"Ian White GM3SEK" wrote in message
...
For example, audio/LF designers tend to deal in noise voltages and also need
to think about source and load resistances. RF designers think more in terms
of noise power, noise factor (ratio) and noise figure (dB); and since
performance tends to be specified and measured in a 50-ohm system, it often
isn't necessary to know the individual source and load impedances.

These days using a regular old op-amp as an HF amplifier can often be
attractive, although when you go through the math you find out that it's very
difficult to obtain a low enough noise op-amp such that it has a noise figure
less than about 10dB (and even obtianing 20dB requires some care -- you can
easily end up with 40dB if you're not careful!). Texas Instruments has a
good application note on this: focus.ti.com/lit/an/slyt094/slyt094.pdf .

Hence op-amps are pretty much out for LNAs, but can be quite useful by the
time you're hitting an IF and already have some reasonable amount of gain
ahead.

---Joel