TF EIMLab14 Writeup

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The Common Emitter Amplifier


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

TF EIM Lab14a.png

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

a. Try [math]R_E \approx 220 \Omega[/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

[math]V_{CE}(max)[/math] = 40 Volts

so [math]V_{CC} \lt 40[/math]

To set this collector current you ned to choose [math]V_{CC}[/math] and [math]R_C[/math] to give 0.5 mA and be in the middle of the load line.

From previous lab we should try [math]V_{CE} = 1 Volt[/math]

Loop theorem \Rightarrow

[math]V_{CC} - I_CR_C - V_{CE} - I_ER_E = 0[/math]
[math]V_{CE} = V_{CC}/2[/math]
[math]\Rightarrow V_{CC}/2 = I_C R_C + I_E R_E[/math]
Since [math]I_C \approx I_E[/math]
[math]V_{CC} = 2I_C (R_C+R_E)[/math]

Unknowns are[math] V_{CC}[/math] and [math]R_C[/math]

[math]R_C = \frac{V_{CC}}{2 I_C} - R_E = \frac{V_{CC}}{2 0.5 mA} - 200 \Omega[/math]

Let V_{CC} = 10 Volts

[math]R_C = 100 k\Omega - 200 \Omega \approx 100 k \Omega[/math]

e. [math]V_{BE} \approx 0.6 V[/math]
f. [math]I_D \approx 10 I_B \lt 1[/math] mA

[math]R_E = 200.0 \Omega[/math]

[math]R_C = 2k\Omega[/math]

[math]R_1 = 2k\Omega[/math]

[math]R_2 = 15 k\Omega[/math]

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)


  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)