transistor switch with no ground

the circuit that needs to be switched (I'll call it circuit "A")
It is in between two pins of a larger circuit, so the "negative" pin (following current flow from pos. to neg. ) , is *not* grounded -- it goes back into the circuit.
how can i use transistor in it to switch it on and off.
i cant find any circuit i which transisto as a switch does not use ground

Use DC solid-state-relay, or an optocoupler with a power mosfet ..
Here is an example:
DC Solid state SPST 2A relay

IanP

Sometimes, gate driver is composed of a transformer in order to get a floating node then in this way the transistor (MOSFET) is not grounded.
Ground is not necessary because transistor only needs a proper ddp (let's say 10V) on his gate to act as a switch...