Operational Amplifiers (Op Amps) | Analog Devices
Frequency Response of an amplifier or filter shows how the gain of the there is an important relationship between the use of these reactive components and the This range of frequencies, for an audio amplifier is called its Bandwidth, (BW). Transimpedance Amplifiers (TIA): Selecting the Best Amplifier for the Job . Operational Amplifier Gain Bandwidth Product (GBWP), Transimpedance gain .. application or other design advice, services or information, including, but not limited to, modify any individual TI Resource only in connection with the development. Q: What is a current feedback amplifier and what is its “black box” The well- known gain relationships remain unchanged: the inverting In contrast, in CFB op amp circuits the gain and bandwidth can be independently set.
Inverting amplifier In an inverting amplifier circuit, the operational amplifier inverting input receives feedback from the output of the amplifier. Assuming the op-amp is ideal and applying the concept of virtual short at the input terminals of op-amp, the voltage at the inverting terminal is equal to non-inverting terminal.
The non-inverting input of the operational amplifier is connected to ground. As the gain of the op amp itself is very high and the output from the amplifier is a matter of only a few volts, this means that the difference between the two input terminals is exceedingly small and can be ignored. As the non-inverting input of the operational amplifier is held at ground potential this means that the inverting input must be virtually at earth potential.
The feedback is applied at the inverting input. However, the input is now applied at the non-inverting input. The output is a non-Inverted in terms of phase amplified version of input. The gain of the non-inverting amplifier circuit for the operational amplifier is easy to determine. The calculation hinges around the fact that the voltage at both inputs is the same.
This arises from the fact that the gain of the amplifier is exceedingly high. Depending on the chip you're using, this may be harmless, or it could mean that there are volume settings where DC offset rises to harmful levels. You will have to crunch the numbers to find out if this will work. The Bottom Line The amp's gain and the values of the pot, input cap, input resistor, and gain resistors all interact with each other.
Lowering any of the resistor values requires that the other two be lowered by the same factor to keep all else equal. That in turn means that the input cap has to be raised by that same factor. You'll frequently find that changing one value means the required change in one or more of the others is impractical.
This careful balancing act is the soul of engineering. There are no truly perfect solutions in engineering, only holistically acceptable collections of compromises. The voltage from the pot is presented at the op-amp's noninverting input, the feedback resistors tell the op-amp how much gain you want, and the op-amp puts out what you gave it magnified by that gain.
So straightforward and neat. Oh, wouldn't it be wonderful if that's how things worked in the real world! In a real circuit, current doesn't just go from A to B, following the traces laid down on the printed circuit board.
Current finds other sneak paths past your feedback loop, too, such as nonzero power and ground impedances, parasitic components in the feedback loop, etc. And those are the easy ones. If things are really getting weird, you have still other paths, like RFI through the air.
Why does this matter? It all comes down to op-amp action and phase shift. The thing that makes an op-amp an op-amp is that it always tries to make its two inputs have an equal voltage. We call this op-amp action. The purpose of adding resistors and such in the feedback loop is to modify this op-amp action.
Let's see how op-amp action works to produce voltage gain. Imagine that we have 0. Imagine then that this voltage goes straight through the op-amp completely untouched: This voltage goes around the feedback loop, which contains a simple voltage divider in an amplifier like the CMoy pocket amp. Now the output is 1. We have voltage gain. Now imagine some force comes along — its nature doesn't matter here — and forces the op-amp's output voltage down a smidge.
Through the feedback loop, this forces the inverting input down, too. The op-amp sees that the inverting input is trying to go lower than the noninverting input, so again op-amp action comes into play, forcing the output upward to counteract our mystery force. This is why op-amps have such low distortion and low output impedance when operating in their normal range. Op-amp action forces corrective measures to be taken any time there is a deviation from the expected behavior.
The only reason op-amps have distortion at all is that we cannot create perfect op-amp action in the real world. Okay, so what about the real world, then? Let's imagine what happens if our amplifier is presented with a sine wave instead of a simple DC voltage. Let's also say there is some delay in the circuitry within the amplifier, such that it shifts the sine wave by degrees as it goes through the op-amp.
In a sine wave, degrees of shift reverses the voltage: Let's say that the sine wave has 1 V peaks, and that a positive peak is at the amplifier's input. Due to the delay within the op-amp, that means that the output is only now putting out what happened degrees ago, which is -1 V. For now, we'll ignore gain and put -1 at the inverting input, too. The two inputs are unequal, so op-amp action kicks in to try and correct it.
The noninverting input is higher than the inverting input, so the op-amp tries to raise its output voltage a smidge. But because there is degrees of phase shift, this actually results in the output going down by a smidge instead! The cycle repeats endlessly. Congratualations, you have created an oscillator. So obviously it would be Really BadTM to make an op-amp with degrees of phase shift. But in the real world, we cannot make an op-amp with 0 degrees of phase shift.
Real op-amps always have some phase shift. The difference between the amount of phase shift and degrees is called the op-amp's phase margin. If nothing else in your circuit adds a delay, you can get away with 0. But life isn't that simple, of course. Remember all those sneak paths I listed above that avoid your carefully planned negative feedback loop?
If conditions are right, any of these can set up a positive feedback loop and create an oscillator.
Inverting & Non-Inverting Amplifier Basics | Online Learning Corner
In a real op-amp, phase shift varies depending on frequency. Since real signals like music are composed of many wanted frequencies plus unwanted ones like noise, the way a circuit reacts to a signal can be very complex.
A circuit can operate without any oscillation in some circumstances, but if just the right frequency enters into the circuit, it can take off into oscillation. As a rule, the faster an op-amp is, the less phase margin it will have.
Furthermore, wider bandwidth means there are a greater range of signals that the op-amp pays attention to. If you present an 8 MHz amplifier with 21 MHz noise, it's more or less going to ignore it. But if you present that same noise to a MHz op-amp, it will amplify it right along with the rest of your signal. If that magnified noise happens to be of just the right frequency to trigger oscillation, your choice to use a faster chip will be fateful.
How to Fix It, Part 1: Power Supply Bypassing An ideal power supply has zero impedance. It would be a perfect voltage source.
Op-Amp Bandwidth, Gain Bandwidth Product & Frequency Response
The same goes for your ground: In the real world, the power supply and ground always have a nonzero impedance. We try to get as close to zero as possible, but it is not possible to construct a perfect voltage source. This means that your circuit's varying current draw sets up tiny voltage ripples in the power and ground signals. It's easy to see why the stability of the ground path matters: As for the power rails, an op-amp doesn't have infinite power supply rejection ratio, so some ripple on the power rails makes it into the op-amp's output, and so can cause oscillation.
One way to lower the power and ground impedances is to add bypass capacitors. Capacitors are energy storage devices. When you place them in parallel with a power circuit, the capacitor tries to smooth out any ripples in that circuit. The art of using bypass capacitors is subtle, however.
You can't just throw a bunch of caps on the board and expect all the oscillations to go away. The reasons are complex. First, the opposite of capacitance is inductance, and all wires have inductance. Making wires shorter and thicker reduces inductance, but it has practical limits.
Because inductance counteracts capacitance, too much wire between a bypass capacitor and what it is supposed to be bypassing will render that cap ineffective.
relation between gain & bandwidth. - The Student Room
Therefore, a bypass capacitor should go as close to the device being bypassed as possible. For critical work, you must even consider the length of the capacitor's lead. This is one reason why surface mount technology has become so popular: That multi-gigahertz computer on your desktop wouldn't be possible without the miniaturization afforded by surface mount technology.
You simply could not build the same circuit twice the size and have it work: The most popular bypass capacitors are 0. For op-amp audio circuits, it's best to add two per op-amp, one from each power rail to ground. The leg going to ground can be as long as it needs to be; get the other leg as close to the power pin as possible.
Because the effectiveness of capacitors for bypassing relative to frequency goes up as the value goes down, you may need to go down to 0. You can also use film capacitors instead of ceramics.
Ceramics are better at high frequency, but films have higher linearity, which makes audiophiles happy. The linearity is of no value if the circuit doesn't work because the bypass capacitor is ineffective at the problem frequency, however, so don't rule out ceramics.
A nice compromise is the C0G or NP0 type ceramic, which has nearly as high a linearity as a film cap but has the high speed advantages of a ceramic. Also helpful can be some larger tantalum capacitors.
They don't handle the higher frequency noises, so they don't have to be as close to the chip as the ceramics. As with the ceramics, it's better in analog circuits to use two caps from ground, one going to each power rail. But, you might get away with using just one from rail to rail.
You can let one big tant or a pair of them serve several ICs. Bigger is better, especially if each IC doesn't have its own tant sbut don't get crazy. How to Fix It, Part 2: Bandwidth Limiting You'll recall that one of the factors increasing the risk of oscillation is excess bandwidth.
They're right that it has enough bandwidth, and sufficient slew rate to pass a decent audio signal, as long as you don't load it down too heavily. But if you listen to your headphones instead of those engineers, you will hear something very different: When we use a modern high-speed op-amp like the MHz LMx for audio, it's not because we need to amplify signals beyond the 20 kHz audio bandwidth or because we need unholy fast slew rates. What we're more after are things like the chip's higher precision and its ability to drive low impedance loads.
Chips with a wide closed-loop bandwidth also tend to have a wide region of flat open-loop bandwidth, which means the feedback factor is linear over a wider range of the audio bandwidth. These and other features add up to lower distortion in that critical lower 20 kHz.
It's possible to have our cake and eat it, too. If you drop a small capacitor across the feedback resistor R4 in most of the amps discussed on this sitethe gain of the amp starts dropping as the frequency rises. You can use the RC filter equation above to determine this filter's -3 dB point.
Since voltage gain is almost always a factor in the onset of oscillation, this bandwidth limiting cap — also called a phase lead cap — can cure oscillation.
Let's work an example. Let's try a 10 pF bandwidth limiting cap. The RC filter formula above tells us that the op-amp will be acting like an RC filter with a corner frequency of about 26 kHz with these values. In other words, the amp's gain is reduced by 3 dB from its nominal value at 26 kHz. That's not far above the audio bandwidth, so we might choose a slightly smaller cap value, or we might somehow finagle smaller resistor values to allow us to use a larger cap value.
Putting the corner frequency somewhere in the 30 kHz to kHz range is probably best. Since this cap is directly in the audio path, quality matters. The absolute best caps for audio use are polypropylenes, but the smallest I've ever seen are 33 pF, and those are hard to get. Next best are polystyrenes, which are made in all values suitable for this purpose, but they, too, are hard to get.
The third best choice isn't bad at all, and they're pretty easy to find: If you cannot get those, or you cannot afford the board space they take, there's only one level lower you should ever go here: These two types are the same.
Never use any other ceramic type in the audio path. Change the circuit another way to avoid oscillation before you break down and use, say, an X7R ceramic in the audio path.
There is an important situation where a bandwidth limiting cap will make things worse instead of better. Because a bandwidth cap rolls off the amp's gain toward 1 at high frequencies, it doesn't work correctly with op-amps that aren't unity gain stable.
For instance, the NE requires a gain of at least 3 to be stable. Using a bandwidth limiting cap with such an op-amp will frequently create an oscillator. How to Fix It, Part 3: Things to Try Before Resorting to Voodoo If the op-amp still oscillates, try these things, in this order: Add a small resistor to the op-amp's output, either inside or outside the feedback loop.
I prefer putting it inside the feedback loop, because op-amp feedback will counteract some of the bad effects of having the resistor, yet the resistor will remain effective. Others recommend putting it outside the loop, so that it can protect both the OUT and the -IN op-amp pins.
This raises the amplifier's output impedance, though, which is not without consequences. Do the same as in the previous step, except use a ferrite bead or chip ferrite instead of the resistor. At low frequencies, the resistance of a ferrite is basically that of the wire going through it.
The resistance rises as frequency rises, and it's at high frequency that oscillation usually occurs.