain fixed values. The output voltage peak-to-peak (Vout,pp) must be greater than 2.5V, and the Total Harmonic Distortion (THD) must be less than 1%.

The values of VDD, C, L, and Vsig must remain as shown in the provided diagram. The parameters that can be adjusted are RG1, RG2, W (with a maximum of 20m), RD1, and RS1. Additionally, the sum of RG1 and RG2 is limited to a maximum of 5MEG. Lambda and gamma are both zero.

Due to the large amplitude of Vsig and the bypass capacitor, even a slight increase in RD causes waveform distortion. The circuit shown in the picture represents my efforts over several days to achieve Vout > 2.5V while minimizing THD. However, despite my best efforts, the THD exceeds 4%.

I am unsure how to design the circuit to achieve THD < 1% while maintaining a peak output voltage greater than 2.5V. Could you please help me with this?

enter image description here

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  • \$\begingroup\$ kn' = 200u, so if W = 250U, L = 0.5U -> kn = 0.1. \$\endgroup\$
    – Mul
    Commented May 17 at 9:13
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    \$\begingroup\$ "Cs" is not a commonly accepted abbreviation for "common source" please write it out. \$\endgroup\$ Commented May 17 at 10:19
  • \$\begingroup\$ A peak output voltage > 2.5V with a 5V DC supply? \$\endgroup\$
    – Raonoke
    Commented May 17 at 12:57

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enter image description here

To save time I cheated a bit by initially using the model of a BS170 to get the bias point and component values for a THD < 1%. After that I replaced the MOSFET with a generic model with rounded W and L values taken from the BS170. The gate bias voltage needed some readjustment. THD is at 0.65% with an output voltage of 2.65Vpp.


I just saw that L = 0.5u is a requirement so the circuit above needs some minor revision. THD is now at 0.43%.

enter image description here

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  • \$\begingroup\$ Where is CC4? Granted, the requirements seem odd to me; don't know if it can even be done without changing CC4 value. \$\endgroup\$ Commented May 18 at 9:29
  • \$\begingroup\$ @ Tim * No CC4 because the THD doesn't go below 1 % @ >2.5Vpp out without some kind of negative feedback to cap the gain and increase linearity. At least I wasn't able to do it. \$\endgroup\$
    – Raonoke
    Commented May 19 at 6:00

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