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Re: gEDA-user: Haunted BF982
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?
Vds max is 12 V on that device, I might be a bit nervous with a supply
that hits that limit.
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.
-Dan
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