# Class F amplifier analysis

I can't figure out why drain voltage is rectangular in the following circuit:

I tried this:

$$V_{ds}=V_{DD}-I_{fund}R_lsin(\omega_0t)-I^{(2)}Z_{in}^{(2)}sin(2\omega_0t)-I^{(3)}Z_{in}^{(3)}sin(3\omega_0t)...$$ $$I^{(k)}= \int i_{d}sin(k\omega_0t)dt$$ $$Z_{in}^{(2k)}=0\\Z_{in}^{(2k+1)}=\infty\rightarrow I^{(2k+1)}=0$$ $$\rightarrow V_{ds}=V_{DD}-R_lI_{fund}sin(\omega_0t)$$ So the output I get is sinusoid but the reference I studied states it is rectangular. Would anyone please help me with this?

Edit: The transistor is assumed to act as switch.

• You must also state your assumptions for Q of each filter and drain current for each level and linked reference to whom said it. Jun 17 '19 at 4:34
• I'm refering to "The design of CMOS radio frequency integrated circuits" by Thomas H.lee. The input is at w0 frequency. "Q is assumed high enought to act as short at all frequency outside bandwidth". And transistor is assumed to act as switch. Jun 17 '19 at 6:17