Re: Tek 576 noise and looping

peter bunge

The post by Dennis talking about Miller capacity prompted me to run a test
with slow transistors and RF transistors which showed no looping with a 900
MHz transistor. You won't see loping at higher currents (10 mA) with either.
The base current is constant for each curve. The collector voltage is a
full wave rectified sine wave and rises to a maximum then returns along the
same path to 0 V. The Collector current should shoot up to a set level and
return along the same path but it does not at very low currents. I think
the miller capacity feeds the decreasing voltage as a bucking charge into
the base turning off the transistor and reducing the current creating the
loop. A fast transistor has lower capacity and does not do this. Try it
with two transistors and see.
The curve is collector current in the vertical plotted against collector
voltage on the horizontal. Ideally the line should jump from zero
instantaneously to a current and stay there as a horizontal line as long as
there is any collector voltage. But nothing is ideal.
I don't believe either one of my Curve Tracers has a fault, I just want to
complete the adjustments which are stuck at the noise test.

On Fri, Nov 15, 2019 at 4:15 PM Brad Thompson <brad.thompsonaa1ip@...>

peter bunge wrote on 11/15/2019 3:27 PM:

No, I have not been working on the 576 curve tracer for 8 years but i
been curious about the looping for at least that long.
Thanks for your explanation which I tested by comparing curves with slow
and fast transistors. I had some trouble trying to find a couple with
matching gain but the high freq (900 MHz) transistor performed perfectly
with nice curves at low current. On the other hand the slow transistor
huge loops. I would like to concentrate on the noise problem next. It mat
be finger trouble.
<good info snipped>


This is a long shot, but I wonder whether there's a thermal issue
involved? If the
device's die is poorly attached to its header or if there's a poor
thermal connection between
internal lead wires and the die attachment(s) that might cause an offset
between the
increasing and decreasing base current.

If it's a leakage effect, I'd expect that germanium transistors
(remember those<g>)
would show fat loops.


Brad AA1IP

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