Re: Yet another use for a curve tracer


Sean Turner
 

Dennis,

That is super cool. Definitely going to have to find one and try that out.

I was happy to find a D1, particularly a D1 that works with the crt in decent shape (a couple of burns from someone leaving a bright dot, but otherwise very crisp and bright)!

Sean

On Wed, Dec 9, 2020 at 01:24 PM, Dennis Tillman W7pF wrote:


Hi Sean,
Lucky you to own a 577D1!

One clever thing you can do with the 577D1 that no other Tek curve tracer can
do is find the perfect bias point for a junction FET. You should bias a FET
right at the point where the temp coefficient is zero but how do you find that
point except by trial and error? FETs are very temperature sensitive little
critters with a very unusual temperature coefficient. It is positive up to a
certain point and then it turns negative. For your circuit's best temperature
stability you need to bias a FET right at the point where the temp coefficient
is zero but how do you find that point except by trial and error?
The 577D1 can find that exact zero temperature coefficient point to bias the
FET in less than 30 seconds.

Here is how to do it: Display a set of typical FET curves on the 577D1 with
storage turned on. Heat the FET up with your finger, or with anything hot for
just a second. As the FET die warms up the gate voltage curves will begin to
move up above a certain bias voltage and they will move down below that
voltage.

The result on the CRT will be that one gate voltage curve will not move so it
will remain sharp. All the other bias voltage curves will smear as they move
away from the perfect point to bias the FET.

You can also use something cold like an ice cube touched to the FET. The
results will be the same except the smears will go in the opposite direction.
But one bias curve right in the middle of the smeared ones will be sharp
because it didn't move at all.

Give it a try. It is fascinating to watch and it only takes a minute to do
this and it is a perfect example of yet another use for a curve tracer. But in
this case it is hard to do without storage so that is another reason why my
favorite curve tracer is the 577D1.

Dennis Tillman W7pF
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