Re: Why JFET amp instead of Non-inv Op amp? Do they both not have high Imp?
- From: Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 12 Apr 2005 16:15:02 GMT
Hello Stephen,
If I could trouble you guys once more...
I am building a preamp/amp for the 60khz WWVB signal. Where should I do my filtering... after the JFET preamp and before the second stage amp? Or should I filter last (after 3rd stage)?
LC filter before the first stage, crystal filter after the first stage.
In general, do you filter before or after your gain sections? Why?
LC before because any high amplitude noise that can saturate the first amp would render the whole receive chain useless. Plenty of this around: Switch mode supplies, TV horizontal stages, big noisy brush motors, local AM stations, arc welding gear and so on.
A crystal filter needs a well-controlled impedance on both sides which makes it tricky to place right between antenna and amp. The antenna isn't a very controlled impedance.
The old RF rule is to place filters as far to the front as possible.
BTW, regarding opamps: As Jim pointed out there are some really great ones. But in commercial designs cost is paramount. While a very good FET can be had for under 5 Cents suitable opamps often cost more than ten times that. And they'd use more power. At 60kHz you might be able to find a decent opamp though, that's not a high frequency. I would still use a transistor but then again I am so used to designing with them.
Regards, Joerg
http://www.analogconsultants.com .
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