bababui
Junior Member level 3
Hello,
I designed an LC VCO, with cross coupled transistors to act as a -R in order to make up for the R losses in the LC tank. I have both NMOS and PMOS crosscoupled pairs in order to increase -R. I have a few problems with my circuit.
One problem is that the output of my VCO is not oscillating from 0 to VDD (Vdd=1.2V). It is oscillating from -100mV to about 900mV.
My understanding is that the reason that the output is not reaching VDD might be that the pmos transistors might be cutting off, i'm not sure on that. However, I dont understand why swing goes into negative, It makes sense to me that the inductor is the culprit here, but I tried reducing the value of L and the output was still swinging into the negative. Seems that I need to shift the DC point of the oscillation upward.
Any help appreciated
I designed an LC VCO, with cross coupled transistors to act as a -R in order to make up for the R losses in the LC tank. I have both NMOS and PMOS crosscoupled pairs in order to increase -R. I have a few problems with my circuit.
One problem is that the output of my VCO is not oscillating from 0 to VDD (Vdd=1.2V). It is oscillating from -100mV to about 900mV.
My understanding is that the reason that the output is not reaching VDD might be that the pmos transistors might be cutting off, i'm not sure on that. However, I dont understand why swing goes into negative, It makes sense to me that the inductor is the culprit here, but I tried reducing the value of L and the output was still swinging into the negative. Seems that I need to shift the DC point of the oscillation upward.
Any help appreciated