The differential input signal is ac coupled into the cascode
amplifier stage. A cascode amplifier is used to increase the output
resistance of the first stage so it will not load down the tank circuit.
The spiral inductors used have a diameter of 320 um and have 6 turns.
The width and pitch of the traces are 7 um and 27 um, respectively.
The inductance was found to be around 5.35 nH using a spiral simulation
program developed here at Kansas State University. With the aid of
the simulation program the Q of the inductor was found to be about 12 for
this process. The size of the inductor was found using a technique
currently being developed at K-State. The 350 fF capacitor shifts
the resonant frequency of the tank circuit down to the Bluetooth band.
Digital frequency tuning is implemented with capacitors that can be switched
in or out of the circuit. The capacitors are made using MOSFETS (MOScaps)
and there are four cells ranging in value from 5-40 fF. Analog tuning
is performed by adjusting the inductance of an active inductor. FET
M3 drops the voltage below Vdd to allow room for voltage swing. Level-shifting
is performed by M5 acting as a source follower to allow the negative resistance
FETs (M10) to be properly biased. The negative resistance is formed
by cross-coupling a differential pair. There are four cells of negative
resistance providing digital Q tuning and there sizes are 12.8, 6.4, 3.2,
and 1.6 um with a minimum length of 0.5 um. Analog Q tuning (not
shown) will be implemented by varying the current in a cross-coupled long
channel pair to vary the transconductance. The output is buffered
by a source follower amplifier to prevent loading or de-tuning of the tank
circuit.