${\displaystyle {\frac {v_{o}}{v_{i}}}=({\frac {R_{2}}{R_{2}+R_{3}}})({\frac {R_{4}}{R_{3}}})}$

Electronics switch

When ${\displaystyle R_{3}=R_{4}}$, Transistor operates as Electronics switch

${\displaystyle {\frac {v_{o}}{v_{i}}}={\frac {R_{2}}{R_{2}+R_{3}}}}$

${\displaystyle {\frac {v_{o}}{v_{i}}}=0}$ . ${\displaystyle R_{2}=0}$

${\displaystyle {\frac {v_{o}}{v_{i}}}=1}$ . ${\displaystyle R_{1}=0}$

Electronics amplifier

When ${\displaystyle R_{1}=0}$, Transistor operates as Electronics amplifier

${\displaystyle {\frac {v_{o}}{v_{i}}}=A}$ . ${\displaystyle R_{4}=AR_{3}}$

${\displaystyle v_{o}=Av_{i}}$

Transistor's Rectifier

A PNP Transistor only allows half of a negative cycle of a voltage or signal wave to pass and grounds any positive cycle of a voltage or signal wave.