• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
Lecture 9
Transistors
•  Physics
•  Transistor/transistor logic
•  CMOS logic
CA 1947
http://www.extremetech.com/extreme/164301-graphenetransistors-based-on-negative-resistance-could-spell-theend-of-silicon-and-semiconductors
http://en.wikipedia.org/wiki/Moore's_law
Robert R. McLeod, University of Colorado
100
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
The Transistor
Discrete
Integrated
Packages
http://www.uobkupartnership.talktalk.net/
page13.htm
http://en.wikipedia.org/wiki/File:Diopsis.jpg
Physics
http://en.wikipedia.org/wiki/Bipolar_junction_transistor
Symbols
Robert R. McLeod, University of Colorado
PNP
P channel
NPN
N channel
BJT
MOSFET
101
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
NPN Bipolar Junction
(BJT) transistor
Symbol
C = Collector
E = Emitter
B = Base
Package
Top-down view
Construction and use
This shows the “common
emitter” mode which is how we
will use the BJT. Concentrate
on the BE junction which looks
like a diode. When this is
forward biased, a large current
can flow from C to E.
http://en.wikipedia.org/wiki/Bipolar_junction_transistor
Hydraulic analogy
The BJT is a current-controlled current amplifier. That is, a
small current into the base results in a large current from
collector to emitter.
Cool hydraulic
analogies. One error –
the CE current is really
larger than the base.
Note that there are
“normally on” and
“normally off” versions.
Robert R. McLeod, University of Colorado
http://www.mine-control.com/zack/transistor/transistor.html
102
http
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
BJT Inverter
“Pull up resistor” causes
VO to be high if IC is ~0
but allows VO to be low if
the bottom of the resistor
is grounded.
Plot of VO vs. VI
high
Current limiting resistor
Input
Output
1
2
low
high
low
Switch, either
“open” or “short”
Simplified
operation
http://jpkc.njau.edu.cn/szdzjs/ywjc2.htm
If we give names to the input and output
levels, we can construct a reduced
description of the plot called a truth table:
Input
Output
Low = 0
High = 1
High = 1
Low = 0
Robert R. McLeod, University of Colorado
So the output is high if it is
NOT the case that the input is
high. We thus call this a
NOT gate or Inverter.
http://ocw.mit.edu/courses/electrical-engineering-andcomputer-science/6-071j-introduction-to-electronics-signalsand-measurement-spring-2006/lecture-notes/19_bjt_1.pdf
103
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
Transistor/transistor logic
(TTL) NAND gate
Consider a circuit like the previous one but now with
two BJT transistors in series:
+Vsource
0 or +Vsource
0 or +Vsource
0V
A current can flow from top to bottom only if both A and B are
high. The output will drop to a low voltage in this case,
otherwise it will remain high.
A
B
Out
0
0
1
0
1
1
1
0
1
1
1
0
So the output is high if it is NOT the
case that A AND B are high. We thus
call this a NOT AND gate or NAND
for short.
Robert R. McLeod, University of Colorado
http://cpuville.com/logic_gates.htm
104
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
TTL NOR gate
OK, what if we arranged the two BJT transistors in
parallel:
Output
Input A
Input B
A current can flow from top to bottom only if either A or B are
high. The output will drop to a low voltage in this case,
otherwise it will remain high.
A
B
Out
0
0
1
0
1
0
1
0
0
1
1
0
So the output is high if it is NOT the
case that A OR B are high. We thus
call this a NOT OR gate or NOR for
short.
Robert R. McLeod, University of Colorado
http://ocw.mit.edu/courses/electrical-engineering-andcomputer-science/6-071j-introduction-to-electronics-signalsand-measurement-spring-2006/lecture-notes/19_bjt_1.pdf
105
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
Complementary metal–oxide–
semiconductor (CMOS) logic
TTL worked well, but it has a
problem. Note that when the
transistor is “on” and IC is high,
there is power dissipated in RC.
This is OK occasionally, but
not in a million logic gates.
The “pull up resistor” is thus a
bad idea in general.
Here is a complementary
inverter that happens to be
made with metal-oxide fieldeffect transistors (MOSFETs).
For our purposes, they act like
switches but with two
polarities.
http://en.wikipedia.org/wiki/CMOS
By replacing the pull-up resistor
with a complementary
transistor, the power dissipation
is greatly reduced. CMOS is
the dominant electronics
platform and this is a major LOW
reason.
Robert R. McLeod, University of Colorado
HIGH
HIGH
LOW
106
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
Quiz 9.1
A closer look at the BJT
Active region above
cutoff and below
saturation
Q: When we use the BJT as a switch, we take advantage of the fact that it
switches between cutoff (off, reverse biased) to saturation (on, forward
biased) very quickly. The intermediate region is called the “active” mode.
Consider using a BJT transistor in this “active” mode. Given the plot above
and assuming the x and y axes have the same scale, how would you
describe the output if the input is confined to operate in the active region
A. The output is an attenuated version of the input.
B. The output is an amplified version of the input.
C. The output is an inverted version of the input.
D. A and C
E. B and C
The slope is large, thus the output is larger than the input = “amplified”.
The slope is negative so the output will go down when the input goes up = “inverted”.
Robert R. McLeod, University of Colorado
107
• Lecture 9: Transistors
ECEN 1400 Introduction to Analog and Digital Electronics
Quiz 9.2
A modification of the inverter
http://www.interfacebus.com/Glossary-of-Terms-bjtamplifier-circuits.html
Q: Assuming the BJT is ideal in that it switches between a perfect open
circuit and a perfect short circuit with an infinitesimal active region, what is
the function of this device?
A. Logical buffer – the output is a binary copy of the input (1 gives 1)
B. Logical inverter - the output is a binary inversion (1 gives 0)
When input is high and BJT is “on” and thus nearly a short, the output is
shorted to Vcc and thus high. When the input is low and BJT is “off” and
thus nearly an open, the output is pulled low by the pull-down resistor RE.
The assumption is that little current flows to the output thus there is little
voltage drop across RE by Ohm’s law, thus the output is nearly grounded.
Robert R. McLeod, University of Colorado
108