Initial Experiments with the MPF102 JFET

Video looking at some initial experiments with the MPF102. It covers a test circuit to establish the Idss and Vp for a specific device. It then uses those values to build a simple buffer amp. No definitive conclusions are made from the experimental results. I'll leave that for another day.

[@MrKoenvandijken]

Hi Charlie, as always, a nice video.
My experience with these kind of JFET amplifiers is that a large voltage swing on the drain causes the Vgs to be less than expected. Remember that many JFETs are 'supposed' to be usable in two directions (with source and drain switched). A common source JFET amplifier is an inverting amplifier, an increase in gate voltage causes a decrease in drain voltage, which gives less Vgs (by symmetry of source and drain) than expected. Expecting a voltage swing of 11 volts on the drain may seriously impact the pinch off behaviour in this manner. Maybe the suggested formulas are only usable for small signal analysis. I agree, this line of thought may be kind of wobbly, but may be of help understanding the behaviour.

[@CharlieMorrisZL2CTM]

Thanks Koen, that's really interesting. Good food for thought! Cheers.

[@joeyscott4299]

Thanks for sharing Charlie ! Great video ! !

[@CharlieMorrisZL2CTM]

Cheers Joey. Quite different from a BJT!

[@W1RMD]

Straight forward excellent explanations. Thanks! Liked and subscribed. 73's.

[@bundylovess]

Good video Charlie big thumbs up

[@CharlieMorrisZL2CTM]

Thanks Allan.It was certainly an interesting experiment. Quite a different approach than a BJT.

[@Pixelwaster]

Nice video ,thanks for making these. Just wonder why an MPF102? Just some in the junque box? They are hard to find from reliable sources. Radio Shack in the US stopped restocking them around 2010 I think. A J310 (plastic U310) is a good replacement and is better suited to common gate and large signal handling. Besides, there is a tariff on 102s here in the US. Junque box is just a box full of old electronic parts 😉

[@CharlieMorrisZL2CTM]

That's interesting. I thought they were a common part and the J310 not so much, which is why I selected it from the junk box. Looks like I need to repeat the process with a J310, as I have a stack of those. I'll see what I can do.

[@robingist6666]

enjoying your videos, charlie. 73 de K4VU/6

[@CharlieMorrisZL2CTM]

Thanks for the feedback Robin. Much appreciated.
73s
Charlie ZL2CTM

[@ivanlam5783]

Helpful video. Did you end up with unity gain?

[@CharlieMorrisZL2CTM]

Hi Ivan. Yes I did on the actual experiment, but not on paper. It was getting quite late at that stage. I would like to revisit it at some stage and see where I might have gone wrong.

[@docfoot316]

Good performer on the bands Charlie ,will be trying this circuit ,how do you get on with broadcast band loading the front end .i have radio Rema just down the road from us.

[@CharlieMorrisZL2CTM]

Agreed Dave. As for broadcast band loading, not sure sorry. What you see in the video was the layout and performance. I wonder if I should put on a filter and retest it. I have a spare broadcast band filter lying around.

[@JurekPrzezdziecki]

Very interesting. Could you put a T1 specification? Thanks.

[@CharlieMorrisZL2CTM]

I think what you are after is at the 12:30 mark. It's on the paper, right hand side.

[@JurekPrzezdziecki]

@@CharlieMorrisZL2CTM I see, thanks.

[@chrisyln1]

Hi Charlie I am trying to make a tuned rf amp like the one you started at the start of your video using a J310.is the tap on the coil about a third of the total number of the coil and is it from the earth end of the coil

[@CharlieMorrisZL2CTM]

Hi Chrisyln. Yes the tap is a approx. 1/4 up from the earth end. The input impedance (magnitude) of the grounded gate JFET should be around 1/gm.

[@RafalitoFalito]

The main fet circuito is not conectado yo ground

[@user-vj4hq2ij3o]

1:07 gate connected to DC GND ?? ... like the vedio ...

[@CharlieMorrisZL2CTM]

Yes that's correct. The positive voltage at the source makes the gate look negative with respect to it, i.e., a negative Vgs.

[@philip2501]

can you make for next projekt radar

[@CharlieMorrisZL2CTM]

I'll add it to the list Philip. No promises though.