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Discrete opamp bjt front end

E

Eeyore

Jan 1, 1970
0
Jan said:
Is the current not also limited by beta x the current drive?

The drive current (bottom transistor) is 0.7 / 110 = 0.6mA.

How did you get that ?

Looks more like 7mA to me !

The beta of the MJE-171 is perhaps about 50, so the I max is 30mA.
At half supply 24V makes .03 x 24 = 720mW in the pulldown.
Pullup will be about the same.

Revise your calculations.

Graham
 
J

Jim Thompson

Jan 1, 1970
0
Nope. Bad shot, Graham.
These diodes are there to pull up the base in case of inductive kickback
current. This is better than putting them to the rails, because then the
transistor breaks down with negative Vbe.
If they were for current limiting, they had to be turned around and
doubled(or a red LED) to allow also for the Vbe of the transistor.

For a change, Graham is correct, and you, Ban, are wrong ;-)

...Jim Thompson
 
E

Eeyore

Jan 1, 1970
0
Jim said:
For a change, Graham is correct, and you, Ban, are wrong ;-)

...Jim Thompson

It also means you're wrong Jim ! See your earlier post.

Graham
 
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