As a side remark, because the original post was asking about internal operation of 6N137, do we actually know the details?
I assumed a bipolar process, mister_rf expected a CMOS buffer. Most 6N137 datasheets (the device is available from different manufacturers) are explicitely describing a schottky clamped NPN output transistor, only Vishay says to use NMOS. According to input current and thresholds, the enable input is apparently a TTL-like bipolar logic input. Preamplifier technology isn't said, so we are effectively guessing.