Setting the DC voltage at Q115 so that the amplified signal is AC centered on 0 V.
Q115 and Q114 form a Sziklai pair, essentially becoming a really nice single transistor. The Sziklai pair is used as an emitter follower whose load, Q116,117, is a constant current source. The whole circuit is a buffer (with high-pass filter): no voltage gain but lots of current gain. (AC gets through, but DC is blocked.)
Yes, specifically it's called a bias servo.
Setting the DC voltage at Q115 so that the amplified signal is AC centered on 0 V.
Q115 and Q114 form a Sziklai pair, essentially becoming a really nice single transistor. The Sziklai pair is used as an emitter follower whose load, Q116,117, is a constant current source. The whole circuit is a buffer (with high-pass filter): no voltage gain but lots of current gain. (AC gets through, but DC is blocked.)
http://web.archive.org/web/20230610070306/https://old.reddit.com/r/AskElectronics/comments/73f8d2/microwave_control_panel_keypad_stops_working_when/