Lab 14 RS

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The Common Emitter Amplifier

Circuit

1.)Construct the common emitter amplifier circuit below according to your type of emitter.

2.)Calculate all the R and C values to use in the circuit such that

a. Try [math]R_B \approx 220 \Omega[/math] and [math]I_C \approx 100 \mu A[/math]

b. [math]I_C \gt 0.5[/math] mA DC with no input signal

c. [math]V_{CE} \approx V_{CC}/2 \gt 2[/math] V

d. [math]V_{CC} \lt V_{CE}(max)[/math] to prevent burnout

e. [math]V_{BE} \approx 0.6 V[/math]

f. [math]I_D \approx 10 I_B \lt 1[/math] mA

Let's [math]V_{CC} = 20\ V[/math], [math]R_E = 200 \Omega[/math] and [math]R_C = 1.8\ k\Omega[/math]. The load line equation becomes:

[math]I_C = \frac{V_{CC}}{R_E+R_C} + \frac{V_{CE}}{R_E+R_C} = \frac{20\ V}{2\ k\Omega} + \frac{V_{CE}}{2\ k\Omega} = (10 + 0.5 \cdot V_{CE})\ mA[/math]

This load line pretty high and give me the wide range of amplification the input signal.

3.)Draw a load line using the [math]I_{C}[/math] -vs- [math]I_{CE}[/math] from the previous lab 13. Record the value of [math]h_{FE}[/math] or [math]\beta[/math].

4.)Set a DC operating point [math]I^{\prime}_C[/math] so it will amplify the input pulse given to you. Some of you will have sinusoidal pulses others will have positive or negative only pulses.

5.)Measure all DC voltages in the circuit and compare with the predicted values.(10 pnts)

6.)Measure the voltage gain [math]A_v[/math] as a function of frequency and compare to the theoretical value.(10 pnts)

7.)Measure [math]R_{in}[/math] and [math]R_{out}[/math] at about 1 kHz and compare to the theoretical value.(10 pnts)

How do you do this? Add resistor in front of [math]C_1[/math] which you vary to determine [math]R_{in}[/math] and then do a similar thing for [math]R_{out}[/math] except the variable reistor goes from [math]C_2[/math] to ground.

8.)Measure [math]A_v[/math] and [math]R_{in}[/math] as a function of frequency with [math]C_E[/math] removed.(10 pnts)

Questions

1. Why does a flat load line produce a high voltage gain and a steep load line a high current gain? (10 pnts)
2. What would be a good operating point an an [math]npn[/math] common emitter amplifier used to amplify negative pulses?(10 pnts)
3. What will the values of [math]V_C[/math], [math]V_E[/math] , and [math]I_C[/math] be if the transistor burns out resulting in infinite resistance. Check with measurement.(10 pnts)
4. What will the values of [math]V_C[/math], [math]V_E[/math] , and [math]I_C[/math] be if the transistor burns out resulting in near ZERO resistance (ie short). Check with measurement.(10 pnts)
5. Predict the change in the value of [math]R_{in}[/math] if [math]I_D[/math] is increased from 10 [math]I_B[/math] to 50 [math]I_B[/math](10 pnts)
6. Sketch the AC equivalent circuit of the common emitter amplifier.(10 pnts)

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