Dear Mike, Chris, LF group,
I would agree with Chris' comments regarding Op-amp suitability - the preamp
circuit appears to be designed for 40dB voltage gain, which it won't achieve
at MF with an OP-27 or even less a 741. The article gives values of gain
from 34dB at 25kHz down to 10dB at 500kHz, although it doesnot say under
what conditions this was measured. I'm sure the circuit "works" in the sense
that it gives some useful gain, but operating it in this way means linearity
is poor, input impedance is reduced and output impedance increased, i.e. you
lose all the advantages of using an op-amp in the first place. The 9V supply
is a bit low for these op-amps also.
With a reasonable receiver sensitivity, you would not need much gain with
this kind of loop arangement - assuming a loaded Q of 100, I calculate the
voltage at the loop terminals at resonance will be about 1.3uV for a 1uV/m
field strength. The noise in a 500Hz bandwidth at the loop terminals would
be about 0.4uV just due to the loop itself, with a similar level due to
external band noise on a quiet band. The internal noise of better receivers
would be swamped even with 0dB preamp gain, i.e. just a buffer, although you
might want 10 - 20 dB if your RX is a bit deaf at 500k.
The remarks about the loop bandwidth in the article are a bit mysterious -
it implies the bandwidth is wide and that tuning is not critical, but my
experience with this type of loop is that Q of 100 - 200 is normal, giving a
bandwidth of a few kHz at 500k, so tuning is critical.
My experience with op-amps as RF preamplifiers is that they can be made to
work quite well, but are not any simpler or better performing than discrete
component circuits in this frequency range. Check out the variable-gain loop
preamp on G3YXM's "circuits" page for example. Video op-amps can be used
well into the HF range, but like all high-frequency, high gain devices with
feedback, care is needed to avoid instability!
Cheers, Jim Moritz
73 de M0BMU
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