What are these resistors for in this PI TX?

[FatBob]

I was looking at some older PI designs and came across this;



It uses a bipolar transistor instead of a FET but has a couple extra resistors.

Does anyone know what they are for? The 1K across the coil might be to protect the transistor.

The 10k to ground at the opamp input just kills the signal it seems...

[Point]

nvm i shold read next time befor i post... my bad

[FatBob]

So I guess the 2 1K's divide the input in half then the 1Meg and 10K give a gain of 100?

Maybe that's the key to a good performing PI?

[Carl-NC]

R17 is the damping resistor for the coil. The gain of the amp is set by R21/R18 = 1000. In normal signal conditions, R19 has no effect because it's connected to virtual ground. But in overvoltage conditions where the diodes turn on, it may help bleed off the diode voltage a little faster down where the diode is almost off and going high-impedance. R20 cancels input bias current effects.

[FatBob]

Ahh I see. Thanks for the info Carl!