Circuits i Have Known

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A Brief introduction to all the circuit elements useful in hardware Design.

Text of Circuits i Have Known

Circuits I Have KnownSimplified Explanations of Analog Circuits The Nuts-and-Bolts Approach With Personal Observations By Ronald W. Parker, B.S.E.E., M.S.E.E., P.E. December 17, 2005 Copyright 2005



In the Beginning In the late 1950s and early 60s most of the circuitry was what we today know as Analog. The vacuum tube was king. But it wasnt long before Mr. Shockleys transistor was making break-throughs. The RCA Tube Manual was the bible of the day. However, a new bible was on the horizon. It was called the G.E. Transistor manual. This was followed shortly thereafter by the logic circuit. ANDs, NANDs, NORs ORs and OREs were the order of the day. IBM mainframes were made of many, many simple logic circuits. When it came to programming, assembly language and basic assemblers was the order of the day. Every month there were new innovations in increased logic density and software compilers. For the logic designers the new bibles were the Texas Instruments TTL Handbook (Volume 1) followed shortly by the Motorola CMOS Book. But almost at the same time, Operational Amplifier came into the spotlight. National Semiconductor seemed to lead the way with a whole series of analog circuits. They had their own bibles known as the National Semiconductor Data Books. Analog Devices was not far behind. The op-amp opened a whole new world of design. Then, the op-amp got better and better, every month. There was an attempt to merge the analog and digital world. This was done through the FET switch. Using the FET switch, an analog circuit could be gated just like a logic circuit. It could be synchronized with a timing and gating pulse to produce analog signals that were digitally gated. So what could top this? Well, along came the microprocessor. The first ones were 4-bit and 8 bits jobs, but shortly thereafter came the 16 and 32 bit processors. Manufacturers started incorporating a lot of features into the micros, such as timers, counters, I/O interfaces, A/D and D/A converters, and ultimately the kitchen sink. These became known as microcontrollers. In the mean time, the op-amps were getting better and better in every parameter. The analog/digital WAR was going hot and heavy. At about this time, the CPLD (Complex Programmable Logic Device) came upon the scene. PALs had been around for a while, but they were primarily used to reduce the combinatorial logic, such as NANDs and NORs. Although still a digital logic device, it lent itself to decision making, much as a microprocessor or microcontroller did, although in a greatly reduced capacity. Its claim to fame was the state machine, or decisionmaking portion in combination with combinatorial or popcorn logic that seemed always to be required in most digital application whether microcontroller-based or not. Many followers of the mixed logic application embraced this new decision making and timing generating device. However, the result was the formation of the basis for the two


great electronic religions, the ANALOGIANS (ana-lo-g-en) and the DIGITALIANS. Only because there are certain things that each discipline can do, that the other cant is the battle still taking place today, although greatly reduced in intensity. Analogians have a modest respect for Digitalians and Digitalilans have a slightly higher regard for Analogians. This is only because there has always been somewhat of an undeserved mystique attached to analog, probably because it has an infinite number of states. I have neglected to discuss the creation of what has turned out to be the greater part of digital engineering, namely programming. My reason for neglecting this group is that most programmers are not concerned with the nuts and bolts of electronic hardware but rather float above all of this fray in an ethereal mindset that divorces the engineer from reality. Case in point, C++. During my career in electronics I have gained expertise in both analog and digital hardware and a basic knowledge of programming. I have also had my share of difficulty in understanding some of the aspects of electronic circuit design. Because of this, I have had to take a nuts-and-bolts approach to understanding circuits. The following circuit explanations are meant to cover many fundamental circuits that I have found useful. The explanations are, as mentioned, nuts-and-bolts. Only when absolutely necessary are any mathematical explanations, other than basic algebra, trigonometry and minimal calculus used in the circuit descriptions. I hope that you enjoy some of these explanations and get a better feel for electronics rather than whats left after the usual stiff and stilted presentations. The Author.


Table of ContentsAn Operational Amplifier Circuit with Gain and Offset....6 Caught in a Layoff....9 Oscillators...11 Why Analog? ..16 Comparator Circuits.....17 Engineers Egos...22 Precision Full Wave Rectifier...23 Finding a Job...26 Operational Amplifier Power Supply..27 The Bad Boss...33 Current-to-Voltage Circuit using Transistors....34 Losing the Job at the Interview.....35 The Schmitt Trigger Input with Bi-Polar Transistors...36 The Good Job..42 The Howland Current Pump....43 The Improved Howland Current Pump..47 Software Guy Just the specs please, just the specs!.......52 Just Another Interesting Circuit..52 VHDL State Machine Development, Conventions and an Example using CPLDs.53 Getting a Degree While Youre Working..74 Secrets of the PID Control Loop......75 The RIGHT Circuit.....94 Relay Ladder Logic...95 The Microprocessor Story....100 Calculations for a Bridge Circuit.......102 On Working and Getting Older...111 Capacitance Independent Frequency-to-Voltage Converter...112 Things to Consider when applying Flexible Automation (Robotics)....119 Digital Logic in Analog Applications.....121 Traveling for your Work..125 Uni and Bi Directional Series Current Limiter Circuits.....127 Hard Lessons Get the Specs!.................................................................................129 Simplest Motor Speed Control Circuit..130 Traveling to Work.131 Precision Difference Voltage Circuit.132 Your Own Business...136 Detecting Zero Crossings in a Three-Phase System.....138 Dealing with Head Hunters......141 Flow Chart for Binary Division.....143 Some Lessons Learned while working for Myself..146 Binary Multiplication..148 On Working Overtime..149 Serial A/D, D/A Circuit with Selectable Bit Resolution...151 Your Co-Workers..156


Voltage Doublers and Negative Voltage Circuits.157 Strobe Firing Circuit...159 Three-Phase-Plus-Neutral Motor Control Device....165 Analog Profile Generator for Motion Control..172 Changing Move Parameters on the Fly (During the Move)177 Moving On......185 Detecting Open Potentiometer Wires....186


An Operation Amplifier Circuit with Gain and Offset The use of an offset current in an op-amp circuit is really quite simple. The whole idea is that a constant current into or out-of the summing node at the input of the amp will produce a constant offset in the output voltage. An operation amplifier circuit that inverts the input and multiplies it by two is shown below. With 5 Volts in, the output is -10 Volts. The summing node is at the - input to the amp. Remember that the inputs always want to be at the same potential. Since one is tied to ground, the other will also try to be at ground potential. The only way to keep this point at ground potential is to change the output current. This current always passes through R2, so R2 produces the voltage drop that appears at the output of the amp.

Now lets add an offset current to the - input of the amplifier. The resulting circuit looks like this.


The current produced by the input voltage, IN will be the voltage at IN/10KOhms. In this example let IN = 2 Volts. Therefore the current through R1 will flow from IN toward pin 2 of U1A. This current will be 2/10,000 or 2exp-4 Amps. This same current flows in R2 and causes a voltage drop of 2exp-4 * 20KOhms or -4 Volts. With one end of R2 at ground, the amp output (OUT) will be at -4 VDC. At the same time, a current flows from 5V, through R3 toward pin 2 of U1A. This current is 5/100KOhms or 50 microAmps. Because pin 2 is always at ground potential, this is a constant current of 50 microAmps. Pin 2 of the op amp is a high impedance input so no current flows into or out of pin 2. The 50 microAmps has to flow through R2 (because U1A pin 1 has the most negative potential). This produces a voltage drop across R2 of 50exp-6 * 20KOhms or -1 Volt (this current flows from Ground to OUT. Therefore it is negative at U1A, pin 1 or OUT). When this voltage drop is added to the -4 Volts the result is -5 Volts OUT. With a negative voltage IN, the current will be from the node at pin 2 of U1A to IN. Use -2 Volts IN as an example. So the current flowing through R1 will be -2/10KOhms or 2exp-4 Amps. Again, this current flows through R2, only in the opposite direction,


keeping the voltage at pin 2 of U1A at Ground potential. This voltage drop is -2exp-4 * 20KOhms or +4 Volts at OUT. At the same time, a current of 5/100KOhms or 50 microAmps flows toward pin 2 of U1Am through R3. This current must be accounted for by the output of the op-amp. It is canceled by a current flowing from OUT to pin 2 of U1A. This current produces a voltage drop across R2 of 50 microAmps * 20KOhms, or 1 Volt. The +4 Volts already across R2 is reduced to +3 Volts. So the result of the offset current is a constant offset of -1 Volts at pin 1, the OUT pin of op-amp U1A. In any event, the offset is constant. In this example, -1 volt is always added to the output of the op-amp. How about changing the gain? Lets make the feedback resistor (R2) equal 30KOhms. Ignoring the current contribution from the offset path (R3) for a moment, let the voltage input to the op-amp be +1 Volt. This puts 1 volt across R1 which is still a 10KOhm