SimSmith – looking both ways – an LNA design task

This article shows the use of SimSmith in design and analysis of the input circuit of an MGF1302 LNA.

The MGF1302 is a low noise GaAs FET designed for S band to X band amplifiers, and was very popular in ham equipment until the arrival of pHEMT devices.

An important characteristic of the MGF1302 is that matching the input circuit for maximum gain (maximum power transfer) does not achieve the best Noise Figure… and since low noise is the objective, then we must design for that.

The datasheet contains a set of Γopt for the source impedance seen by the device gate, and interpolating for 1296MHz Γopt=0.73∠-10.5°.

Lets convert Γopt to some other useful values.

The equivalent source Z, Y and rectangular form of Γopt= will be convenient during the circuit design phase.

The other important characteristic is Zin of the gate in the final circuit configuration, and that is derived from a model of the device in circuit. The value is 7.8-j164Ω. At low frequencies, the FET gate looks like an extremely high impedance, at higher frequencies more like tiny capacitance with very low equivalent series resistance (ESR), and still higher frequencies the capacitive reactance is lower and ESR higher, and still higher frequencies lead inductance comes into play and the gate looks inductive with even higher ESR.

A model of the antenna to gate circuit was built in SimSmith v16.9.

Above, the SimSmith model which includes a swept length of LDF4-50A transmission line.

The standard display was supplemented with impedance, admittance and Γ looking from the gate back towards the antenna. These make it easier to adjust the L and C components for the desired outcome. The G component of Y is most sensitive to adjustment of L and the B component to adjustment of C. So, they are both adjusted to approximately obtain Γopt=0.7178-j0.1330 from the conversions done earlier.

The behavior demonstrates the complex interaction of source, transmission line and load… worth studying.

The discussion is about a low noise receiving system where optimal results come from an input circuit that is not designed for maximum power transfer.