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< _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
