CHAPTER 14. ELECTRONICS 14.4
pletion band, the narrower the channel. The narrower the channel, the harder it is for
electrons to flow from source to drain.
FACT
No-one would build a
computer with billions
of bipolar transistors —
the current in each tran-
sistor’s base might be
small, but when you add
up all of the base cur-
rents in the millions of
transistors, the computer
as a whole would be
consuming a great deal
of electricity and mak-
ing a great deal of heat.
Not only is this wasteful,
it would prevent manu-
facturers making a com-
puter of convenient size.
If the transistors were
too close together, they
would overheat.The voltage of the gate is not the only factor affecting the current of electrons between
the source and the drain. If the external circuit has a low resistance, electrons are able
to leave the drain easily. If the external circuithas a high resistance, electrons leave
the drain slowly. This creates a kind of ‘traffic jam’ which slows the passage of further
electrons. In this way,the voltage of the drainregulates itself, and ismore or less
independent of the current demanded from thedrain.
Once these two factorshave been taken into account, it is fair to say that the positive
output voltage (the voltage of the drain relativeto the source) is proportional to the
negative input voltage (the voltage of the gate relative to the source).
For this reason, the fieldeffect transistor is knownas a voltage amplifier. This contrasts
with the bipolar transistor which is a current amplifier.
Exercise 14 - 5
- What are the two types of bipolar transistor?How does their construction differ?
- What are the three connections to a bipolartransistor called?
- Why are very few electrons able to flow from emitter to collectorin an NPN
transistor if the base is not connected? - Why do you think abipolar transistor wouldnot work if the base layer were too
thick? - “The bipolar transistor is a current amplifier.”What does this statement mean?
- Describe the structure of a FET.
- Define what is meant by the source, drain and gate. During normal operation,
what will the voltages of drain and gate be withrespect to the source? - Describe how a depletion layer forms when the gate voltage is mademore nega-
tive. What controls the width of the depletion layer? - “The field effect transistor is a voltage amplifier.” What does this statement mean?
- The amplifier in a cheap radio will probably contain bipolar transistors. A com-
puter contains many field effect transistors. Bipolar transistors are morerugged
and less sensitive to interference than field effect transistors, which makes them
more suitable for a simple radio. Why are FETs preferred for the computer?
More practice video solutions or help at http://www.everythingscience.co.za(1.) 01jr (2.) 01js (3.) 01jt (4.) 01ju (5.) 01jv (6.) 01jw
(7.) 01jx (8.) 01jy (9.) 01jz (10.) 01k0