Laboratory 8 - Transistors

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Nov 2, 2013 (3 years and 10 months ago)

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Laboratory 8
-

Transistors


Goal:

-

Investigate some basic characteristics of bipolar transistors

-

Generate curves to characterize an NPN transistor

-

Build two different types of simple transistor amplifier


Background:


A transistor can be thought of as

a device which amplifies current, or as a type of transducer in
which voltage changes at the input result in current (or voltage) changes at the output. There are
two types of bipolar transistor, NPN and PNP, which take their respective names from the
co
nfiguration of the semiconductor junctions of which they are made. Circuit symbols for the
two types are shown in Fig. 1.


The chief characteristic of a transistor is the
amplification of current such that the ratio of
collector current, I
C
, to base curr
ent, I
B
, is
large. This ratio is designated by h
fe
, the
forward
-
current transfer ratio. Although h
fe

is
well
-
defined for a given transistor, it is not a
reliable design parameter, as it may vary from
piece to piece of the same type of transistor.


In
this Laboratory we will be making
measurements of h
fe

in both static and
dynamic configurations. In addition, two
different amplifier configurations will be built
and tested, each of which has its advantages and disadvantages.


Procedure:


Identify the pi
ns on the case of the transistor to be used (2N2219A). As viewed from the pin side
the assignment of pins is as shown in Fig. 2


There is one very simple way to blindly check the current
amplification ratio h
fe
. The DMM’s that we have been using
in clas
s have this capability.


(1) What is your DMM measured value for h
fe
?


Set up the circuit shown in Fig. 3a. To make the
measurements, the resulting circuit on the Proto
-
Board will
translate into Fig. 3b.


(2) Which recent experiment does this resemble?




Run the program EXP27
-
1.BAS.


(3) What quantities are being
monitored?


(4) Over what voltage range does
the input to the transistor base
change?


Make a graph in Excel of the data
you gathered. Comment on the
graph with regards to our
discussions in cl
ass of diodes.


(5) Is the result for this graph what you would expect? Explain.


Set up the circuit shown in Fig. 4. You will use this to gather data for the collector current as a
function of the collector
-
emitter voltage, I
C

vs. V
CE
.


The variable
voltage
at the base is taken
from 0
-

5 Volts
using a potentiometer
on the Proto
-
Board.
The collector voltage
is generated by
DAC0. Current and
voltage data are
taken using a similar
arrangement to the
previous part. Set
the base voltage for
0.5V, 1.0V,

1.5V, 2.0V and 2.5V. Run the program EXP27
-
2.BAS to collect data.


Plot your data using Excel.


(6) To which figures in your text do your data
compare?


(7) What would you estimate to be the current
“gain”, h
fe
, using these curves. Explain.


Build a ba
sic “common emitter amplifier” as
shown in Fig. 5.


(8) What are your results for the voltage gain
(G=V
out
/V
in
) with an input signal at a frequency
of 10kHz?