Proposed Oscillator
Topologies
A Colpitts oscillator (CO) is made up of an amplifier and a feedback
network, consisting of an inductor and two capacitors. The feedback
network is employed to direct a portion of the output signal back to the
input through one of the reactive elements. Usage of feedback network
provides both the frequency selection and signal reinforcement (positive
feedback). For a CO, the oscillation frequency is dependent on its tank
circuit elements L and the capacitive divider circuit, as given below.
\(f_{\text{osc}}=\frac{1}{2\pi\sqrt{LC_{t}}}\) and\(C_{t}=\frac{C1\ C2}{C1+C2}\)
The MOS based o8oscillator can be analyzed by using its tap ratio that
is given by:
\begin{equation}
n=1+\frac{C_{2}}{C_{1}}\nonumber \\
\end{equation}Integer order Colpitts
Oscillator
Initially, in this work, we design and simulate a conventional MOSFET
(32 nm technology node) based single ended CO, as shown in Figure 8. In
simulations a 32nm PTM model of MOSFET (L=32 nm and W=180 nm) has been
used, along with RL=1kΩ, C1=C2=0.3n and Ibias=5mA. The
value of inductor has been varied (keeping other parameters same) to
check its effect on the oscillation frequency. The oscillation frequency
is 1.2GHz for L=100p and 13.11 GHz for L=1p. Scaling down one of the
elements of the tank circuit has a huge impact on its circuit
oscillation frequency. On performing the transient analysis with both
L=100pH and L=1pH, decaying oscillations can be observed with peak at
approximately 0.999 Volts, as shown in Figure 9(a,b).