# Ever thought that diodes could still be used in this way?

2023-05-06Archive

Using unidirectional conduction of diodes, you can create fun and practical circuits.

This article will analyze how clipping circuit and clipping circuit are implemented with diodes.

Limiter Schematic

As shown in figure below, when VIN is greater than or equal to 0.7V in positive half cycle, diode operates in forward direction. At this point, VOUT will be fixed at 0.7V.

When VIN is less than 0.7V, diode is in a cutoff state. In negative half cycle, current is reversed and diode is also in cutoff state. At this time, VOUT= VIN, and waveform of VOUT follows VIN.

Scheme of radiation limiting circuit

According to principle of radiation limiting circuit described above, following two-way radiation limiting circuit can be designed.

Schematic diagram of a two-way radiation limiting circuit

However, sometimes 0.7V voltage is not adequate, so how can another clipping voltage be generated? A bias voltage VBIAS is added to circuit and diode can only be turned on when VIN is greater than or equal to VBIAS. At this time, VOUT is fixed and its value is 0.7V + VBIAS, as shown in figure below.

Displacement limiter circuit diagram

Clamp Outline

The following is a clipping circuit implemented by a diode in combination with a capacitor. The conduction voltage drop of diode is ignored in analysis, assuming that time constant RC is large enough that output waveform is not distorted.

(1) Closing principle

When input value Vin is negative in negative half cycle, current is indicated by red arrow in figure below. The diode turns on and capacitor gradually charges up to V, during which Vout=0.

When Vin input is positive in positive half cycle, current is indicated by blue arrow. The diode turns off and Vout equals voltage across capacitor plus positive half-cycle voltage V, and at this time Vout=2V.

Clamping chain principle

(2) Bias Limiting Circuit

Similar to limiter circuit, in order to obtain required clamping value, a bias voltage must be added to circuit, as shown in figure below.

Displacement limiting scheme

When applied bias voltage matches conduction direction of diode, clamp value will be increased by V1, Vout=2V+V1.

（3）Application example of bi-directional diode clamp

Some circuits use clamping effect of two diodes for protection, as shown in figure below, assuming that 0.7V corresponds to conduction voltage of D1 and D2.

Vin is greater than or equal to Vmax, D1 is enabled, and Vout will be fixed at Vmax.

When Vin is less than or equal to Vmin, Vout is fixed at Vmin.

Diode Clamp Protection Circuit