[Lowfer] JFET (J310) Biasing

[John Andrews]

Clive,

Having a pot as John Davis describes is a way to set the bias, and hence 
the drain current. It can be used to minimize IMD, particularly the 2nd 
order variety from the mixing of strong AM broadcast signals.

As John D. points out, a resistive divider lowers the input impedance, 
and the extra resistor to the gate would raise it back up.

Some excellent work on these preamps may be found at Chris Trask's site:
http://home.earthlink.net/~christrask/
The material is in the bottom sections of the page. As he correctly 
points out, the bipolar stages used to drive a 50 ohm line are 
frequently the weak link in IMD performance.

John, W1TAG

On 6/28/2014 1:20 PM, Clive S Carver wrote:
>
> I am curious of the pro's and con's of three different ways of biasing a
> J310 for use as an e-Probe front end stage.
>
> 1. For example, the PA0RDT mini-whip uses two 1Mohm resistors in a potential
> divider across the +12ve to ground supply, with a third 1Mohm resistor
> between the potential divider mid-point and the J310 gate.
>
> 2. Some designs omit the third 1Mohm resistor referred to above, and connect
> the potential divider mid-point directly to the J310 gate.
>
> 3. I have also come across a design where there is a single 1Mohm resistor
> between the +12ve rail and the J310 gate.
>
> So, what are the advantages/disadvantages in the three methods (besides
> using less 1Mohm resistors!).
>
> 73's
>
> Clive
> GW4EYO
>
>
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