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

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

\frac{v_{o}}{v_{i}} = A

.

R_{4} = A R_{3}

v_{o} = A v_{i}

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.