AD797 guts


Could one of you guys (Scott Wurcer:D) explain the purpose of Q50/Q64 in AD797 simplified schematic, thank you?


  • ad797 sch.gif
    ad797 sch.gif
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As far as I understand the circuit:
-at normal operation, both Q50 and Q64 are kept below threshold Vbe
-Q19 and I3 form the main, final VAS
-collector of Q17 drives the Q18 buffer and this drives Q19 VAS
- at positive overload Q17 collector to I3 upper node voltage gets high, here Q64 starts to conduct
- at negative overload Q50 conducts

I understand it as two anti-parallel diodes at current mirror.