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Attached is a level shifter circuit that converts a positive going digital signal into the control of a P-MOSFET that switches the resistors which determine the negative output voltage of the LM337.
Note that if the negative supply voltage is greater than the maximum gate-source voltage for the P-MOSFET (typically 20V) then you will need to add another gate resistor to form a voltage divider that will limit the maximum gate voltage.
Also note that the input has inverted logic. A logic high (+5V) turns the P-MOSFET off and a logic low (0V) turns the P-MOSFET on.
You can replace the MOSFET with a PNP but all the current carried by the base resistor must go through the collector and emitter of the first transistor when you want to turn it off, and thus must be supplied by the input driver. So it depends upon how much current your input driver can provide. Note that the input current is Vin/Rin and that must be equal to or greater than V-/Rb (Rb is base resistor).
It's for that reason that I used a MOSFET since the gate resistor can be large and thus so can the input resistor.
What is the value of the minus voltage you will be connecting to the base resistor?
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