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discrete opamp current sources?

J

John Larkin

Jan 1, 1970
0
Found this discrete audio type opamp from 1977

http://es.geocities.com/mart_in_medina/918opamp.jpg

Question
The designer used two sets of diodes for setting the bias for Q3, Q4

I would normally just use 2 diodes common to Q3, Q4

What's the advantage in using 2 sets of diodes?


martin

There's probably a latchup mode. If they shared and the output rails
high, Q4 could saturate and steal all the bias from Q3, turning off
Q6-Q7, and it's dead forever.

Old original LM324s suffer from shared bias supply problems, but at
least they recover.

John
 
R

Robert Baer

Jan 1, 1970
0
martin said:
Found this discrete audio type opamp from 1977

http://es.geocities.com/mart_in_medina/918opamp.jpg

Question
The designer used two sets of diodes for setting the bias for Q3, Q4

I would normally just use 2 diodes common to Q3, Q4

What's the advantage in using 2 sets of diodes?


martin
You forgot Q5...
Better to use matched transistor pairs in same package (for better
thermal coupling), and use a resistor in place of that "extra" diode
where the resistor matches (in value) that used for Q3, Q4 and Q5.
 
M

martin griffith

Jan 1, 1970
0
There's probably a latchup mode. If they shared and the output rails
high, Q4 could saturate and steal all the bias from Q3, turning off
Q6-Q7, and it's dead forever.

Old original LM324s suffer from shared bias supply problems, but at
least they recover.

John

Thanks, I'd hadn't thought about fault conditions, maybe I'll
LTspiceit, and see if that happens, but I still dont trust (LT)spice
for wierd stuff, but thats probably just my ignorance


martin
 
H

Helmut Sennewald

Jan 1, 1970
0
martin griffith said:
Found this discrete audio type opamp from 1977

http://es.geocities.com/mart_in_medina/918opamp.jpg

Question
The designer used two sets of diodes for setting the bias for Q3, Q4

I would normally just use 2 diodes common to Q3, Q4

What's the advantage in using 2 sets of diodes?


martin

Hello Martin,

If the opamp is driven near saturation (Q6 off), a lot of
current will flow into the base of Q4 and thus lowering the
voltage across the two diodes. This would be a large
unwanted change in the current for the differential input stage.
The base current of Q4 will also change in the normal
operation with every change of the output voltage.
This will modulate the bias current source of the input
pair too. Think on additional distortion here.

Over all two good reasons for seperate bias voltages.

Best regards,
Helmut
 
J

John Larkin

Jan 1, 1970
0
Thanks, I'd hadn't thought about fault conditions, maybe I'll
LTspiceit, and see if that happens, but I still dont trust (LT)spice
for wierd stuff, but thats probably just my ignorance


martin

The three 27K resistors could be reduced to two.

The input diodes prevent another latch mode, zenering an input b-e
junction. It may have some other hazards if connected as a follower.

John
 
T

Tony Williams

Jan 1, 1970
0
Found this discrete audio type opamp from 1977

Question The designer used two sets of diodes for setting the
bias for Q3, Q4
I would normally just use 2 diodes common to Q3, Q4
What's the advantage in using 2 sets of diodes?

Probably to avoid a latchup if the -IN voltage ever
gets high enough to bottom Q3. With joined bases the
Q4 cc-source would then suddenly change current.
 
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