Question 6.13: We want to design a resistive feedback amplifier having a lo......

Microwave Electronics [1102891]

We want to design a resistive feedback amplifier having a low-frequency $S_{21f}=20\text{ dB}$ for a FET with $R_0=50 \; \Omega$ matching. Estimate the minimum $\text{g}_m$ needed to implement it without series feedback and the resulting $S_{21}$ .

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We have $\left|S_{21 f}(0)\right|=10^{20 / 20}=10$ , from which:

$R_p=R_0\left(1+\left|S_{21 f}(0)\right|\right)=550\; \Omega ,$

i.e., the minimum transconductance is:

$\text{g}_m=\frac{R_p}{R_0^2}=220\text{ mS} .$

The low-frequency open-loop transmittance is $S_{21}=-2 R_0 \text{g}_m=-22$ , equal, as it should be, to the limit value $2\left(1+\left|S_{21 f}(0)\right|\right)=22$ (we have $R_S=0$ in fact).

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