OP AMP Spec - noise gain configuration?? - Page 1
Forum section: electronics / beginners

author: innkeeper
author_rank: Unknown
timestamp: May 10, 2019, 01:33:51 pm
content: I ran into something on a spec sheet for an opamp that I do not understand what it is or what it means. "noise gain configuration"What are they referring to when they talk about "noise gain configuration"? Can someone explain this to me?from the spec sheet:The LTC®6090/LTC6090-5 are high voltage, precisionmonolithic operational amplifiers. The LTC6090 is unitygain stable. The LTC6090-5 is stable in noise gainconfigurations of 5 or greater. Both amplifiers feature highopen loop gain, low input referred offset voltage and noise,and pA input bias current and are ideal for high voltage,high impedance buffering and/or high gain configurations.

author: Zero999
author_rank: Super Contributor
timestamp: May 10, 2019, 03:09:25 pm
content: https://www.analog.com/en/analog-dialogue/raqs/raq-issue-91.htmlhttps://www.analog.com/media/en/training-seminars/tutorials/MT-033.pdfhttps://www.edn.com/electronics-news/4054393/Noise-gain-influences-op-amp-choice

author: Zero999
author_rank: Super Contributor
timestamp: May 11, 2019, 08:55:52 pm
content: Here are two op-amp circuits. Both have a unity inverting signal gain.A basic inverting amplifierAn inverting amplifier with 30k resistor connected between the op-amp's inverting input and 0V.At first it may seem like the one with R3 aught to have a lower gain and it does by a small margin, especially at high frequencies, but the inverting input is a virtual ground node, so it's not that straightforward. There are two effects which cancel one another out.R2 and R3 form a potential divider, which should divide the input signal by 5. VOUT= VIN*R3/(R2+R3) and when VIN= 1V:VOUT= 1*30k/(120k+30k) = 0.2V.But the output impedance of the potential divider is equal to the value of the two resistors in parallel.ROUT= (120k*30k)/(120k+30k) = 24kThis can be simplified to an equivalent circuit, showing 0.2V in series with 24k.When connected to the op-amp circuit, the gain is now 5.So we have an attenuator dividing the signal by a factor of 5, connected to an op-amp with an inverting gain of 5, giving a total gain of 1.Now, imagine the input is connected to zero and a noise voltage is injected into the inputs. To simplify matters assume it's the non-inverting input. Say the noise voltage is 10nV. The circuit is now a non-inverting amplifier. Remove R3 and it has a gain of Av = 1+ 120k/120k = 2, keep it and you have a gain of 6. So assuming there are no other noise sources, the output noise will be 20nV, without R3 and 60nV with it in place.So why might an op-amp only be stable when the noise gain is over a certain figure?The op-amp has phase shift and if there's still gain, when the phase shift is 180º it will oscillate, since negative feedback, will become positive. The resistors in the feedback network, attenuate the op-amp's output, before it's fed back into the negative input. If the op-amp has a gain of 5, when the phase shift is 180º, then it will oscillate if the resistive divider attenuates the signal by less than a factor of 5. In the circuit with the noise gain of 6, but signal gain of unity, the resistive divider is attenuating the output by a factor of 6, so it won't oscillate, if the op-amp has a gain of 5, with a 180º phase shift.In practice, the circuit can still ring, even if the gain us under unity, when the phase shift is 180º, as it will be under-damped, so op-amps are designed to have a phase margin or less than that, so they're over-damped.To reduce the noise at lower frequencies, yet still avoid oscillation, R3 can be connected to 0V, via a capacitor. Here's a demonstration of a noise analysis. I've made the input noise of the op-amp stupidly high, at 10mV, (en = 10mV) so it dominates. At low frequencies the noise gain is 2, at higher frequencies it's 6, followed by a roll-off at much higher frequencies, due to the op-amp running out of gain.

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2019, 04:02:27 am
content: The simple answer is that the noise gain is equal to the non-inverting gain of the circuit. So an inverting configuration with a gain of -1 has a noise gain of 2 if it was treated as a non-inverting amplifier. A unity gain follower is an non-inverting amplifier with an infinite shunt resistance so its noise gain is 1 and therefore the least stable configuration.

author: Zero999
author_rank: Super Contributor
timestamp: May 12, 2019, 08:42:42 am
content: A simulation showing an op-amp, which isn't fully unity gain stable. It rings when the noise gain is 2, but is made stable, with a noise gain of 12, at high frequencies. To minimise noise, R3 is AC coupled, so the noise gain only increases at high frequencies.The above circuit's step response. It's fully stable, no ringing, with a noise gain of 12.The above circuit with C1 and/or R3 deleted. It's no longer stable and rings on the edges of the square wave.

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2019, 09:48:39 am
content: Quote from: Zero999 on May 12, 2019, 08:42:42 amI have done this exact thing to use OP-37s in OP-27 circuits when I was learning and this brings up the disadvantage over other methods. The output noise really is proportional to the noise gain so this is a method of last resort. It is almost always better to use a suitably compensated operational amplifier instead.One example where it might be done is with an audio power amplifier. They are usually designed to operate at a minimum closed loop gain and if you wanted to decrease this, then raising the noise gain will allow it at the expense of *not* decreasing the added output noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2019, 09:52:24 am
content: Noise gain is, quite simply, the amount of gain between output voltage and input noise.Basically it's the gain ratio of the feedback network and topology, which is also the signal gain.Signal and noise gains don't need to track: for example, a resistor between input pins does not affect signal gain, but increases noise gain.They are not talking about a "noise gain configuration", but a configuration in which the "noise gain" is so-and-so. The note is important because reduced-compensation amplifiers offer much greater GBW from essentially the same device, but are not stable at low gain (in this case, below 5). They are chosen when you need a modest amount of gain (say 5 to 100 or more), and can't afford the cost* of a conventional (unity-gain-stable) amp with the same GBW.*Cost, broadly speaking. Faster amps may consume more supply current, or input bias current, or have more input offset or noise, or are more restricted in voltage ranges, or... In short, more things that you have to consider in your design, which may lead to real knock-on costs (added components, worse performance, etc.).Note that, since you can increase noise gain independently of signal gain, it's still possible to use such an amp at unity gain, if you really need to. But the noise is (by definition) that much worse, so you would rarely do this in practice.Tim

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2019, 10:38:13 am
content: Quote from: T3sl4co1l on May 12, 2019, 09:52:24 amSometimes the only difference is less Miller compensation capacitance so they could in principle be less expensive (capacitors take up a lot of space) but in practice do not benefit from economy of production because demand for decompensated parts is so low. Externally compensated operational amplifiers can be compensated to operate ideally at the closed loop gain but nobody likes the inconvenience and extra bandwidth is not always desirable anyway; it also means more noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2019, 11:20:40 am
content: I assume they use the same die but cut off some of the Miller comp, and maybe a few resistor tweaks to fine tune the second pole. And yeah, the quantity sucks, so they often aren't actually much cheaper at all, a shame.Tim

author: David Hess
author_rank: Super Contributor
timestamp: May 12, 2019, 11:29:18 pm
content: Demand for the decompensated OP37 or LT1037 is so much lower than the OP27 or LT1007 that it would not be economical to make a separate die. At least by sharing the same die, the OP37 and LT1037 gain some economy of scale in production; otherwise they would be even more expensive. It seems National did the same thing by making their LM308 share thesame die as their LM11.

author: Zero999
author_rank: Super Contributor
timestamp: May 13, 2019, 08:26:27 pm
content: In most low gain applications the extra noise isn't a big deal, since the signal is already quite strong anyway.For completeness I've added the unity gain buffer, because it's something which isn't immediately obvious to most people. R2 is now the value of R2 and R3 in parallel 10|100k = 9k1, to the nearest E24 preferred number.One thing to be wary of is the impedance of V3, at high frequencies, can't approach R2. Ideally R2 and R1 should be much higher. Another option is to connect C1 to 0V, but then the circuit gain will rise above unity at higher frequencies, so a low pass filter would need to be added after V3, if that's a problem.That's not a problem, since the inverting input is still connected to the non-inverting, at high frequencies, via C1, irrespective of the impedance of V3.