Electronic – Replace transistor used as diode


in the schematic on page 5 of this document:

enter image description here

The note says:

T2a is connected as a diode (clipping diode) to protect the power
source against higher Vcc voltages.

I assume from this I can replace T2A with a simple diode, with one side connected to pins 4/5 of T2A and the other connected to TRKR (pointing towards TRKR) – am I correct, or have I overlooked something?

Best Answer

The schematic of the TRKI and TRKR signals is a differential output where TRKI have the same polarity of TXD and TRKR have the inverted polarity of TXD. Obviously the TRKI signal must have an external pull-up resistor and the TRKR signal must have an external pull-down resistor. The purpose of this differential driver is to only drive the lines when TXD is 0, and to release the lines (tri-state) when TXD is 1. This is why T2A "diode" is required: it prevent to drive any current on the TRKR line when TXD is 1.

You can safely exchange T2A with a diode if you wants. The schematic take advantage of a single BC847BV package to make two functions into it.

QUCS simulation schematic QUCS simulation result