Diod across emitter and base?

[u666sa]

Transistors are R25, and you are looking at VHF and UHF receiver tract. In stock form there are no diodes. But, someone just added them as part of a sensitivity mod. What is going on? What these diodes do?

[madires]

BJTs have usually a base emitter reverse breakdown voltage around 5 V. The diode limits the reverse voltage to the diode's Vf drop and protects the BJT.

[TimFox]

A diode from base to emitter is a common prophylaxis against reversing the base-emitter diode, for example during power-up.
Even if the collector-base breakdown voltage exceeds 50 V, the base-emitter diode breakdown is usually only 6 V or so, and if that diode breaks down it reduces performance of the surviving transistor.
Supposedly, one can "anneal" that damage by running large forward current through the base-emitter diode, but I have never tried that.

[u666sa]

How does it relate to sensitivity?

I see an improvement in replacing stock R25 transistors to BFU550AR, or BGA427's, and it's been done on same tract on different radio. But diodes, would that improve sensitivity? Stock sensitivity is around -116 dBm, and we want -130 dBm and better.

[TimFox]

If anything, the parasitic conductance and capacitance of the added protection diode would make the sensitivity worse.
Noise-critical applications sometimes have a switch to remove the diode after the operation as settled to avoid the effect.