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parkagendaElectronics - Devices

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Lab 15


Common
-
Emitter Amplifier Impedance, Power,
and Phase Relationships


12/15/2008


Richard Vander Brug




Introduction


In this lab we will
learn a lot about the relationship between the common
-
emitter amplifiers
impedance, power, and phase relationships
.

We will also learn to measure the input and output
impedance of a CE amplifier
.

We will also learn to determine the decibel power gain of a CE amplifier
while observing, with an oscilloscope and
its

probes, the

phase of the input and output signal voltages of
a CE amplifier
.




Procedure


We started this lab off by gathering the nece
ssary materials
.

They are listed below
.





Power supply



Oscilloscope



Multimeter



Sine wave generator



470 ohm resistor



560 ohm resistor



1k ohm resistor



4
.
7k ohm resistor



8
.
2k ohm resistor



18k ohm resistor



2 25uF capacitors



1 100 uF capacitor



2N2222A capacito
r



SPST switch



5k ohm potentiometer


After we gathered the necessary materials we had to create the circuit shown below in Figure 1
.















Figure 1 below shows the circuit we
used for the lab. Later on we
removed the potentiometer.


After we
created the circuit, we adjusted RX to 1000 ohms
.

We then closed S1 and adjusted the
AF sine wave generator for 1000 Hertz and set the generator level control for 70 percent of the
maximum undistorted
output

(Figure 2)
, as observed with an oscilloscope
.

Wi
th an oscilloscope we
measured and recorded in Table 1 the peak
-
to
-
peak voltage of VAC across AC

VBC, or VIN across BC and
Vout in the output
.

We computed VX across RX by subtracting VBC

and
Vac
.

We recorded it in Table 1
.

We computed and recorded Iin and
Rin
.

We showed our
computations
.

Next we adjusted Rout until the
measured output signal equals one half the output measured in step 3c
.

`We removed Rout from the
circuit and measured and recorded it’s resistance
.

We then computed and recorded in Table 1 th
e
voltage gain and power gain of the circuit under load
.

Next we removed the bypass capacitor from the
circuit
.

With an oscilloscope we observe the output signal Vout
.

The output has dropped dramatically
because the bypass capacitor gives the output its va
lue
.

With C3 still out of the circuit, we increased the
generator output until Vout
equaled

1 volt peak to peak
.

We repeated steps 3 to 7
.

Then we opened S1
and replaced C3 into the circuit
.

We closed S1 and with an oscillos
cope observed 2 cycles of the ou
t
put
signal waveform
.

We
showed

this waveform in
Figure 3 below
.













Steps

V p
-
p

I in

Rin

Rout

Gain

Vac

Vbc

Vout

VX

Voltage

Power

Vin

Vac
-
Vbc

mA

ohms

ohms

dB

2 to 7


1
.
01

.
07



4

.
05


30


1000

800


20
.
7


15


8


1


.
08

.
1



.
01


40


1000


650


1


15

In Table 1 we recorded all the results
we got from the lab. They are below.

Figure 2 above shows 70 percent of
the maximum undistorted output
needed for this lab.

Figure 3 above shows the output
waveforms. They are 180 degrees out
of phase.

Discussion


In this lab we had to use a couple of formulas
.

One was for the
amplifier
s

voltage gain,
the other
was power gain and the other was impedance
.

The formula for the impedance is Rin= Vin/Iin
.

It’s just
Ohm’s Law for finding resistance, but with all the input values, like to find the impedance in, you’d use
the voltage coming in d
ivided by the current coming in
.

The other formula we used was power gain
.

The
formula for this is (Vout/Vin)^2 times Rin/Rout
.

The last formula we used was voltage gain
.

The formula
for this is simple
.

It’s just output over input
.

So, the voltage coming o
ut divided by the voltage coming
in
.


Another thing that should be mentioned is the use of a different transistor
.

We used a 2N2222A
transistor instead of the 2N6004 transistor that was recommended for the lab
.

We looked up some of
the information between
the two transistors and found out that they were very similar, so the transistor
was fine for the lab
.




Questions


1.

A
.

If you wished to measure VX directly, why would it be necessary to use a “floating” oscilloscope,
that is, an oscilloscope whose case is not grounded to the electrical system?

B
.

Why is the use of a floating instrument generally not recommended?


a.

You’d
have to use a floating oscilloscope because one whose case was grounded to the circuit
wouldn’t give you the correct reading
.

b.

The use of a floating instrument is generally not recommended because it can mess up things in
the circuit
.

Grounding instruments
is always the right answers
.


2.

What is the effect on input impedance of removing your bypass capacitor C3 in Figure 1?

The effect on input impedance when removing your bypass capacitor would be a decrease
.

3.

A
.

What is the phase relationship between the inpu
t and output signals of a CE amplifier?

B
.

Was this relationship confirmed by the results of your experiment? Explain how
.



a.

The phase relationships between the input and output signals in the CE amplifier is that they are
180 degrees out of phase
.

b.

This w
as indeed confirmed by the experiment
.

It is shown in Figure 3 above
.


4.

Is the output impedance of a CE amplifier a fixed quantity?

Yes, the input impedance of a CE amplifier was a fixed quantity
.








Conclusion



In this lab we learned a lot about the

input and output in a common emitter amplifier.
This lab
helped reconfirm some things we already learned in related. An example would be the formulas for
impedance and voltage gain. We briskly went over the decibel voltage gain and this lkab helped a bit
with that. We also reconfirmed that the voltages were 180 degrees out of phase in a common
-
emitter
amplifier. Other than that, we just reestablished some of the aforementioned objectives in the
introduction.