Cyril Mechkov

Cyril Mechkov
Technical University of Sofia | TU · Department of Computer Systems

Ph.D. in Electrical Eng.

About

44
Publications
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Introduction
I have been teaching analog, digital and microcomputer electronics since 1986 at the Computer Systems Department of the Technical University of Sofia. My pursuit is to reveal the fundamental ideas behind circuits and to show them to students and web readers. I do this relying more on my imagination, intuition and emotions than on the pure logic and reasoning. I prefer to understand and explain instead to learn and tell circuits to students. Visit also my blog https://circuitstories.blogspot.com.
Additional affiliations
January 2011 - present
Education
September 1975 - July 1981
Technical University of Sofia
Field of study
  • Electronics, Computer systems

Publications

Publications (44)
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A few years ago, when I started receiving the first notifications from RG team, I did not pay much attention to them. At the time, I was frantically writing comments and had no time for "side" activities. Thus I gradually accumulated a lot of materials, my popularity grew and I started receiving notifications about my achievements almost every week...
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This is a summary of all comments to the question below: https://www.researchgate.net/post/What_is_the_truth_about_the_exotic_current_feedback_amplifier_Is_it_something_new_or_just_well_known_old_Is_it_really_a_current_feedback_device
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Looking at Wikipedia from the outside (as readers), it seems perfect with its highest Google rank. But if we look at it from the inside (as contributors), we will see that although it is a great idea (like Google and social networks) Wikipedians (Wikipedia editors) in the area of electronics are not so great. I found this from my bitter experience...
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I have extracted my RG questions and answers related to the negative impedance phenomenon and collected them in this file.
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There is hardly so controversial, misunderstood and denied phenomenon in circuitry as negative impedance (resistance). Many questions can be asked about the mystical phenomenon: What is it? Is it possible at all? Does it violate natural laws? Does it exist at all? If so, how do we make it? What is the use of the true negative impedance? What is the...
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Negative impedance elements are amazing and extremely useful electronic devices (circuits)... but there is only one small problem:) - there are not such elements in nature; there are only humble passive elements with "positive" impedance (resistors, capacitors, inductors and memristors:( So, we have to make them... and this is an extremely interest...
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When I first met the exotic RTL gate, I was amazed since I could not imagine how it is possible humble ohmic resistors to perform logic operations. Until then I knew that logic operations were realized only with electrical switching elements – diodes, tubes and transistors. For example, if we had to realize a 2-input OR gate, I thought we should co...
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For quite a long time I have discovered that there is a close connection between the input parts of the transistor-transistor logic (TTL), diode-transistor logic (DTL) and diode logic (DL). Thus TTL includes DTL... and DTL includes DL... or DL have evolved to DTL... and then DTL evolved to TTL... So, it seems to understand what TTL is, we have firs...
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For me there is no more beautiful thing in the world from the new idea – to come up with your own new idea or to explain an existing else's great idea. In this material, I have tried to reveal in an intuitive manner the great idea behind the legendary Wien bridge oscillator. I have done it in the time domain at the lowest level of thinking.
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This is a short story about the transimpedance amplifier. I have told here what happened when, a few years ago, I wrote a comment to the Bob Pease's article "What's All This Transimpedance Amplifier Stuff, Anyhow?" published in Electronic Design in Jan. 8, 2001.
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In this story, I have tried to reveal in an intuitive manner the great idea behind the legendary Wien network. I have done it in the time domain at the lowest (intuitive) level of thinking. I hope my detailed examination of the circuit operation visualized by voltage bars will attract your attention as well.
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In this material, I have collected my answers to questions so far related to the nature of sinusoidal oscillations. I do not arrange them systematically but chronologically. I will periodically update the content of the material by adding the new posts. To see and join the full discussions, click on the respective questions.
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I wrote this story in May 2007 as two consecutive comments to the EDN's article "Consider The 'Deboo' Integrator For Unipolar Noninverting Designs". There I showed how we can convert, in two different ways (Miller's and Deboo's), the passive RC integrator into an active op-amp one. To my much surprise, six months later I received an email from the...
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In this material, I have collected my answers to questions so far related to creating various op-amp inverting circuits with parallel negative feedback.
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I wrote this personal story in 2001 when I began creating with a great ehthusuasm the site of circuit-fantasia.com (How do we understand, present and invent electronic circuits).
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I wrote this circuit story in 2001 when I began creating with a great ehthusuasm the site of circuit-fantasia.com (How do we understand, present and invent electronic circuits). Then I wasted a few months of my life to create an animated version of this story as well – a kind of a fairy “story tale”:-) As naive now seeming to me then I did it all t...
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I wrote this personal story in 2001 when I began creating with a great ehthusuasm the site of circuit-fantasia.com (How do we understand, present and invent electronic circuits).
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From my experience, I have found that, in the most cases, circuit phenomena have dual versions: voltage - current, resistance - conductance, positive resistance - negative resistance, capacitance - inductance, positive feedback - negative feedback, amplifier - attenuator, integrator - differentiator, low-pass - high-pass, series - parallel, etc......
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I have extracted my RG questions and answers related to the virtual ground phenomenon and collected them in this file.
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In this material, I have collected and hierarchically arranged links to my ResearchGate questions and my answers to them with the purpose to help the navigation. I will continuously update and rearrange it.
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I have started the website Circuit fantasia in 2002 with the purpose to share my teaching circuit philosophy with people. It is based on a few basic principles: 1. Electronic circuits are based on clear and simple basic ideas, which may be derived from our routine. 2. In order to really understand electronic circuits, we - human beings - have first...
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I established Circuit Idea wikibook in 2007 with the purpose to reveal the fundamental ideas behind circuits. The philosophy behind this e-book relies more on human imagination than on logical reasoning. It considers the circuitry more as art than science and the creation of electronic circuits as a result of human fantasy, imagination and enthusia...
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In classic electronics courses, the electronic circuits are presented in their complete, final and perfect form. Only, in order to really understand circuits, we need to know the relation between passive and active versions and how passive circuits have been converted into active ones. The novel interactive multimedia tutoral described in this pape...
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I joined electronics Wikipedia in 2006 with great enthusiasm. I was noted that Wikipedia articles in this area were formal and theoretic; there had not introductory sections saying what the circuit idea actually was. Thus I came with clear and obvious purpose - to reveal the basic ideas behind circuits by clear and obvious explanations based only o...
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In this paper, the basic ideas behind emitter-coupled logic (ECL) gates are revealed step-by-step by applying heuristic means. First, the problem of saturation is considered and the possible remedies for preventing it are shown. Then, an equivalent electrical circuit of the basic ECL gate is suggested to show the basic ECL idea. Finally, a generic...
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In this paper, the operation of the basic emitter-coupled logic (ECL) gate is investigated by applying heuristic means. Five states of the input signal are considered: below the low voltage threshold, low input voltage, low-to-high transition, high input voltage and above the high voltage threshold. The circuit operation is visualized by specific h...
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The paper is dedicated to negative impedance converters with voltage inversion (VNIC). The operation of the circuit is investigated step-by-step and it is visualized by voltage bars and current loops superimposed on the circuit diagrams. Equivalent electrical circuits are used to explain the circuit operation at every stage.
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The paper reveals the secret behind the unique feature of the popular Wilson current mirror circuit to eliminate the Early effect that is inherent for the simple current mirror. The general idea is revealed heuristically by reinventing step-by-step the famous circuit. A negative feedback approach is used to explain the circuit operation.
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This paper reveals the secret behind the unique feature of the popular Wilson current mirror circuit to equalize the input and output currents. The general idea is revealed step-by-step by applying a heuristic approach. First, the disadvantages of the basic current mirror circuit are shown. Then, three possible solutions of the equalizing problem a...
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In this paper, a new educational approach for teaching negative resistance phenomenon is proposed. In opposite to the traditional approach, it relies mainly on human imagination and intuition. In the material proposed, negative resistance circuits are not analyzed as ready-made circuit solution by using formal explanations and definitions. Instead,...
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In this paper, a new educational technology for teaching the fundamentals of electronics is proposed. In opposite to the traditional approach, it relies mainly on human imagination, intuition and emotions. In the heuristic course proposed, circuits are not analyzed as ready-made circuit solution. Instead, first basic ideas behind circuits are revea...
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In this paper, first a general heuristic principle “conflict causes amplification” is revealed. For this purpose, a few analogies of the conflict phenomenon are generalized into a block scheme of a negative feedback follower disturbed from the output. Then the conflict principle is used as a tool for analysing various electronic circuits with dynam...
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In [1], a universal heuristic principle “conflict causes amplification” lying in the root of circuits with dynamic load was established. In this work, the conflict principle is improved in order to cover the more sophisticated circuits with controlled dynamic load. First, a general heuristic principle “dramatic conflict causes high amplification” i...
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The web-based building course on analog electronics (english and bulgarian versions) described in this paper is designed as a supplementary means to the classical analog electronics courses. It is implemented during the spring term of 2004 in the exercises with the students from the Faculty of Computer Systems and Faculty of Communication Technolog...
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The novel interactive multimedia tutoral described in this paper is designed as an alternative to the classical courses on analog electronics. It is intended for creatively thinking students, teachers and inventors in the area of electronics. Here, the electronic circuits are not presented as ready-made circuit solutions. Instead, they are built sy...
Conference Paper
Full-text available
In the classic electronics courses, the electronic circuits are presented in their complete, final and perfect form. Only, in order to really understand circuits, we need to know the relation between passive and active versions and how passive circuits have been converted into active ones. The novel interactive multimedia tutoral described in this...

Questions

Questions (128)
Question
It is well known that there is a linear relationship between the BJT collector and base current (Ic = beta x Ib). It would be interesting to explain it in an intuitive way. The power of such a "philosophical" approach is that it can explain various specific implementations.
The idea. In the BJT, the base-emitter voltage controls the collector current in an exponential manner. So, if we directly drive the base-emitter junction by a perfect voltage source, the bare transistor can be thought as an "antilog voltage-to-current converter".
If we drive the base-emitter junction by a voltage source through a resistor or by a perfect current source, we will control indirectly the base-emitter voltage... and it will control the collector current as usual. So the base-emitter junction serves here as a "log current-to-voltage converter".
The conclusion is that, in this arrangement, there are two cascaded (reverse and direct) non-linear convertors (see the attached picture)... and the overall relation between the output quantity (collector current) and input quantity (base current) of this "aggregate" is linear - Ic = beta x Ib.
Conceiving the idea. I came to this insight after analyzing in detail the BJT current mirror (see the attached picture) and so I realized the role of the input transistor T1 (the so-called "active diode"). Here is the evolution of my reasoning...
The key point here is that it is not the "active diode" that makes the current mirror... and it is not the one that provides the linearity; both they are provided by the humble second transistor. Indeed, it is not exactly a current follower; it is rather a linear current amplifier since Ic = beta x Ib = beta x Iin. That is why the input "active diode" is connected in parallel to the base-emitter "diode" - to divert the excess input current and leave only the small base current so Ic = Iin. It is not connected to ensure the input log conversion since the second transistor does it itself by its base-emitter junction; it is connected to equalize the input and output currents.
Generalization. This conclusion (linear relation) applies to every transconductance converter which input behaves as the opposite converter... or, in other words, it applies to every 2-port voltage-to-current (transconductance) converter which input behaves as a 1-port current-to-voltage converter. In our example, the BJT is an antilog transconverter (a "transdiode" controlled by voltage at one place that produces current at other place) which input is a log converter (a diode controlled by current that produces voltage at the same place).
Note the ordinary transconductance (voltage-to-current) converter does not obey this idea since it can have a true voltage input (with extremely high resistance). Its input must be conductive... and with the opposite relation. So we can emulate it by connecting (embedding) an external element (diode) in parallel to the input. For example, imagine the MOSFET transfer characteristic was exponential like BJT. The problem is that its input is extremely high resistive (open circuit). So we can connect a diode in parallel to the gate-source input to immitate the BJT base-emitter junction; thus the whole combination will act as a "BJT" with a linear relation Id = k.Iin.
Variations. It seems the opposite is possible as well but not so useful - to make a voltage-to-voltage converter by cascading a voltage-to-current and current-to-voltage converters. Interesting... it seems the op-amp inverting amplifier is such an example where R1 acts as the input voltage-to-current converter and the rest (R2 and the op-amp) acts as a current-to-voltage converter (transimpedance amplifier). The transfer ratio is linear. But it would be linear even we replace the ohmic R1 and R2 with nonlinear resistors (e.g., diodes)... a quite odd and somehow conflicting configuration... In this case we will apply input voltage and will obtain output voltage... and the relation between them will be linear.
I would like to know your opinion if this is a reliable viewpoint at the bipolar transistor and its current transfer characteristic I = f(Ib).
Question
In contrast to diodes (that maintain a relatively constant voltage when the current through them varies (https://www.researchgate.net/post/How_does_a_diode_maintain_a_constant_voltage), a fundamental property of all types of transistors (BJT, FET...) is to maintain a relatively constant current when the voltage across them varies. How do they do that?
As in the case of diodes, this question can be answered specifically by considering the processes in the semiconductor device. But again it would be interesting to explain this on a conceptual level by revealing the basic idea. As I have already said, this "philosophical" approach has several advantages: first, it does not require in-depth knowledge of semiconductor devices; second, it would be applicable to all 2-terminal devices that have this property. I will do this using the concept of "dynamic resistance".
Generally speaking, a transistor behaves like a resistor that interferes with current creating a voltage drop and heat loss. In the initial steep part of its output IV curve, this "resistor" has a relatively constant low resistance. And if it was really a resistor, the curve would continue in the same direction.
However, when the current threshold of the respective transistor is reached (set by the base-emitter voltage or current), the curve changes its slope and becomes almost horizontal. The voltage continues to change but the current stops changing. Why? Here is my simple explanation…
When the voltage V increases, the transistor also increases its static resistance R with the same rate of change. So, the current through the transistor I = V/R does not change.
This is a simple arithmetic trick where we change the numerator and denominator of a fraction in the same direction and with the same rate of change; as a result, the quotient of the division does not change (see the attached picture).
Thus, by changing its resistance in the same direction with the current, this "dynamic resistor" maintains a constant current. This clever "transistor trick" can be graphically illustrated (https://en.wikibooks.org/wiki/Circuit_Idea/Group_67b#A_transistor_acting_as_a_current-stable_resistor) - see the second attached picture below.
Tomorrow I have an online exercise on Semiconductor Devices with my students of group 48b, ITI, FCST of Technical University of Sofia, that is about transistor applications. Maybe I will illustrate to them this viewpoint by means of ZOOM pen drawing moving IV curves. And again, to get the attention of my students, I would tell them another fun story - that I could mimic any transistor they wanted with just a variable resistor ("potentiometer")... and that if I hid in a big box, they would think that there is a transistor inside:-)
The next day... Done! Here is a part of the video record (https://photos.app.goo.gl/EqonSNBjzT6Cw2Dj7, in Bulgarian) and the attached screenshot.
It would be interesting for me to know your opinion on my explanation.
Question
A fundamental property of all types of diodes (silicon, germanium, Schottky, LEDs, Zener ...) is to maintain a relatively constant voltage when the current through them varies. How do they do that?
This question can be answered specifically by considering the processes in the semiconductor PN junction. Also, it would be interesting to explain this on a conceptual level by revealing the basic idea. This "philosophical" approach has several advantages: first, it does not require in-depth knowledge of semiconductor devices; second, it would be applicable to all 2-terminal devices that have this property. I will do this using the concept of "dynamic resistance".
In the most general sense, a diode behaves like a resistor that interferes with current creating a voltage drop and heat loss. In the initial sloping part of its IV curve, this "resistor" has a relatively constant high resistance. And if it was really a resistor, the curve would continue in the same direction.
However, when the voltage threshold of the respective diode is reached, the curve abruptly changes its slope and becomes almost vertical. The current continues to change but the voltage stops changing. Why? Here is my simple explanation…
When the current I increases, the diode decreases its static resistance R with the same rate of change. So, the voltage drop across the diode V = I.R does not change.
Thus, by changing its resistance opposite to the current, this "dynamic resistor" maintains a constant voltage. This clever "diode trick" is illustrated by the graphical representation in the second attached picture.
During the last online exercise (25.11.21) on Semiconductor Devices with group 48a, ITI, FCST of Technical University of Sofia, I used the ZOOM pen to draw moving IV curves on the existing picture in Nicole's lab report; see the video record (https://photos.app.goo.gl/zhCDUVddKFA9RL4D8) of the ZOOM meeting (in Bulgarian) and the attached screenshot.
To get the attention of my students at this late class, I told them that I could mimic any diode they wanted with just a variable resistor ("potentiometer")... and if I hid in a big box, they would think that there is a diode inside:-)
It would be interesting for me to know your opinion on my explanation.
Question
Once visualized the voltages at the internal circuit points (https://www.researchgate.net/post/What_are_voltages_in_circuits), we can go even further and "enter" inside resistors to visualize the voltages across the resistive film. This will allow us to see some interesting points (for example, virtual ground), the operation of well-known electrical devices (potentiometer, resistive summer) and electronic circuits (inverting, non-inverting, differential and instrumentation amplifiers).
This idea came to me in the early 90's. Then, for the purposes of intuitive understanding, I began representing local voltages inside resistors by a set of vertical segments whose length (height) was proportional to the local voltage magnitude (I named it voltage diagram). This notion of voltage came from a hydraulic analogy (a garden hose with evenly drilled holes - https://en.wikibooks.org/wiki/Circuit_Idea/Walking_along_the_Resistive_Film#Hydraulic_analogy:_Pressure_diagram) which I had seen in an old Electrical Engineering textbook. See, for example, the attached yellowed sheet of paper from my archive where, in 1990, I drew the voltage diagram of a resistor voltage summer.
It is more convenient to display only the shell of the diagram. Thus a triangle is obtained in which the vertical leg is the voltage V, the horizontal leg is the resistance R, and the angle between the hypotenuse and the horizontal leg represents the current I (another geometric representation of Ohm's law, see the Flash picture below).
I have developed various kinds of voltage diagrams (https://photos.google.com/share/AF1QipM8_1gR4NooKALdwJ3E8bZpVelrWD-0gYnt_hjBzUSGRGuhtk2CErlxbEzKTyHT6Q?key=NDNTVTRjbFB2bUlnSWx2UW8xQ29jMzhKb3VwS3hn). I colored them in red (an association with pressure) to easily distinguish them from circuit diagrams drawn in black.
Later, I drew more sophisticated current pictures by means of Corel Draw. See, for example, the attached circuit of a resistor summer where the input voltage varies (I made it in the late 90's).
In 2000, I made Flash animated circuit tutorials (https://www.circuit-fantasia.com/collections/circuit-collection/circuits/old-circuits/v-to-i-old.html) with "live"voltage diagrams that were changing proportionally to the voltage magnitude. You need Ruffle Flash emulator (https://ruffle.rs/) to see this movie because Adobe Flash player is no longer supported.
But then I had the desire to make "real live" voltage diagrams that use actually measured voltages at several base points in the circuit. For this purpose, I used MICROLAB data acquisition system; see the Wikibooks story about Ohm's experiment (https://en.wikibooks.org/wiki/Circuit_Idea/Walking_along_the_Resistive_Film#How_to_visualize_the_voltage_diagram_on_the_screen) and the detailed 5 min movie (https://photos.google.com/share/AF1QipMgYJd1CrJPgfOmfm91AIwsFmNyrt9tD442-dS-JGyzhjoNRrbdcCsWlqx_GoB4BA/photo/AF1QipOFMh8_oVuXGqRv7ZNwt0lUjNDn05vRYkhGEJcM?key=WTg0Z2F3Z1NVYTZhR3VYOHVLWW5FclN1WW1pdEF3).
But, as in the case of current loops (https://www.researchgate.net/post/Where_do_currents_flow_in_circuits2) and voltage bars (https://www.researchgate.net/post/What_are_voltages_in_circuits), I finally came to the conclusion that content was more important than form... and I began to draw circuit diagrams with superimposed voltage pictures by hand. For example, in the Wikibooks story about Ohm's experiment (https://en.wikibooks.org/wiki/Circuit_Idea/Walking_along_the_Resistive_Film), I have drawn the voltage diagrams by color fiber pens on a white sheet of paper (see the attached picture).
As a conclusion:
What do voltage diagrams actually represent compared with voltage bars? The voltage diagram is a further development of the voltage bar representation. Voltage bars are a one-dimensional way of representing voltages while the voltage diagram is a two-dimensional way. The voltage diagram is a set of voltage bars.
The voltage diagram is based on a linearly distributed resistance along the length of the resistor. In most cases this representation is artificial, because ordinary resistors in electronic circuits are discrete; but it allows to illustrate the circuit operation with voltage diagram. In many cases, quantitative parameters (transfer ratios, etc.) can be directly seen.
What is the difference between a voltage diagram and voltage oscillogram at a given point in the diagram? The difference is significant, they have nothing in common. The voltage diagram shows voltage distribution along a resistance film, ie what local voltages inside a resistor are. The resistor is considered not as a "point" (without dimensions) but as a "line" (at each point of this line, in a certain step, a section perpendicular to the line is drawn, with a height proportional to the voltage at this point).
An oscillogram is a set of points that represent voltages (by a vertical displacement) at successive points in time through. So, the particular segments of a voltage diagram represents the voltages at particular points of the resistive layer at the same time while the oscillogram shows the voltages at one point but at different points in time.
As I have said in the stories about current loops (https://www.researchgate.net/post/Where_do_currents_flow_in_circuits2) and voltage bars (https://www.researchgate.net/post/What_are_voltages_in_circuits), a year ago, when we switched to online learning, I started using the ZOOM pen to draw on existing web circuit diagrams. This turned out to be a very powerful didactic technique that I managed to improve even more on the last exercise. I will show you how this happened at the final part of the last exercise (11.11.21) on Semiconductor Devices with group 49a, ITI, FCST of Technical University of Sofia. I make full video recordings of ZOOM meetings with my students and, with their permission, I will use a link to the latest record (https://photos.app.goo.gl/aB8Mfv7b3W9q2K9B8, in Bulgarian). I have attached also two snapshots of ZOOM whiteborad (really, pictures are not so beautiful... but attractive:-)
It will be interesting for me to know your opinion about this didactic technique. You can see this story in a more user friendly version in my blog: