Lab 14 RS

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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]


About R1, R2 and RE, C1, C2 and CE see my calculations below.

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 reported values of β in lab report #13 was β150. But this because we have used the approximation VE0 because small resistor value RE=100 Ω was used.

If we will do the accurate calculation of β based on my lab report #13 measurements using exact formula:

[math]I_B = \frac{(V_{bb}-V_{BE}-V_E)}{R_B}[/math]

we will end up with value of β:

 [math]\beta \approx 170[/math]

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_{BE} = (0.50 + 0.68)\ V = 1.18\ V[/math]


To set up the above operating point 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 ID=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 end up with all my resistor values I can easily set up.


Now when we know all resistor values we are ready to calculate capacitor values.

1) To prevent decreasing in gain we need to make capacitance CE large enough so the XC will become small enough compared to RE to make AC signal grounded. Also note that the DC bias is not affected because CE will not pass the direct current.

[math]X_C = \frac{1}{\omega C_E} \ll R_E[/math]

So

[math]C_E \gg \frac{1}{\omega_{min}R_E} = \frac{1}{(2\pi)(20\ Hz)(200\ \Omega)} = 40 \mu F[/math]

I choose the electrolytic capacitor:

[math]C_E = 100\ \mu F[/math].


2) Now we need to block the amplifier from input and output signal to avoid change the DC bias. We can do it by placing the "blocking" capacitor, in series in-front and in-back of the amplifier. They both form the high-pass filter. And if we want to amplify the frequency more than 20 Hz we need to choose the breakpoint frequency to be:

[math]\omega_B = \frac{1}{RC} \ll (2\pi\ 20)\ Hz[/math] 


For the output high-pass filter from the ac equivalent circuit we have R=RL where the resistance RL is the resistance of the output device. Let's RL=10 kΩ

So

[math]C_2 \gg \frac{1}{(2\pi\ 20)\ R_L} = \frac{1}{(2\pi\ 20)\ 10\ k\Omega} = 0.8\times10^{-6} F[/math]


For the input high-pass filter from the ac equivalent circuit we have

[math]\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_{BE}} =  \frac{1}{2\ k\Omega} + \frac{1}{16.25\ k\Omega} + \frac{1}{2.6\ k\Omega} = 0.94\ k\Omega [/math]

So

[math]R = 1.06\ k\Omega[/math]

And

[math]C_2 \gg \frac{1}{(2\pi\ 20)\ R_L} = \frac{1}{(2\pi\ 20)\ 1.06\ k\Omega} = 7.5\times10^{-6} F[/math]


To be safe I have choose the following values for C1 and C2

[math]C_1 = C_2 = 100\ \mu F[/math]

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 VB=VE+VBE, VC=VE+VEC and Vcc=VB+VR2, Vcc=VC+VRC


Also all my predicted values are in agreement with my measured DC voltage values except of the fact that my measured VBE=0.683 V instead of VBE=0.680 V as I initially assumed. That gives me the correspondent corrections to VB. 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 Aν as a function of frequency and compare to the theoretical value.(10 pnts)

In the table below are my input and output voltage measurements and voltage gain calculation. Here the Vin is the peak-to-peak value of the input voltage, the Vout is the peak-to-peak value of the output voltage, and Aν=VoutVin is the voltage gaine.

Table gain.png


My theoretical voltage gain are:

[math]A_{\nu} = \frac{\beta R_C}{R_{BE}}[/math].

Here I do not really know the values of RBE. But typically RC and RBE are the same order of magnitude so the approximate voltage gain are:

[math]A_{\nu} \approx \beta = 173[/math].


As we can see from the table above my theoretical voltage gain are in good agreement with the measured ones in wide range of frequencies from the low frequency f=300 Hz up to high frequency f=200 kHz


The decreasing of measured gain in low frequencies can be explained from the fact that capacitors C1 and C2 actually form the high-pass filter for input and output AC signals. And because I have measured Vin before C1 and Vout after C2 that will drop down my measured gain at low frequencies.

Curcuit06.png


The decreasing of measured gain at high frequency can be explain from the fact that the transistor have some nonzero capacitance value CBE. So the transistor by itself will behave as low-pass filter. Because this CBE capacitance should be small enough the breaking point frequency ωB=1RC should be high enough. As the result we have the attenuation of measured voltage gain at high frequencies.

Curcuit062.png

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.


Input impedance Rin measurements and comparing with theory

To measure the input impedance of my amplifier I have set up the circulant below. Here I have attached the resistor RL in-front of capacitor C1 and I replaced all my input internal resistor of amplifier by equivalent resistor Rinp. Here I have draw the capacitor C1 only for clearness and it does not included in calculation below. My input signal frequency is 1 kHz


Inputresistor.png


Using Ohm's Law:

[math]I = \frac{V_A-V_B}{R_L}[/math]

and

[math]R_{inp} = \frac{V_B}{I} = R_L\ \frac{V_B}{V_A-V_B}[/math]


Below is the table with my measurements and Rinp calculation. I did that calculation for several values of RL. Here VA and VB are the RMS values.

Rinp table 03.png


The minimum error for Rinp I have for the case RL=4030 Ω.

So my best estimate for Rinp is:

[math]R_{inp} = (1727.1 \pm 35.7)\ \Omega[/math]



The theoretical values of input impedance can be described using the so called h parameter as:

[math]R_{in} \approx (h_{ie}||R_1||R_2)[/math]

where

[math] h_{ie} \cong \frac{\beta kT}{e I_C}[/math]

or using β173, T=300 K and IC=2.5 mA


[math] h_{ie} = 1.79\ k\Omega[/math]


So,

[math](R_{in})_{theory} \approx (1.79\ k\Omega||2.00\ k\Omega||16.26\ k\Omega) \approx 0.89\ k\Omega[/math]



This result is in not really well agreement with my measured values of

[math](R_{inp})_{exp} = (1727.1 \pm 35.7)\ \Omega[/math]

The discrepancy probably can be explained by approximate nature of the theoretical formula and maybe assumed values of temperature T=300 K which I do not really measured. Maybe my method of measuring input impedance was not good. Anyway this big discrepancy need more detailed study.

Output impedance Rout measurements and comparing with theory

To measure the output impedance of my amplifier I have set up the circulant below. Here I have attached the resistor RL after of capacitor C2 and I replaced all my output internal resistor of amplifier by equivalent resistor Rout. Here I have draw the capacitor C2 only for clearness and it does not included in calculation below.


Outresistor.png


Here to find out the output impedance we can use the battery method from lab#1. By Kirchoff voltage law:

[math]V_B=V_A - I\cdot R_{out}[/math]

and

[math]I = \frac{V_B}{R_L}[/math]


So by graphing the current on the x-axis and the measured voltage VB on the y-axis for several values of the resistance RL we can find the output internal impedance of our amplifier as the slope of the line VB=VAIRout


Below is the table with my measurements and current calculation. Here VB is the RMS values.

Rout table.png


And below is my plot of output voltage VB as function of current I:


L14 outimpedance.png


The line equation is VB[mV]=p0[mV]+p1I[μA] The slope of this line is p0=(1.971±0.004) mVμA=(1.971±0.004) kΩ.


So my measured output impedance is:

[math] R_{out} = (1.971\pm 0.004)\ k\Omega[/math]



The theoretical values of output impedance can be described as:

[math]R_{out} \approx \left(\frac{h_{ie}}{\Delta_e h}\right)||R_C[/math]

I do not really know here the Δeh but assuming the values of Δeh2102 and using hie=1.79 kΩ calculated above

[math]R_{out} \approx (89.5\ k\Omega)||R_C[/math]

Using the actual values of RC=2 kΩ we get finally for the theoretical values of output impedance:

[math]\left(R_{out}\right)_{theory} \approx (89.5\ k\Omega||2.0\ k\Omega) = 1.96 k\Omega[/math]


This theoretical values is in very well agreement with my measured values

[math] \left(R_{out}\right)_{exp} = (1.971\pm 0.004)\ k\Omega[/math]

that tell me that the method I have used to measure the output impedance is pretty good and accurate.

Measure Av and Rin as a function of frequency with CE removed.(10 pnts)

To measure Av and Rin as a function of frequency with CE removed I have set up the following circuit:

Curcuit051.png


Here the box represents the equivalent circuit for my amplifier (with removed CE). Here I have draw the capacitor C1 and C2 only for clearness and it does not included in calculation below.

The input impedance Rin is as before:

[math]R_{inp} =  R_{load}\ \frac{V_b}{V_a - V_b}[/math]

and the voltage gain Aν is as before

[math]A_{\nu} =\frac{V_{out}}{V_{inp}}[/math]



In the table below are presented my measurements and Rin and voltage gain Av calculation. Here VA, VB and VC are the RMS values. I have measured peak-to-peak values by oscilloscope and than convert them to RMS. It's why I have fractional values for voltage errors.

Table rinp03.png


As we can observe from the table above the measured voltage gain for the case with removed CE is much less than the measured voltage gain values for the case with CE. It's in completely agreement with the main function of emitter conductance as CE to avoid decreasing the gain of amplifier.

Questions

Why does a flat load line produce a high voltage gain and a steep load line a high current gain? (10 pnts)

If we have a flat load line then by changing a little the input signal (so we will change the IB and will move up or down in load line) we will change a lot of VCE. As the result we have high voltage gain.


If we have a steep load line then by changing a little the input signal (so we will change the IB and will move up or down in load line) we will change a lot of IC. As the result we have high current gain.

What would be a good operating point an an npn common emitter amplifier used to amplify negative pulses?(10 pnts)

The operating point should be as higher as possible in the load line so we will have the freedom of moving down in the load line to amplify the negative pulses. But it still has to be in saturation region of transistor.

What will the values of VC, VE , and IC be if the transistor burns out resulting in infinite resistance. Check with measurement.(10 pnts)

With burned transistor (removed) I will end up with the DC following circuit:


Curcuit0723.png


As we can easily see because we have the open loop there are no current in collector and emitter and all voltage Vbb will drop between collector and emitter. As the result:

[math]V_C = V_{bb} = 11\ V[/math]
[math]V_E = V_C - V_{BE} = (11 - 11)\ V[/math]
[math]I_C = 0\ A[/math]


My measured values are:

[math]V_C = (11.00 \pm 0.01)\ V[/math]
[math]V_E = (0.0 \pm 0.0)\ V[/math]
[math]I_C = (0.0 \pm 0.0)\ A[/math]

that is in total agreement with my predicted values

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)

With burned transistor (with zero resistance) I will end up with the DC following circuit:


Curcuit0733.png


Now we have the perfect voltage divider:


[math]V_C = V_E = \frac{(R_E||R_1)}{(R_E||R_1) + (R_C||R_2)}\cdot V_{bb}[/math]

where

[math]R_E||R_1 = \frac{R_E R_1}{R_E + R_1} [/math]
[math]R_C||R_2 = \frac{R_C R_2}{R_C + R_2} [/math]

and using my actual values of resistors

[math]R_E = (200.0 \pm 1.0)\ \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]

I end up with

[math]R_E||R_1 = (0.1818 \pm 0.0009)\ k\Omega  [/math]
[math]R_C||R_2 = (1.7825 \pm 0.0002)\ k\Omega [/math]
[math]V_C = V_E = \frac{(0.1818 \pm 0.0009)}{(0.1818 \pm 0.0009) + (1.7825 \pm 0.0002)}\cdot (11.00 \pm 0.01)\ V  = (1.018 \pm 0.005)\ V [/math]
[math]I_C = \frac{V_{bb}-V_E}{R_C} = \frac{(11.00\pm 0.01)V-(1.018 \pm 0.005)V}{(2.002 \pm 0.001)k\Omega} = (4.986 \pm 0.009)\ mA[/math]


My measured values are:

[math]V_C = V_E = (1.021 \pm 0.001)\ V[/math]
[math]I_C = (5.00 \pm 0.01)\ mA[/math]

that is in total agreement with my predicted values

Predict the change in the value of Rin if ID is increased from 10 IB to 50 IB(10 pnts)

Taking my biased base current value calculated above IB=14.4 uA my corresponded ID are:

[math](I_D)_1 = 10 \cdot 14.4\ uA= 144\ uA[/math]
[math](I_D)_2 = 50 \cdot 14.4\ uA = 720\ uA[/math]

My change in VB becomes:

[math](V_B)_1 = (I_D)_1 \cdot R_1 = (144\ uA \cdot 2\ k\Omega) = 0.288\ V[/math]
[math](V_B)_2 = (I_D)_1 \cdot R_1 = (720\ uA \cdot 2\ k\Omega) = 1.440\ V[/math]

and corresponding change in VE becomes:

[math](V_E)_1 = (V_B)_1 - V_{BE} = (0.288 - 0.68) = -0.392\ V[/math]
[math](V_E)_2 = (V_B)_2 - V_{BE} = (1.440 - 0.68) = 0.76\ V[/math]


Here I ended up with the negative (VB)1 value that is impossible for my case. The point here that my R1 was calculated to be able to conduct current ID=590 uA14.4 uA. So the case of increasing ID from 10 IB to 50 IB is just impossible for my circuit just because I can get so low ID=10 IB values.


To make some reasonable estimate I will pick up the change in ID current from 40 IB to 200 IB, so my relative change is 5 times as before. Now I have:

[math](I_D)_1 = 40 \cdot 14.4\ uA = 576\ uA[/math]
[math](I_D)_2 = 200 \cdot 14.4\ uA = 2880\ uA[/math]

My change in VB becomes:

[math](V_B)_1 = (I_D)_1 \cdot R_1 = (576\ uA \cdot 2\ k\Omega) = 1.152\ V[/math]
[math](V_B)_2 = (I_D)_1 \cdot R_1 = (2880\ uA \cdot 2\ k\Omega) = 5.760\ V[/math]

and corresponding change in VE becomes:

[math](V_E)_1 = (V_B)_1 - V_{BE} = (1.152 - 0.68) = 0.472\ V[/math]
[math](V_E)_2 = (V_B)_2 - V_{BE} = (5.760 - 0.68) = 5.08\ V[/math]

and the IE current change:

[math](I_E)_1 = \frac{(V_E)_1}{R_E} = \frac{0.472\ V}{200\ \Omega} = 2.36\ mA[/math]
[math](I_E)_2 = \frac{(V_E)_1}{R_E} = \frac{5.08\ V}{200\ \Omega} = 25.4\ mA[/math]

Now I can use formula for transistor input impedance:

[math] h_{ie} \cong \frac{\beta kT}{e I_C}[/math]

and because IEIC

[math] (h_{ie})_1 \cong \frac{\beta kT}{e (I_C)_1} = 1.89\ k\Omega[/math]
[math] (h_{ie})_2 \cong \frac{\beta kT}{e (I_C)_2} = 0.18\ k\Omega[/math]

Now I can use formula for the amplifier input impedance:

[math]R_{in} \approx (h_{ie}||R_1||R_2)[/math]

and using corresponding values of resistors I will end up with:

[math](R_{in})_1 \approx (1.89||2.0||16.26)\ k\Omega = 0.92\ k\Omega[/math]
[math](R_{in})_2 \approx (0.18||2.0||16.26)\ k\Omega = 0.16\ k\Omega[/math]

So if I increase my ID current from 40 IB to 200 IB I will decrease my amplifier input impedance from 0.92 kΩ to 0.16 kΩ

Sketch the AC equivalent circuit of the common emitter amplifier.(10 pnts)

The point here is to remove the resistors RE which is shorted out by capacitor CE and remove all capacitors C1, C2 and CE which are have zero resistance for ac circuit. Also because Vbb terminal of power supply is also ac grounded we can combine the R1 and R2 into one resistor (R1||R2) and we can connect the resistor RC to the ground. So the ac equivalent circuit of the common emitter amplifier becomes:


Circuit08.png


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