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Small Signal Amplifier

Load the PSPICE file bjt-amp1.sch, Figure 37. Your task is to design values of R_B1, R_B2 and R_E for good gain and input level.

  

Figure 37: Small signal amplifier.

Exercise:

1.

Using the relations

$$V_{BB} = \frac{V_{CC} R_{B2}}{R_{B1}+ R_{B2}}, \ \ \ R_B = \frac{ R_{B1} R_{B2}}{R_{B1}+ R_{B2}},$$

select (standard) values of R_B1 and R_B2 so that $V_{BB} \approx 2.0$ V, $R_B \approx 15$ k$\Omega$ (as in the previous bias section). Set these values in PSPICE.

2.

Now choose R_E so $I_C \approx 1.2$ mA (for maximum possible AC swing). Set in PSPICE.

You may use your curves from the DC bias exercise above, section 7.4, to find R_E.

3.

Simulate and check that your DC bias point is correct. Record your values.

Draw the load line on the characteristic graph (I_C vs V_CE) for your value of R_E. Plot the DC operating point Q on this load line.

Check for consistency with the DC bias exercise above, section 7.4.

4.

Enable transient analysis and simulate. Observe the input and output waveforms. What is the voltage gain from the source voltage v_s to the output load voltage v_out (A_s,out)? Compare with theory.

Lecture Notes : BJT Transistor Circuits : Small Signal Amplifier : AC Analysis

To measure the gain, measure the peak-to-peak values of v_s and v_out and calculate the ratio v_out/v_s.

5.

Repeat the transient simulation for increasing magnitudes of the source voltage v_s, and determine the maximum magnitude of the input signal before the output waveform is clipped. Note any distortion.

6.

Set the magnitude of v_s back to its original value, and now vary the source R_S (increase it) and load R_L (decrease it) resistances (individually). Note the influence on gain. Discuss.

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