$A$ thermoelectric refrigerator works on

The energy of a photon of sodium light $(\lambda = 589 \text{ nm})$ is equal to the band gap of a semiconductor. Find the minimum energy $E$ in $eV$ required to create an electron-hole pair.

An oscillator is which of the following types of amplifier?

Which of the following is used to produce radio waves of constant amplitude?

An amplifier has a voltage gain $A_v = 1000$. The voltage gain in $dB$ is...........

The voltage gains of two amplifiers connected in series are $8$ and $12.5$. If the voltage of the input signal is $200 \mu V$, then the voltage of the output signal is