Practical Electronics/Bipolar Transistors/NPN Transistor/Transistor circuit

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

Electronics switch

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

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

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

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

Electronics amplifier

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

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

$v_{o}=Av_{i}$

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.