Semiconductor Devices

# Signal processing diode applications ### Limiter

A limiter is a diode circuit that is designed to keep voltages at the load in a certain voltage range ${V}_{min}<{V}_{L}<{V}_{max}$, so the maximum dangerous voltage levels for a circuit will not be reached. It is obtained by cutting the initial AC signal to a certain level. Figure below depicts the scheme of the limiter with inverse parallel diodes. Here the diode ${D}_{1}$ cuts a positive peak, and diode ${D}_{2}$ cuts a negative peak of the initial circuit.

### The clamp diode

The purpose of this diode is to clamp the AC signal to the DC value. The principal scheme of the clamp diode is depicted below. As long as the diode is conducting in the forward direction, the capacitor will charge only at the positive half-wave of the AC signal. The capacitor will charge until the value , of the capacitor is shifted to ${V}_{source}\left(t\right)–{V}_{peak}$.

The positive peaks are now shifted, and it is important that the $RC>T$ where $T$ is a period of the signal. Figure below depicts the AC signal in the initial state and is clamped. The clamp diode can also work in the reverse direction, and the capacitor will charge to the voltage -, and the output voltage will be ${v}_{out}\left(t\right)={v}_{source}\left(t\right)–{V}_{peak}$. Negative voltage peaks will be shifted to 0V, as all the graphs will be moved to ${V}_{peak}$. The signal clamp can also be made by using a battery ${V}_{DC}$, where ${V}_{DC}<{V}_{peak}$. This circuit is called the biased diode clamp.

### Photodiodes

Semiconductor diodes can also be useful because of their ability to react on the light energy, when the light photons are reaching the depletion region, they stimulate the hole-electron generation process, which is called photo-ionisation. The top-material is transparent. The reverse saturation current is dependent on the light intensity, $–\left({I}_{photo}+{I}_{0}\right)$ where ${I}_{photo}$ is a current caused by photo-ionisation. The $I–V$characteristics for this diode are depicted below, and it is shifted down to the value of , and also we can see the diode operation points for each diode.