Question 12.6: LOW-NOISE MOSFET AMPLIFIER DESIGN An Infineon BF1005 n-chann...
LOW-NOISE MOSFET AMPLIFIER DESIGN
An Infineon BF1005 n-channel MOSFET transistor having C_{gs} = 2.1 pF and g_{m} = 24 mS is used in a 900 MHz low-noise amplifier with inductive sourcedegeneration, as shown in Figure 12.10. Determine the source and gate induc-tors, and estimate the bandwidth of the amplifier. Assume a source impedance of Z_{0} = 50 Ω.
From (12.62), matching the input resistance to Z_{0} determines the source inductor as
Z=\frac{V}{I}=\frac{g_{m}L_{s}}{C_{gs}}+j(\omega L_{s}- \frac{1}{\omega C_{gs}} )L_{s}=\frac{Z_{0}C_{gs}}{g_{m}}=\frac{(50)\times (2.1 \times 10^{-12} )}{0.024} =4.37 nH
The net reactance at the input is jX=j(\omega L_{s}-\frac{1}{\omega C_{gs}})= -j59.5 \Omega, so the required series inductance for matching is
L_{g}=\frac{-X}{\omega }=\frac{59.5}{2\pi (900 \times 10^{6})}=10.5 nH,From (12.63) we can estimate the Q as
Q= \frac{\omega L_{g}C_{gs}}{g_{m}L_{s}}=1.2,so the bandwidth of the amplifier could be as high as 80%. This value is probably higher than what would be obtained in practice, due to the approximations that have been made in our analysis.