In the circuit shown, the voltage source $V(t)=15+0.1\sin(100t)$ volts. The $PMOS$ transistor is biased such that it is in situation with its gate-source capacitance beign $4 nF$ and its transconductance at the operating point beign $1 mA/V$. Other parasitic impedances of the MOSFET may be ignored. An external capacitor of capacitance $2 nF$ is connected across the PMOS transistor as shown. The input impedance in mega ohm as seen by the voltage source is ____________ $M\Omega$.
