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

$\frac{v_o}{v_i}=A$ . $R_4=A{R}_3$

$v_o=A{v}_i$

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.