Difference between revisions of "Lab 14 RS"

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And <math>R_2</math> we can find from Kirchhoff voltage Low:
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And <math>R_2</math> we can find from Kirchhoff Voltage Low:
  
 
  <math>V_{CC} = I_2 \cdot R_2 + V_B</math>.
 
  <math>V_{CC} = I_2 \cdot R_2 + V_B</math>.
  
And
+
and Kirchhoff Current Low:
  
 
  <math>I_2 = I_1 + I_B</math>
 
  <math>I_2 = I_1 + I_B</math>

Revision as of 20:58, 21 March 2011

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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 RB220Ω and IC100μA
b. IC>0.5 mA DC with no input signal
c. VCEVCC/2>2 V
d. VCC<VCE(max) to prevent burnout
e. VBE0.6V
f. ID10IB<1 mA


Let's VCC=11 V, RE=0.2 kΩ and RC=2.0 kΩ.

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 IC -vs- IEC from the previous lab 13. Record the value of hFE or β.

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 β170 based on my previous lab report #13

Set a DC operating point IC 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 VBE and β 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 VB=1.18 V.

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 IR1 IB

Let's IR1=590 uA IB=14.4 uA

So

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


And R2 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 Vcc and I1 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 voltage are:

We have from operating point:

[math]V_{EC} = 10\ V[/math] 
[math]I_C \approx I_E = 5\ mA[/math]

And because we have silicon transistor:

[math]V_{BE} = 0.6\ V [/math]

Now

[math]V_E = I_E \cdot R_E = 5\ mA \cdot 200\ \Omega = 1\ V[/math]
[math]V_C = V_E + V_{EC} = (1 + 11)\ V = 12\ V[/math]
[math]V_B = V_E + V_{BE} = (1 + 0.6)\ V = 1.6\ V[/math]


My measured DC voltage are:

[math]V_{EC} = (11.25 \pm 0.01)\ V[/math]
[math]V_{BE} = (0.69 \pm 0.01)\ V [/math]
[math]V_E = (0.88 \pm 0.01)\ V[/math]
[math]V_C = (12.11 \pm 0.01)\ V[/math]
[math]V_B = (1.56 \pm 0.01)\ V[/math]

First note that my VB=(1.56±0.01) V is close to predicted values VB=1.6 V.


Also note that my VEC=(11.25±0.01) V is a little higher than my initial operating point VEC=10 V. The main reason is that my actual values of VBE=(0.69±0.01) V instead of VBE=0.6 V as I was assumed initially. That will reduce my VE voltage that reduce my IC current. So I will shift to right my operating point from IC=5 mA to lower values and correspondingly will increase my VEC.


By direct measurements my operating point now is IC=(4.34±0.01) mA and VEC=(11.25±0.01) V

Let's check do my operating point is still on my load line. I have RE=(0.20±0.01) kΩ, RC=(1.80±0.01) kΩ. So from load line equation:

[math]I_C = \frac{V_{CC}-V_{CE}}{R_E+R_C} = \frac{((20.01\pm 0.01)-(11.25\pm 0.01))\ V}{((1.80 \pm 0.01))+ (0.20 \pm 0.01))\ k\Omega} = (4.38 \pm 0.04)\ mA[/math].

So my operating point lies in my load line within experimental error.

Measure the voltage gain Av as a function of frequency and compare to the theoretical value.(10 pnts)

Measure Rin and Rout at about 1 kHz and compare to the theoretical value.(10 pnts)

How do you do this? Add resistor in front of C1 which you vary to determine Rin and then do a similar thing for Rout except the variable reistor goes from C2 to ground.

Measure Av and Rin as a function of frequency with CE 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 npn common emitter amplifier used to amplify negative pulses?(10 pnts)
  3. What will the values of VC, VE , and IC be if the transistor burns out resulting in infinite resistance. Check with measurement.(10 pnts)
  4. What will the values of VC, VE , and IC 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 Rin if ID is increased from 10 IB to 50 IB(10 pnts)
  6. Sketch the AC equivalent circuit of the common emitter amplifier.(10 pnts)




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