This is 5V to 12V boost converter circuit or higher using transistor circuit,When if you want a negative voltage from a positive power supply.
This is a DC to DC Buck converter working principle. We often use it as a step down DC to DC converter circuit. Because it is a switching mode power supply.
Can we build the switching dc power supply circuit with 2 transistor? This may be a good choice for you.
I would suggest the IC-555 DC boost converter circuits is a key equipment that it is popular and easy to uses so can be used in many circuits.