Electronics is Fun

Below is a simple circuit that demonstrates the use of a transistor.  The transistor drives an LED from a photocell at the input bias circuit. The intensity of the LED light depends upon the intensity of the light from the environment (ambient light) hitting the photocell. The greater the  ambient light, the greater the LED intensity. If light is totally blocked on the photocell, very little or almost no glow coming from the LED. Figure 1                                         This simple circuit can be expounded for many exciting applications in the real world. The photocell is just one of representations of sensors one can use to control the output of the transistor.  The output can be other than an LED, say a motor whose speed can be variably controlled, or it can be an input to another circuit that does some other functions that depends on the input from the sensor. Basic Operation: The photocell has resistance that ranges from 5k (strong light) to 20k (dark).

In the first part of the series we made an introduction about the noise and how we can categorize them into device noise, emitted noise, and conducted noise. We first discussed about the most common noise, the Johnson noise coming from a resistor . This noise is also a white noise, due to the fact that it is present at all frequencies. This noise is produced whether or not a resistor is connected in a circuit. When a resistor is connected to a circuit, say to an amplifier, this resistor will be a source of noise to the circuit. Let's take an inverting operational amplifier as an example that has two resistors connected to it, one at the input and the other one as feedback. The noise due to the resistors can be modeled as noise sources in series with the resistors. Figure 1 Resistor Noise In order to appreciate its effect in the circuit, we must be able to understand how these noise affect the behavior and output of the amplifier. Modeling the noise sources as in the f

I am back from a little hiatus. I've been meaning to write something about noise in the circuits, because I know it's one of the least understood topics in electronics. It's something that one can easily attribute to when something goes odd in the circuit, yet so few have the courage to delve into it. In audio applications, it's easy to describe what noise is. We know if the music is clear and pure, it is free of noise. Anything that is unwanted we can call noise, well, in a broad sense of the word. Noise comes in different forms and from different sources, but in the hope of making some order out of chaos we can attempt to categorize them in the following: 1. Device Noise - noise coming from active and passive devices 2. Emitted Noise - those coming from power lines, motors, radio trasmission, etc. 3. Conducted Noise - those that are already in the circuit, either from the device or transmitted into Electronics components are indeed sources of noise in the ci

With the availability of many modern instrumentation amplifiers today, it has become quite an easy task to build a very precise current source. Current source is an important electronic building block that is supposed to deliver a constant defined current regardless of the load. If to a voltage source the low output impedance is the important characteristic, high output impedance would be for the current source. High output impedance means that as the load voltage changes due to current being sourced or sunk, there is very small change to the current. Ideally we would want the current to remain unchanged, because it is what it is supposed to do, to be a constant current source. My Digilent Discovery Kit includes an AD8226 in-amp, and together with a buffer, OP02 , also included in the kit, I made a very simple but very precise current source.  What I like about this in-amp is it is a low-power, wide input and supply range instrumentation amplifier at +/-1.35 to +/18V, very flexib

by DarwinT Well I've been taught in school only one thing about the bandwidth of an op amp, or perhaps it just didn't register enough in my brain that there's another kind of bandwidth, and all of a sudden I hear this large signal bandwidth, or sometimes referred to as the full-power bandwidth, about op amps. At first I thought that this "large signal bandwidth" thing should be pretty obvious and a no-brainer, until I got confused some and I realize it can't be ignored anymore. That I already know, and I wanted to know more about it. How come nobody explained this thing to me! When you look at an op amp's datasheet (e.g. ADA4895-1 ), the manufacturer often publish the bandwidth on the front page, otherwise it can always be found in the collections of plots inside. It's basically the unity gain frequency of the open loop gain of the amplifier. What is not so obvious is that this doesn't apply to all level of signals you intend to use your amp

Ever saw an op amp circuit oscillating and behaving weirdly? We basically have just two options: give up to the idea it's not really going to work and try another circuit, or the better option would be is to figure out what is wrong and make it work. As some well known author has said, every op amp sits there waiting to oscillate [1]. I discussed in one of my articles here, titled What's The Feedback Around Op Amp All About?, the concept of negative feedback and how the large open loop gain makes the overall closed loop gain stable and predictable. It is when this negative feedback becomes not negative anymore, that is, if it becomes positive, that the circuit becomes unstable and starts to oscillate. Figure 1 General Feedback Topology How can that possibly happen? To put it in simplest terms, it happens when enough phase shift is introduced in the complete feedback path which includes the amplifier itself, enough to make the feedback turns180 degrees around, thus ma

This is the last installment of the grounding series articles I have posted. In Part II of this series, we have explained that star ground provides a single or common reference point to a circuit in an effort to avoid sharing of paths that could cause errors and degrade the integrity of the signals. This is accomplished by selecting a single "mecca" point where all ground return paths meet, and that this point should be a low impedance node. But this scheme is not always practical and doesn't solve all the woes of circuit grounding. Long routes going to the mecca point can itself be a source for error as it becomes inductive to high frequency signals. The rule of keeping the routes or wires as short as possible applies to grounding as well. The other scheme that we have described is the use of ground plane, because it presents much lesser resistance and inductance compared to the individual wires and traces. But we have cautioned that there are two paths the signal

Early this year I got my Digilent Analog Discover Kit . It is a wonderful PC based design and experiment kit complete with software measurement and resource tools perfect for students and professionals alike, but students surely will benefit the most out of it.The complete Discovery kit includes many useful exciting components that will enable you  to design your own circuit in your living room! One cute little component it has is the AD654 , a low-cost IC voltage-to-frequency converter. It is a very simple to use IC that outputs a square wave with programmable frequency, set by just a single RC network. The scaling relationship as obtained from its datasheet is: Below is a simple V-F circuit that takes its input from another IC included in the kit, the AD584 , a precision programmable voltage reference. Figure 1 V-F Converter The AD654 accepts full-scale current up to 2mA, but the best linearity is offered at 1mA full-scale. The voltage input is converted to a dr

Building circuits whether on a PC (Printed Circuit) board or a breadboard, grounding have always been important. Careful grounding becomes critical on applications where performance and integrity of the signals are of prime importance. Examples are low-level application such as in audio, in precision sensing and measurement circuits, and in mixed analog and digital circuits . When do we use star ground and ground plane? What is the difference between the two? Which is actually better? We will try to answer first what the difference between the two is. Star Ground Also sometimes called mecca ground, this is based on the concept of creating a single return point in the circuit as a common reference in order to avoid ground loops and thus circuit errors (See figure 1 [1]). In the figure, we could see the input signal return path is shared, assuming very high input impedance and no significant current flowing in it. Those in the path of significant current flow, such as the power supp

Below is a basic op amp configuration of converting a square wave input to triangular waveform. Figure 1 The concept is pretty simple. The square wave input produces a current that is translated to a voltage ramp across the capacitor. The positive cycle of the input produces a negative ramp output, while the negative input produces a positive ramp. The ramp is governed by the following basic equation: Equation 1 where Ic  is produced by the input voltage divided by Rin, since the same current flows in the resistor and in the capacitor. Note that the input has to be centered and symmetrical about the positive input terminal which is connected to ground, in order to produce a symmetrical output. But this basic configuration has some serious problems. Even if the output starts at zero, it will not be centered at zero, and will not stay at the same center or common mode value due to the inherent offsets of the amplifier. Figure 2 A technique in order to hold the ou

I was once asked by a friend/colleague if it is alright to pick any of the grounds in the circuit to make a measurement. I answered it depends on what you are trying to measure, and I reminded that what he calls ground is supposedly a "reference" to his circuit, and will be a reference to his measurement. There will be multiple "ground" points in the circuit, and not all of them are created equal. What we call ground is ideally zero volts but this is often confusing, because there really can only be one point in the circuit that is truly zero volts. Given a ground plane or ground traces in our PC board, they can actually be at varying potentials due to errors introduced in the circuits by what we call ground loops. Figure 1 Ground Loop There will be errors unique to each individual ground points. These errors may or may not affect your circuit's performance depending on the applications. We can perhaps get away with our little concept of ground being

We see these bypass or decoupling capacitors on the supply pins, and maybe we wonder what if we just take them out so we can save a few components on the circuits. Or perhaps out of plain ignorance we forgot about it, and I have seen people do that and be faced with seemingly impossible problem to contend with, like outputs oscillating, or behaving weird even in the absence of inputs. What could be wrong? Only to find out at the end of the day that they missed something critically important in the circuit. So what do they do really? They provide a low impedance path to the ground for noise and any ac components that might be present on the power supply lines. For example, a sudden current change in an IC can cause a large glitch on the supply. The capacitor will help prevent that by providing immediate current requirements. In this manner it helps maintain the supply node to be low impedance. Being low impedance means it can source a varying amount of load current and being able t

Basic Integrator If you strap a capacitor around an op-amp as a feedback in an inverting configuration, instead of a resistor, you wind up with an interesting basic circuit called an integrator. The name comes from the fact that the circuit basically does a mathematically integrating function, expressed in the output transfer function. This circuit can be very confusing to many. I think the best way to understand it is from a practical point of view, by way of analogies and equivalent circuits. And once you are able to get a mastery of its functions and behaviors, there’s a lot of interesting opportunities where the basic circuit can be useful. Current Flow and Output Equation The output is a derivation applying KCL (Kirchoff’s Current Law) at the amplifier’s summing node, and the op-amp’s ideal assumptions namely that the positive and negative terminal are at the same potential, and no input bias current flows. The voltage input creates a current in the resi