Why match an LNA at the input?

input and output impedance matching for lna

Dear all,

I was wondering why an LNA has to be power matched at the input. Is it for low noise performance, or is it to achieve optimum power transfer from the receiving antenna to the rest of the receiver? If the latter is true; what is the use of achieving optimum power transfer? I mean: if the information of the signal is in the voltage or in the current, why do we need the power? An example; an ideal voltage meter or an oscilloscope presents an infinite impedance to the source that is being measured, such that the source is not influenced by the measurement. Why can't we design an LNA with a high input impedance?

Thanks in advance.
Kind regards,

Mark

high input impedance lna

In order to achieve low noise figure, you need to have the input port matched to the preselect filter, which is typically 50 ohms. When the return loss (S11) is less than 10 dB, the measured noise figure will increase.

There are the two separate matching cases: optimal noise matching and conjugate matching. The first one in used to match LNA input to preceding circuit. It may be filter, antenna, transmission line or something else. This kind of matching provides the lowest noise figure, but may introduce some mismatch loss. It used mostly for narrow band matching because it is very hard to do required optimal matching for broadband system. Sometimes optimal noise matching is used for LNA output and sometimes for both input and output circuits.

The second matching is one when source and load impedance have equal resistive part and equal by magnitude and opposite by sign reactance. As results pure resistive match will be achieved in ideal case and this case corresponds to maximum power transfer from source to load. In RF and microwave it is much easier to measure power than voltage or current, so power is used for circuits and systems characterization. But if you know the impedance for point of interest you may use voltage as for low frequency case. Often RF analyzes for RFIC systems use voltage parameters. But when connections between elements are big enough and can change the impedance power is only the choice.

You probably can design high impedance LNA. In this case you need to have transmission line with the such high impedance, then antenna and so on. It may be not possible to build these elements with high characteristic impedance. This is why we use 50 Ohm and 75 Ohm transmission lines.

yeah U can design LNA with high input impedance but still in the bard u will design a matching circuit for the 50 ohm lines

Khouly

Thank you all very much, it's much clearer now!

Mark