How OpAmps Work - The Learning Circuit

the following program is brought to you by element14 the electronics community where you can connect and collaborate with top engineers from around the world join now at element14.com slash presents hi and welcome back to the learning circuit where we learn about basic electronics today we're going to learn about operational amplifiers more commonly called op amps [Music] an operational amplifier is a high-gain DC voltage amplifier meaning it can use a very small voltage difference between its inputs to control a much larger voltage at its output typically hundreds of thousands of times larger op amps come in three basic package types dip with through-hole pins surface mount and teo 5-8 round metal cans a package may contain 1 2 or 4 op amps internally it is made up mostly of transistors and resistors a couple capacitors and sometimes some diodes are thrown in for good measure a typical op-amp has two inputs inverting and non-inverting two DC power supply leads positive and negative an output terminal and sometimes a few other specialized leads used for fine tuning the two DC power supply inputs usually determine the voltage limits of the outputs they may connect to positive voltage and ground as we usually connect our circuits or you may see they connect to matching positive and negative power supplies one way to achieve this is by connecting a voltage divider made of resistors between a single power supply and the op amp by itself and abuse operation is simple the two voltages at the inputs are compared and that determines whether the output saturates towards the positive supply voltage or the negative supply voltage let's say our op amp is connected to a nine volt power supply the two inputs are non-inverting labeled positive and inverting labeled negative if the voltage at the positive input is larger the output would go towards the positive supply voltage nine volts if the voltage at the negative input is larger the output would go towards the negative supply voltage ground or zero volts when the output reaches either of these points it reaches saturation and is at the maximum rated gain gain is what we call the ratio or multiplier between the signal at the inputs versus this signal at the output op amps are high gain devices meaning the difference between the two input voltages the input differential can be very small that's what makes an op amp so powerful a very small voltage change can be to control a very large voltage change one of the most common op amps is the 741 looking at the datasheet the typical gain is 200 volts per millivolts doing the math that comes out to a gain of 200,000 so if our circuit runs on a 9 volt power supply that means a differential of only 45 micro volts is required at the inputs to output volt that's not very much so if an op amp is used in a circuit where the differential voltage is not finely controlled the output pretty much only goes full throttle towards the positive or negative supply voltages that is the case in an open-loop circuit like this virtually any voltage at the input causes the output to fully saturate in this configuration the op amp works great as a comparator comparing the two input voltages and effectively switching the output between high and low as a comparator an Optima could be used to make an AC to DC converter turning an AC sine wave into a DC square wave with one input connecting to AC as the wave flows the output would switch back and forth between saturating positive and negative converting AC to DC but where the op-amp really shines is when the gain can be controlled for this we need to create a feedback loop putting the op-amp in a closed loop configuration feedback is achieved by connecting the output back to one of the inputs this is most commonly done with the inverting input creating negative feedback so the voltage at the inverting input always equals V out as the voltage at V in changes the voltage at V out also changes and the loop self-corrects keeping the voltage at the inverting input near equal to that at V in but just different enough to keep the micro volts differential needed to generate an output so effectively V in equals V outs oh that looks like good equal sign equals not great equals so what's the point of this with the output looping straight back to the input the op-amp acts as a buffer op amp's have high input impedance but low output impedance electrical impedance is the measure of a circuit or components opposition to changes in electrical current using an op-amp as a buffer can prevent the signal source from being affected by load currents but we want control of the gain so we introduce a feedback resistor to the loop changing the value of the feedback resistor changes how much of the output voltage gets fed back to the input effectively controlling the amount of amplification between the input and output if the active signal is connected to the inverting input the signal is amplified but the polarity of this signal is reversed up becomes down and down becomes up but if the active signal is connected to the non-inverting input the signal is amplified but maintains polarity by adding more resistors or capacitors to the circuit abuse can be used to create a number of different computational circuits an adder or summing circuit like this can be used to combine multiple input voltages into a single output voltage summing circuits can be used to make an audio mixer or to convert a digital signal like logic level binary or BCD to an analog signal while most types of op-amp circuits have one input grounded a subtractor or differential amplifier circuit has signals connected to both inputs the voltage difference present between those two inputs is then amplified differential amplifiers can be used to make a Wheatstone bridge or a sensor activated circuit like this one that uses an LDR by replacing the feedback resistor with a capacitor an op-amp can be used to make an integrator an integrator op-amp produces an output voltage that is proportional to both the amplitude and the duration of the input signal integrators can be used to make analog to digital converters and wave shaping circuits by swapping the resistor at the input with the feedback resistor we can make a differentiator sir get where the output is approximately directly proportional to the rate of change the time derivative of the input an active low-pass filter is an example of a differentiator amplifier op amps can also be used in place of a 555 timer to create an a stable multivibrator our resistor and capacitor are connected to the inverting input and a voltage divider is connected to the non-inverting input in order to generate a rectangular output waveform to create a monostable multivibrator circuit the capacitor is given a discharge path to ground to create a one-shot timer where the output generates one rectangular pulse per input trigger the op amp is an extremely useful component but it's important to realize that op amps don't create power out of nothing they don't take a small signal and amplify that into more power you can't take one volt and magically turn it into 100 volts an op amp doesn't create power it just controls the voltage it's already supplied now I just barely touch the surface of what there is to know and learn about op amps I wanted to give you a general idea of how they work and how they can be used but there's so much more if you're familiar with op amps tell us about it what's something weird or interesting you've learned in using them or what's a problem you've come across that you think would be helpful to share post your stories comments questions and anything else you want to say about op amps on the element14 community on element14.com forward slash the learning circuit happy learning [Music]

Channel: element14 presents

Views: 212,618

Keywords: electronics, hardware, gaming, hacking, mods, weekly, element14, maker, engineering, element14presents, Feedback, Negative feedback, gain, Inverting, Non-inverting, Differential, Differentiator, Adder, Summing, Subtractor, Multivibrator, Astable, Monostable

Id: kbVqTMy8HMg

Length: 8min 45sec (525 seconds)

Published: Wed Mar 04 2020