The most important characteristic of diode is unilateral conductivity, which can be used to design many interesting and practical circuits. This paper mainly talks about limiting circuit and clamping circuit.
▉ Positive limiting circuit
When the voltage of Vin is greater than or equal to 0.7V in the positive cycle, the diode will conduct and Vout will be clamped at 0.7V; In the negative half cycle and when the Vin voltage is less than 0.7V, the diode is off, so Vout=Vin, that is, the Vout waveform follows the Vin waveform.
▉ Negative limiting circuit
In the positive half cycle, the diode turns off, and Vout=Vin, that is, the waveform follows; In the negative half cycle, when the Vin voltage is less than or equal to -0.7V, the diode will conduct and the Vout voltage will be clamped at-0.7V.
▉ Bidirectional limiting circuit
Bi-directional limiting is a combination of the above two circuits, using two diodes. In the positive half cycle, the excess is clamped at 0.7V by D1, and in the negative half cycle, the excess is clamped at -0.7V by D2.
In order to generate different limiting voltages, it is sometimes added to the circuit.Bias voltage Vbias，When the voltage of Vin is greater than or equal to Vbias+0.7V, the diode turns on and Vout is clamped.
The negative bias is the same principle. When the Vin voltage is less than or equal to -0.7-Vbias, the diode turns on and Vout is clamped.
▉ Bidirectional bias limiting
Bi-directional bias limiting is two diodes plus two bias voltages. When the positive half cycle is greater than or equal to 4.7V, D1 is turned on, and the excess part is clamped at 4.7V; When the negative half cycle is less than or equal to -6.7V, D2 turns on, and the excess part is clamped at-6.7V.
The above circuits are all circuits without capacitors, which are mainly used for amplitude limiting.
The following are diode clamping circuits with capacitors, which are analyzed as follows.Regardless of the conduction voltage drop of the diode（That is, the forward conduction of the diode is equivalent to a wire, and the reverse cutoff is an open circuit.），The RC time constant is large enough to ensure that the output waveform is not distorted.
▉ Simple positive clamp circuit
When the input Vin is in the negative half cycle (Vin is negative on the upper side and positive on the lower side), the diode conducts, the current is shown by the red arrow, the capacitor is charged to +V (left negative and right positive), and Vout=0V；
When the input Vin is in the positive half cycle (Vin goes up and down negative), the diode turns off, and the current is shown by the blue arrow. The voltage of Vout is equal to the capacitor voltage plus the positive half cycle voltage, so Vout=2V；
▉ Biased positive clamp circuit
Bias clamping circuit is similar to limiting circuit, in which bias voltage is added to increase or decrease the clamping value.
Figure a is a forward bias voltage. When the applied bias voltage is consistent with the conduction direction of the diode, the waveform is upward, that is, the clamping value will increase by V1.
Figure b is a reverse bias type. When the applied bias voltage is opposite to the conduction direction of the diode,The waveform is downward, that is, the clamping value will decrease.V1。
▉ Simple negative clamp circuit
When the input Vin is in the positive half cycle (Vin is positive and negative), the diode is turned on, and the current is shown by the red arrow. The voltage difference across the capacitor is charged to +V (Zuo Zheng is negative right), and Vout=0V；
When the input Vin is in the negative half cycle (Vin is negative on the top and positive on the bottom), the diode is turned off, the current is shown by the red arrow, and the voltage of Vout is equal toNegative (capacitor voltage+negative half-cycle voltage), that is, vout =-2v;
▉ Biased negative clamping circuit
The biased negative clamp is similar to the biased positive clamp,Add bias voltage to the circuit to increase or decrease the clamping value.
Figure C is a reverse bias type. When the applied bias voltage is opposite to the conduction direction of the diode, the waveform is upward, that is, the clamping value will increase by V1.
Figure D is a forward bias voltage. When the applied bias voltage is in the same direction as the diode conduction, the waveform is downward, that is, the clamping value will decrease by V1.
▉ Common bidirectional diode clamp circuit
In some ADC detection circuits, two diodes are used for clamping protection. The principle is very simple. 0.7V is the conduction voltage drop of D1 and D2. When the incoming voltage of Vin is greater than or equal to Vmax, D1 will conduct and Vout will be clamped at VMAX. When Vin is less than or equal to Vmin, Vout is clamped at Vmin, and the positive electrode of D2 is generally grounded.
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