Diode circuit analysis method

1. Graphical analysis

Prerequisite: I − V IV of the diode$I-The\; V$ characteristic curve is known.

Disadvantages:

• When multiple diodes are included in the circuit, disassembly becomes complicated.

2. Simplified model analysis method

It can reflect all the characteristics of the normal working of the diode in forward and reverse bias.

(1) Ideal model

Applicable situation: The power supply voltage is much larger than the forward tube voltage drop of the diode.

① $I-V$ characteristic curve:

• When the diode is forward biased, the tube voltage drop is 0, which is equivalent to a short circuit;
• When the diode is reverse biased, the current is zero, which is equivalent to an open circuit.

The dashed line represents the I − V IV of the actual diode$I-V$ characteristics

② Representative symbol:

(2) Constant pressure drop model

Applicable situation: Diode current $i_D$Approximately equal to or greater than 1mA.

① $I-V$ characteristic curve:

• When the diode is turned on,
  • The pressure drop of the silicon tube is constant at $0.7V$；
  • The voltage drop of the germanium tube is constant $0.2persons$ .
• When the diode forward bias voltage <the conduction voltage or an external reverse voltage, the current is 0, which is equivalent to an open circuit. .

② Circuit model:

(3) Polyline model

Applicable situation: Same as polyline model.

① $I-V$ characteristic curve:

• When the diode is turned on, the tube voltage drop has a linear relationship with the current through the diode.

② Circuit model:

battery voltage = threshold voltage $V_{th}$

Germanium tube: 0.5V
silicon tube: 0.7V

(4) Small signal model

Applicable situation: The diode is forward biased, and $v_D>>V_T$

DC working status

Static operating point

Slightly variable resistance $r_d$
$$r_d=\frac{\Delta v_d}{\Delta i_d}$$

In the case of DC operation, a small change state is superimposed, and only the model established when the small signal changes is considered.

$I-V$ characteristics: