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Low Noise Amplifier and Input/Output Phase

Electrical Engineering Asked by Jay on November 4, 2020

I’ve been designing LNAs for a brief time and I was wondering why the input and output phases is usually never cared for or measured when measuring the specs of the LNA. For example, the only specs we cared about when designing the LNA were gain, intermodulation point (3rd), and power.

I guess this question would also apply to general use of amplifiers such as Op-Amps (with feedback) but why is phase for different frequencies not cared about (except for 180 deg frequency point for checking stability) or is never measured? I would assume that if a signal consisted of a bandwidth from freq ‘a’ to freq ‘b’, you would need the entire bandwidth of frequencies, from input to output, to all have the same phase (i.e., not a single or more frequencies are delayed/ahead) in order to demodulate the entire bandwidth of signal.

2 Answers

LNA phase is cared for in at least one case. Phase matching is critical to successfully combine multiple receive antennas to form an array. Transmission lines must be equal length, combiners phase matched and it is desirable to have LNAs at each antenna to reduce transmission line and combiner losses.

Answered by Tom1975 on November 4, 2020

One does not usually place an LNA inside an RF feedback loop, so as long as the group delay across the frequency range of interest is reasonably constant (It generally is) it is just not that important. Also LNAs cannot in general be assumed to be small compared to the operating wavelength, if I need to tune phase at 24GHz I do it with a short length of transmission line...

Opamps are basically always used inside a feedback loop which makes phase shift in the naked opamp basically only a concern for stability but not one that impacts the overall phase shift of the circuit because the feedback loop defines the phase shift as long as there is sufficient open loop gain (Opamp circuits are pretty much all electrically small).

Answered by Dan Mills on November 4, 2020

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