Using the Practical Model

In many cases, it suffices to use the practical model of the diode, Section 7.2.3. The circuit of Figure 77 can be redrawn as follows, Figure 79:

**Figure 79:** Diode biasing circuit showing practical model.
$\begin{figure} \begin{center} \epsfig{file=images/diodeimg14.eps} % \end{center}\end{figure}$

So v=V_Q=0.7 V since the diode is always forward biased in this circuit, and

$\begin{displaymath}i = \frac{V_S-v}{R} = \frac{5-0.7}{1000} = 4.3 \ {\rm mA} \end{displaymath}$

Therefore the practical model gives i=I_Q=4.3 mA, as shown in Figure 80. Of course, it is very easy to determine the operating point from this diagram.

**Figure 80:** Resistor-diode circuit analysis showing practical diode characteristic and load line.
$\begin{figure} \begin{center} \epsfig{file=images/diodeimg15.eps}\end{center}\end{figure}$

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