Electronics is Fun

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 th...

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...

School has probably taught most of us that an op amp is a black box with a set of ideal and non-ideal characteristics. We often easily derive the output equations using ideal assumptions, which includes infinite open loop gain, zero input offset, zero input bias currents, and zero output impedance, etc. But in order to fully appreciate its capabilities and limitations, it helps to know why they came about in the first place. And it is primarily because its use with negative feedback. It always helps me to think that op-amp is basically a device that takes a differential input and amplifies it with its large open loop gain. One can't go wrong with the inputs that are always labeled with positive and negative. A small differential input would drive the output large positive or negative depending on the polarity of the differential input. Negative feedback is what makes the op amp very useful in applications such as amplifiers and signal conditioning circuits. The follow...