Advantage of using low noise transconductance amplifiers for RF receiver front end

[Tanvi Singh]

Hi

I would like to know waht is the advantage of converting voltage to current in low noise transconductance amplifier when in traditional LNA design output is measured in terms of voltage only.

 

[jiripolivka]

Hi

I would like to know waht is the advantage of converting voltage to current in low noise transconductance amplifier when in traditional LNA design output is measured in terms of voltage only.

I am sorry but in RF low-noise amplifiers power relations are used exclusively. Voltage and current noise has developed only in low-frequency opamps as voltages and currents can be measured. At RF this is not possible, so power is only measurable quantity.

 

[Tanvi Singh]

Okay many thanks but is there any particular advantage of using the circuit in current mode I mean to take output as current rather than voltage even if we measure power at the output because I read somewhere that it simplifies the design of mixers added ahead of LNTA in RF receivers. So this is the only reason .

 

[jiripolivka]

Mixer front-end receivers are now seen mostly at more than 40 GHz where LNAs are still expensive.
Matching mixer IF output to a IF LNA is easy via 50-Ohm short line as both devices can be optimized before joining.
When designing mixers I have not used anything different from 50-Ohm IF, with very good results in all projects.
One of my 35 GHz designs, a mixer plus an IF LNA, featured 2.7 dB DSB noise figure (really measured). It is better than a MMIC LNA receiver, and lower-cost.