Re: gEDA-user: Haunted BF982

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Mon, Oct 30, 2006 at 08:15:51AM -0500, Dan McMahill wrote:
> Karel Kulhavy wrote:
> >I was wondering why something is limiting the bandwidth of my 1MHz square
> >signal from the bottom so the square wave had higher beginning and lower
> >end (significantly).
> 
> Do you have a picture of the waveform you can post?
> 
> >I figured out it is caused by the BF982 transistor. The signal at the
> >input gate is fine, on the output (which is drain directly into a 220 ohm
> >load), it is already deformed. The source is nailed to the ground. The
> >transistor has tons of headroom and runs from 12V. It happens even on
> >microscopically small signals. The operating point of G1 is in the middle
> >of it's almost-linear space, the G2 is at full amplification point at
> >4V, amply blocked to the ground with total of 100nF.
> 
> Have you probed G2 just to be sure it isn't moving around?

Yes, it's flat.

> 
> Vds max is 12 V on that device, I might be a bit nervous with a supply 
> that hits that limit.

It actually isn't 12V, it's 11.9V (after RC filtration) and then the 220
ohm resistors eats something more.

> 
> >The transistor was disconnected from the rest of the amplifier, isolated,
> >nothing was connected to the 200 Ohm workload.
> >
> >Is it possible that as the transistor is optimized for 300MHz or 800MHz
> >operation, they actually managed to make it start amplifying a bit less
> >from 150kHz downwards?
> 
> Not that I know of but I have very little experience in actually using 
> dual gate FET's.
> 
> >As I couldn't find any physical cause that I could control, I implemented
> >a compensation for the deformation in the next stage and now the wave
> >is nicely level. The compensation turns out to be perfect when calculated
> >for 150kHz simple RC high-pass.
> >
> >I think I saw this effect in another receiver populated by BF988, too.
> 
> By any chance do things, like the 150 kHz frequency, change if you stick 
> a heatsink on that transistor?  For the purposes of this test, it 
> doesn't need to be attached very well mechanically as long as its 
> connected thermally.  I'm thinking of just a drop of thermal grease with 
> a large enough piece of aluminum stuck on top.  "Large enough" would be 
> something that appreciably changes the thermal system.

I can't - the transistor is in a hole in a metal shielding partition, two
legs (G1,G2) on one side and the other two on the other (D, S).

CL<


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