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 $Z_{0} = 50 Ω.$

Question

LOW-NOISE MOSFET AMPLIFIER DESIGN

An Infineon BF1005 n-channel MOSFET transistor having [latex]C_{gs} = 2.1 pF[/latex] and [latex]g_{m} = 24 mS[/latex] 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 [latex]Z_{0} = 50 Ω.[/latex]

Accepted Answer

From (12.62), matching the input resistance to [latex]Z_{0}[/latex] determines the source inductor as

[latex]Z=\frac{V}{I}=\frac{g_{m}L_{s}}{C_{gs}}+j(\omega L_{s}- \frac{1}{\omega C_{gs}} ) [/latex]

[latex]L_{s}=\frac{Z_{0}C_{gs}}{g_{m}}=\frac{(50) imes (2.1 imes 10^{-12} )}{0.024} =4.37 nH [/latex]

The net reactance at the input is [latex]jX=j(\omega L_{s}-\frac{1}{\omega C_{gs}})= -j59.5 \Omega, [/latex] so the required series inductance for matching is

[latex]L_{g}=\frac{-X}{\omega }=\frac{59.5}{2\pi (900 imes 10^{6})}=10.5 nH,[/latex]

From (12.63) we can estimate the Q as

[latex]Q= \frac{\omega L_{g}C_{gs}}{g_{m}L_{s}}=1.2,[/latex]

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.

Question 12.6: LOW-NOISE MOSFET AMPLIFIER DESIGN An Infineon BF1005 n-chann...

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