A4Transistor 08 - Vicphysics

heartlustElectronics - Devices

Nov 2, 2013 (3 years and 7 months ago)

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Bruce Carpenter

Science Coordinator,

Bendigo Senior Secondary
College


carpenter.bruce@bssc.edu.au




The transistor
turns 60!

Invented at Bell labs USA December 1947.

About the size of a modern mobile phone.

We can now fit 2 million transistors on a full stop. .


Timeline:

1947 Transistor invented.

1948 Shockley develops first semiconductor transistor

1954 Texas Instruments introduces transistor radio.

1965 Intel co
-
founder Gordon Moore coins Moore’s
Law.

1981 IBM launches the PC

2007 Intel demonstrates chip with 1.9 billion
transistors.


Integral to computers, CD/DVD players, iPods, mobile
phones, digital cameras, video recorders, Plasma and
LCD TVs, in fact almost every modern electronic
device imaginable!


Trans


“across”

…istor

”resistor”



The transistor can be thought of as a
device whose resistance (across collector
-
emitter) varies with input current (at the
base).


A water pipe analogy


The collector (C)
collects water at the
top, the emitter (E)
emits water at the
bottom, and the flow
of water current is
controlled by a small
tap and valve, similar
to the base lead (B) of
a transistor.

Water Pipe model (cont)


When the tap is jammed on (valve
vertical), maximum water flows. Analogous
to transistor
saturation
. I
C

= I
E

at all times.


When the tap is jammed off (valve
horizontal), no current flows. Analogous to
transistor
cut off
.


These two states are used in all digital
circuitry using transistors. Only ON or OFF
(1
-
0) states are possible.

Analogue Transistor Action


If we turn the tap half
on and then
continuously twist the
tap clockwise and
anticlockwise, then
the change in water
current flowing from
collector to emitter will
be proportional to the
twisting action.



This is analogous to a transistor operating
in the linear region. Small changes in the
base current cause much larger but
directly proportional changes in the current
flowing from collector to emitter.


The transistor can be biased using a DC
power supply and resistors to act as an
amplifier
.


Small AC signals are applied via the input
capacitor, which allows AC but not DC to
flow through it.


PART A: TRANSISTOR BIASING


Q1. Use a digital multimeter set to read DC Volts to measure the
supply voltage, the voltage across each of the four resistors, and the
transistor voltages V
BE
, and V
CE
.




V
supply

V
10K
W


(bottom)
V
3.3
K
W

(top)

V
3.3K
W

V
1 K
W

V
BE

V
CE.

9.18v

6.90v

2.27v

4.40v

1.33v

0.66v

3.42v

This is what it looks like on
breadboard

Q1a) Use Ohm’s Law to calculate I
C

and I
E


(in
mA).



I
C

= V
R
C
/R
C

= 4.4/3.3k = 1.33mA


I
E

= V
R
E
/R
E

= 1.33/1.0k = 1.33mA



Q1b) How do they compare? Explain your answer.



They are the same because virtually all
current flows from collector to emitter. The
base current is very small as I
B

= I
C
/

b


b

= Transistor current gain = 100 to 800 for a
BC108 transistor.

Q1c) Explain the purpose of the voltage divider
(combination of 10 K
W

and bottom 3.3 K
W

resistors). Use your measured voltages to
explain your answer.



To provide sufficient quiescent DC operating
conditions, small DC base current and
correct biasing of the base
-
emitter (pn)
junction of the transistor.



Note V
10k

+ V
3.3K

= 9.17volt = Supply voltage


PART B: TRANSISTOR AMPLIFIER


Q2. These DC voltages levels bias the
transistor so that it will operate correctly as
an
amplifier.


Apply a small AC voltage signal of
frequency 1000Hz from a signal generator
to the input of the amplifier and observe
input and output signals on the Cathode
Ray Oscilloscope.



2a&b) Measure and record the peak to peak input
& output signals. (Vertical gain is set to 1 volt/div)


Vpp for input signal
= 1.6 volt

(bottom trace)

Vpp for output
signal = 5.0 volt

(top trace)

Note how the traces
indicate that this is
an inverting
amplifier

2c) Hence calculate and record the Voltage Gain
of this amplifier.


Voltage gain = V
out
/V
in

= 5.0/1.6 = 3.1


Note that this is approximately the same as the
“rule of thumb” for voltage gain,

Av = R
C
/R
E

= 3.3k/1k = 3.3



2d) Explain the purpose of the input and output
capacitors.





The purpose is to
block any DC
voltage from an
input transducer
such as a
microphone which
could alter the DC
biasing conditions
for the amplifier to
operate correctly.


Similar for output.


2e) Increase the input voltage until “clipping”
occurs. Explain why this happens.



Clipping occurs
when Vin peak to
peak is
approximately 1.8
volt.


The output signal
can no longer follow
the input signal
because the
amplifier has moved
out of the linear
region.

More severe clipping occurs as the input signal
increases further




2f) What would be the effect of clipping if his
amplifier was used to amplify music signals (say
from a microphone).




Any musical signals would sound
distorted
.

Further hints


Use a 9 volt battery
and battery snap for
the DC power supply.


This can avoid
earthing problems
when you attach the
CRO and signal
generator to your
amplifier.


Use a cheap
multimeter to
measure DC
voltages. This one
costs approx $10.


The
b

(beta) or
current gain of a
transistor can be
measured using a
digital multimeter.


The BC108 transistor
used in our amplifier
is npn.


Use red and black
alligator clips to
connect CRO and
signal generator to
components on
breadboard.


Use black for 0
volts DC from CRO
and Sig Gen.

Hoping you enjoy the
transistor topic!