A75 protection diodes and Cascode load resistors

[slogan2112]

1) I see that there are two methods to fix the damage to the A75 front end mosfets if clipped. One is signal diodes, the other is Zeners per the attached PDF. Which is the preferred?

2) What is the purpose of the R25 and R26 (outlined in last figure) on the output of the cascode driver stage? Stability, gain, offset....? I have not seen this on similar designs.

Thanks for your help.

-Scott

[Nelson Pass]

Originally posted by slogan2112
1) I see that there are two methods to fix the damage to the A75 front end mosfets if clipped. One is signal diodes, the other is Zeners per the attached PDF. Which is the preferred?

2) What is the purpose of the R25 and R26 (outlined in last figure) on the output of the cascode driver stage? Stability, gain, offset....? I have not seen this on similar designs.

1) Either works fine. When the amp is clipping heavily, the
gain devices on the opposite side are shut down, and this allows
some transient voltage to bleed through the cascode with the
potential of exceeding the Gate to Source voltage rating of 20V.

This prevents that.

2) As you say - Stability, Gain, Offset.