I have built a high side switch using bjt: the circuit is powered with 18V and the base of the NPN transistor is driven by a pulse between 0 and 5V I would like to use a 50mA load. The circuit is in the image attached to the post.
The load is powered only if I remove resistor R1
Here are the calculation I have done:
Ib(Q3) = 5mA as Ic/Ib=10 in saturation
Hypotesys: Ic(Q4)=4mA
Spurious signals will activate the NPN transistor if the circuit is built on a breadboard because all the long connecting wires and many long strips of intermittent contacts are antennas.