Lab 14 RS

From New IAC Wiki
Jump to navigation Jump to search

Go Back to All Lab Reports


The Common Emitter Amplifier

Circuit

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

TF EIM Lab14a.png

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} = 11\ V[/math], [math]R_E = 0.2\ k\Omega[/math] and [math]R_C = 2.0\ k\Omega[/math].

The load line equation becomes:

[math]I_C = \frac{V_{CC}-V_{CE}}{R_E+R_C} = \frac{(11 - V_{CE})\ V}{2.2\ k\Omega} [/math]

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

On the plot below I overlay the output transistor lines (from the previous lab report #13) and the Load Line calculated above.


Load Line 5mA.png


My [math]\beta \approx 170[/math] based on my previous lab report #13

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.

I will set up my operating point in the middle of the load line:

[math]I_C = 2.5\ mA[/math], [math]V_{EC} = 5.5\ V[/math].


Let's calculate all bias voltage needed to set up this operating point. Because the knowing of [math]V_{BE}[/math] and [math]\beta[/math] is very important for this calculation I did the preliminary set up to measure this quantities. They are the only parameters which depends from transistor. I was able to find:

[math]V_{BE} = 0.68\ V[/math]
[math]\beta = 173[/math]

Now

[math]V_E = I_E \cdot R_E = 2.5\ mA \cdot 0.2\ k\Omega = 0.5\ V[/math]
[math]V_B = V_E + V_B = (0.50 + 0.68)\ V = 1.18\ V[/math]


To set up the operating point above we need to set up [math]V_{B} = 1.18\ V[/math].

We have:

[math]I_B = \frac{I_C}{\beta} = \frac{2.5\ mA}{173} = 14.4\ uA[/math].


To get operating point independent of the transistor base current we want [math]I_{R1} \gg\ I_B[/math]

Let's [math]I_{R1} = 590\ uA \gg\ I_B = 14.4\ uA[/math]

So

[math]R_1 = \frac{V_B}{I_1} = \frac{1.18\ V}{590\ uA} = 2\ k\Omega[/math]


And [math]R_2[/math] we can find from Kirchhoff Voltage Low:

[math]V_{CC} = I_2 \cdot R_2 + V_B[/math].

and Kirchhoff Current Low:

[math]I_2 = I_1 + I_B[/math]

So

 [math]R_2 = \frac{V_{CC}-V_B}{I_1+I_B} = \frac{(11-1.18)\ V}{(590 + 14.4)\ mA} = 16.25\ k\Omega[/math]


I tried to adjust my calculation by varying the fee parameters [math]V_{cc}[/math] and [math]I_1[/math] to get good values for resistors I can easily to set up.


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

My predicted DC voltages are: (from the calculation above):

[math]V_{EC} = 5.50\ V[/math] 
[math]V_{BE} = 0.68\ V [/math]
[math]V_E = 0.50\ V[/math]
[math]V_B = 1.18\ V[/math]
[math]V_C = V_E + V_{EC} = (0.50 + 5.50)\ V = 6.00\ V[/math]


My measured DC voltages are:

Here is very important to set up all resistor values as close as possible to my assumed values.

After many tries and errors I was able to end up with the following values of my resistors:

[math]R_E = (200.0 \pm 0.1)\ \Omega[/math]
[math]R_C = (2.002 \pm 0.001)\ k\Omega[/math]
[math]R_1 = (2.004 \pm 0.001)\ k\Omega[/math]
[math]R_2 = (16.26 \pm 0.01)\ k\Omega[/math]

And my measurements of DC voltages looks like:

[math]V_{cc} = (11.00 \pm 0.01)\ V[/math]
[math]V_E = (0.500 \pm 0.001)\ V[/math]
[math]V_B = (1.183 \pm 0.001)\ V [/math]
[math]V_C = (6.03 \pm 0.01)\ V [/math]
[math]V_{BE} = (0.683 \pm 0.001)\ V[/math]
[math]V_{EC} = (5.53 \pm 0.01)\ V[/math]
[math]V_{R_2} = (9.82 \pm 0.01)\ V[/math]
[math]V_{R_C} = (4.97 \pm 0.01)\ V[/math]


All my measurements are in agreement with each other within experimental errors.

I mean here that [math]V_B = V_E+V_{BE}[/math], [math]V_C = V_E+V_{EC}[/math] and [math]V_{cc} = V_B+V_{R_2}[/math], [math]V_{cc} = V_C+V_{R_C}[/math]


Also all my predicted values are in agreement with my measured DC voltage values except of the fact that my measured [math]V_{BE} = 0.683\ V[/math] instead of [math]V_{BE} = 0.680\ V[/math] as I initially assumed. That gives me the correspond corrections to [math]V_B[/math]. But if I will consider only the one significant sign all my predicted and measured DC voltage values are in total agreement.


Below are my current measurements which I did using millivoltmeter and which are also in agreement with each other and with all my previous calculation:

[math]I_E = (2.48 \pm 0.01)\ mA[/math] 
[math]I_C = (2.49 \pm 0.01)\ mA[/math] 
[math]I_B = (14.4 \pm 0.1)\ uA[/math] 
[math]I_1 = (588 \pm 1)\ uA[/math] 
[math]I_1 = (603 \pm 1)\ uA[/math]


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

Table gain.png

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.

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)




Go Back to All Lab Reports Forest_Electronic_Instrumentation_and_Measurement