ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado
• Physics • Transistor/transistor logic • CMOS logic
100
Lecture 9 Transistors
• Lecture 9: Transistors
http://www.extremetech.com/extreme/164301-graphene-transistors-based-on-negative-resistance-could-spell-the-end-of-silicon-and-semiconductors
CA 1947
http://en.wikipedia.org/wiki/Moore's_law
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 101
The Transistor
Packages
Physics
Symbols
http://www.uobkupartnership.talktalk.net/page13.htm
http://en.wikipedia.org/wiki/Bipolar_junction_transistor
BJT
• Lecture 9: Transistors
http://en.wikipedia.org/wiki/File:Diopsis.jpg
Discrete Integrated
MOSFET
PNP
NPN
P channel
N channel
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 102
NPN Bipolar Junction (BJT) transistor
http://en.wikipedia.org/wiki/Bipolar_junction_transistor
C
B
E http://www.satcure-focus.com/tutor/page4.htm
Symbol
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.
C = Collector E = Emitter B = Base
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.
Package Top-down view
• Lecture 9: Transistors
http://www.mine-control.com/zack/transistor/transistor.html
Cool hydraulic analogies. One error – the CE current is really larger than the base. Note that there are “normally on” and “normally off” versions.
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 103
BJT Inverter • Lecture 9: Transistors
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-071j-introduction-to-electronics-signals-and-measurement-spring-2006/lecture-notes/19_bjt_1.pdf
If we give names to the input and output levels, we can construct a reduced description of the plot called a truth table:
1 2
low high
high
low
Input Output Low = 0 High = 1 High = 1 Low = 0
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.
Input Output
“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.
Switch, either “open” or “short”
Simplified operation
http://jpkc.njau.edu.cn/szdzjs/ywjc2.htm
Plot of VO vs. VI
Current limiting resistor
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 104
Transistor/transistor logic (TTL) NAND gate
Consider a circuit like the previous one but now with two BJT transistors in series: +Vsource
0 V
0 or +Vsource
0 or +Vsource
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.
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.
http://cpuville.com/logic_gates.htm
• Lecture 9: Transistors
A B Out 0 0 1 0 1 1 1 0 1 1 1 0
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 105
TTL NOR gate OK, what if we arranged the two BJT transistors in parallel:
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-071j-introduction-to-electronics-signals-and-measurement-spring-2006/lecture-notes/19_bjt_1.pdf
• Lecture 9: Transistors
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.
Input A
Input B
Output
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.
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 106
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.
• Lecture 9: Transistors
Here is a complementary inverter that happens to be made with metal-oxide field-effect transistors (MOSFETs). For our purposes, they act like s w i t c h e s b u t w i t h t w o polarities.
http://en.wikipedia.org/wiki/CMOS
LOW HIGH HIGH LOW
By replacing the pull-up resistor w i t h a c o m p l e m e n t a r y transistor, the power dissipation is greatly reduced. CMOS is the dominant e lectronics platform and this is a major reason.
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 107
Quiz 9.1 A closer look at the BJT
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”.
• Lecture 9: Transistors
Active region above cutoff and below saturation
ECEN 1400 Introduction to Analog and Digital Electronics
Robert R. McLeod, University of Colorado 108
Quiz 9.2 A modification of the inverter
• Lecture 9: Transistors
http://www.interfacebus.com/Glossary-of-Terms-bjt-amplifier-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.
