Because they dont need high voltage but high current, wich could by useful in some scenario's.
amplification
They use transistor as the purpose to transfer the voltage
You can use a PNP or an NPN transistor in a fire alarm circuit. Both will work. It all depends on how you want to energize the alarm, and with what polarity of signal.
Biasing is necessary in a transistor circuit to keep the transistor working. Without proper biasing, the circuit will fail
a transistor circuit for driving the coil of a magnetic relay.
Transistor are DC output, Triac are AC output.
A transistor does not act as an amplifier. It is used as a component in an amplifier circuit.
This mechanism is how a transistor works.
Including a series capacitor in the input and/or output circuit of the transistor. If the capacitor in the output circuit is omitted there will be a dc component in the output.
Use the astable to switch a power transistor and connect the buzzer as the transistor load.
A switch, switching transistor, or circuit breaker usually does.
Selection and setting of a transistor's operating pointto meet circuit functional requirements.