What is the equivalent circuit of a diode?
The micro-change equivalent circuit is when a part of the circuit is replaced with its equivalent circuit, the voltage and current of the unsubstituted part do not change, that is to say, the part with the same voltage and current is only the circuit other than the equivalent part.

Features of the micro-change equivalent circuit:

① The object of the micro-change equivalent circuit is only the change amount. Therefore, the equivalent circuit of the NPN type tube and the PNP type tube are exactly the same.

② The equivalent circuit of slight change is obtained at the correct Q point. If the Q point is set incorrectly, that is, when the Q point is selected in the saturation zone or cut-off zone, the equivalent circuit is meaningless.

③ The static operating point cannot be calculated by the micro-variable equivalent circuit.

④ The voltage and current in the micro-change equivalent circuit are all expressed by the effective value of the alternating current. The direction of the voltage and current is in accordance with the direction defined by the network, and should not be changed arbitrarily.

Diode equivalent circuit
If there is only direct current on the diode, then the volt-ampere characteristic of this direct current is its common volt-ampere characteristic curve. If a small AC is superimposed on this DC, what is the relationship between voltage and current for this small AC signal? Or, for this AC signal, what kind of circuit is the diode equivalent to? This is its slightly changed equivalent Circuit. Of course, for this AC signal, the diode is equivalent to a resistance, and this resistance is also related to DC

Ideal diode equivalent circuit
In the circuit, if the forward voltage drop of the diode when it is turned on is much smaller than the voltage of the component connected with it, and the reverse current when the diode is turned off is much smaller than the current of the component in parallel with it, then the forward voltage drop and reverse of the tube can be ignored The current idealizes the diode as a switch. When a forward voltage is applied, the diode is turned on, and the forward voltage drop is 0, which is equivalent to closing the switch. When a reverse voltage is applied, the diode is turned off, and the reverse current is 0, which is equivalent to The switch is off, and the equivalent circuit of an ideal diode is shown in Figure 4-4. Using the ideal diode to represent the actual diode to analyze and calculate the circuit can get a more satisfactory result, but there are some errors.


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