Hey all,
Ive been designing some FET boosters with a J201 Fet, but the only FET my crappy dealer has is a MPF102, and i just cant bias it to get nearly the amout of gain i was getting with a single stage J201 FET (17db). I can understand it might have less gain, but for some reason i cant get beyond about 7db :? :cry: . Any tips/Schematics out there?
Thanx
Are you using a bypass cap? No? Try a 22uF bypass cap around the source resistor - should get you up into the 20dB+ region. I find that for a 2k2 or 2k4 source resistor, MPF102s generally like about 3 to 6k of drain resistance.
Have you got any schematics of simple FET boosters using MPF102's?
Here's a very simple booster (high gain)
http://members.optusnet.com.au/~jethro.dog/mypic106.jpg
For lower gain, reduce the 22uF bypass cap to a 2.2uF (about 10dB gain), or even leave it out altogether (for about 5dB gain). If you wanted to replace the trimpot with a fixed-value resistor, try about 4.7k (for most MPF102s). For higher (less negative) Vgs jfets (like a 2N5485), use 6.8k or a bit more.
Hope this is of interest.
Oops, there should really be a 1M resistor between the jfet gate and ground (for self-biasing). Tho it'll probably work ok without it, the gate should really be tied to earth.
i actually find that mpf102's distort quite peasingly, but you have to line up a few of them to get to real distortion teritory.
One of my next projects uses two of them. i bread boarded it a while ago. soon i will have one up and running, but right now i am sick, and need to make more stupid boxes. :D
Quote from: brettHere's a very simple booster (high gain)
http://members.optusnet.com.au/~jethro.dog/mypic106.jpg
Brett, you definitely need a gate resistor otherwise the gate is floating and there really isn't a Vgs. It may work but it will be intermittent at best.
Quote from: brettFor lower gain, reduce the 22uF bypass cap to a 2.2uF (about 10dB gain), or even leave it out altogether (for about 5dB gain).
This isn't quite correct. The cap boosts the gain to maximum for all frequencies about 10x above the cutoff point made by the source resistor and the source cap. You would need to add a pot/resistor in series with the source bypass cap to be able to control the amount of gain.
As for the original question, MPF102s have significantly less gain that J201s and a
HUGE variance in their Vgs(off) value, from .5 to 7.5 which makes them incredibly hard to bias consistantly for different parts.
You just can't get as much gain from a MPF102 as you can from a J201. You will probably have to use two or more to get the same gain.
The NTE458 has a ton of gain and should be available to you, though it may be expensive.
Hope this helps,
Jay