How Do We distinguish the DC Resistance and Dynamic Resistance of a Semiconductor Diode?

 

A semiconductor diode is a nonlinear device that exhibits different equivalent resistances for DC and AC (or dynamic quantities). The DC resistance of a diode is the ratio of the terminal voltage to its current when it operates at a point in the volt-ampere characteristic.

DC resistance of a diode

Figure (a) circuit (b) diode volt-ampere characteristics and operating point Q (c) DC resistance of the diode

 

Under the action of the DC power supply V, the point corresponding to the diode current ID and the voltage UD across the diode is called the quiescent operating point, and the DC resistance corresponding to this point is

the quiescent operating point

 

The dynamic resistance is generated by a constant voltage and current (ie, static operating point Q) caused by a change in the voltage and current of the characteristic curve near the Q point by the AC signal ui. If the AC signal ui is low frequency and the amplitude is small (commonly referred to as low frequency small signal), the resulting current variation is also small, and this small characteristic curve can be equivalent by the tangent passing through the Q point.

The dynamic resistance

Figure (a) circuit (b) diode volt-ampere characteristics (c) diode dynamic resistance

 

If a small low-frequency signal is applied on the basis of the Q-point, the amount of voltage change and current change generated at both ends of the diode are as shown in (b), and the diode at this time can be equivalent to a dynamic resistor rd, according to the diode. Current equation available

dynamic resistor rd

Rd is the reciprocal of the tangent slope using the Q point as the tangent point. Obviously, the Q point has a different position on the volt-ampere characteristic, and the value of rd will be different. Current equation based on diode

the reciprocal of the tangent slopeThenthe reciprocal of the tangent slopeThereforethe reciprocal of the tangent slope

 

The ID is quiescent current, and UT=26mV at normal temperature. It can be seen that the larger the quiescent current ID, the smaller rd will be. When UD = 0.7V, ID = 2mA. Thus, the DC resistance RD=350 and the dynamic resistance (AC resistance) are obtained.

the reciprocal of the tangent slope

It can be seen that the two are far apart and must not be confused.

 

According to the above explanation, it should be easy to understand that when measuring the diode with the resistance of the ordinary multimeter, the measured resistance value should indicate the DC resistance of the diode instead of the dynamic resistance.

 

At the same time, when measuring the forward resistance of the diode with the ohmmeter of the ordinary multimeter, the internal current resistance of the meter is different, so that the current flowing through the tube during measurement is different, that is, the position of the working point is different, so the measured RD value is also different.