Switching

Load the PSPICE file bjt-dc1.sch, Figure 33.

**Figure 33:** BJT switching circuit.
$\begin{figure} \begin{center} \epsfig{file=images/clab6img1.eps}\end{center}\end{figure}$

Exercise:

1.

Simulate the circuit for the given values of V_BB and V_CC and note the DC currents and voltages. Is the transistor Q₁ on or off? Why?

Lecture Notes : BJT Transistor Circuits : Introductory BJT Circuits : Transistor as a Switch
On or off refers to extreme operating points corresponding to saturation or cut-off respectively. When on a substantial current flows (order mA), while when off only a small current flows (order few $\mu$ A or less).
Note also the values of V_BE and I_B (add trace).

2.

Enable DC sweep and simulate again (V_BB is varied, and the resulting V_CE is plotted). You should obtain the graph shown in Figure 34.

Also graph I_C versus V_BE.

3.

At what range of values of V_BB and V_BE does switching occur?

4.

Describe the switching behaviour.

**Figure 34:** BJT switching curve (V_C versus V_BB).
$\begin{figure} \begin{center} \epsfig{file=images/clab6img2.eps}\end{center}\end{figure}$

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