Question on input noise current

[john curl]

Folks, let's not complicate the issue. Noise figure is mostly useless. I know, I know, you have been told that it is important, but for audio, with today's circuits, it is mostly useless. However, an IC op amp works just like an individual transistor, or fet, except you have to add the effect of the op amp topology, to get the answer to the noise produced.
If you look at an IC op amp data sheet, you will all be able to find the VOLTAGE NOISE of the op amp. If the op amp is FET input, that is all you need to know.
If the op amp has a bipolar input, then you HAVE to pay attention to the CURRENT NOISE, because the current noise will always be pretty high. This is BECAUSE the voltage noise is low, and that implies the base current is relatively high. Of course, extremely hi beta input devices will lower the base current, BUT there is always the trade-off with base resistivity, that would increase the VOLTAGE NOISE, if taken too far. This is what designers live for. The trade-offs in design for best performance.
So you have to look at the data sheet for the current noise contribution and realize that any source over 1Kohm and even 100 ohms in many cases is going to see the current source noise contribution as important as the voltage noise. More later.

[john curl]

I looked at the AD797 and found the current noise. It is 2pA. The voltage noise is about 1nV. Now, 2pA multiplied by 500 ohms is 1nV, so the best noise figure would be about 500 ohms and it would get significantly noisier if used with a higher source impedance. A 2SK389 can do better than this, but it needs a higher operating current to work well.

[MarcelvdG]

SY,

The problem with base current compensation is that often a single transistor is used to generate the compensating current for both inputs. Often, this transistor has a smaller size and smaller collector current than the input devices. An amplifying current mirror with two outputs is then used to get the right compensating currents.

For example, suppose the input transistors are each biased at 1mA of collector current. You can then compensate for the base currents using a ten times smaller transistor biased at 100uA and a current mirror with two outputs with ten times gain. However, in this case, the base current shot noise from the transistor biased at 100uA and the noise from the input transistor of the current mirror will be amplified and injected (fully correlated) in both inputs.

Measure with equal impedances driving the + and - inputs and you don't see this common-mode noise current. Use unequal impedances, as in most real-life applications, and this current noise term will be much greater than the base current shot noise of the input transistors.

An LT1028 is a good example. The noise current is specified as 1pA/sqrt(Hz) with equal impedances, but you should count on 3.25pA/sqrt(Hz) with unequal impedances...

[SY]

Ah, thanks Marcel, that clarifies things for me.

[PMA]

In most of practical audio circuits, with OA network resistors of 2k - 10k and source impedance (or input series resistor) >= 1k FET input OpAmps like OPA627 give better overall noise than bipolar ones like AD797.

[john curl]

Thanks Marcel, the 797 most likely has bias compensation, because the voltage and current noise does not track the bias current specified.

[Fred Dieckmann]

Bias compensation for op amps is very common the get the both speed and DC precision required for op amp applications.

For an input transistor have 1 mA to have the typical input bias current of 0.25 micro amps (with the AD797), the Hfe would have to be 4.e9 for an uncompensated. This is a good indication of how well the bias current compensation is implemented on this part, This is a reduction of the uncompensated bias current of about 7 orders of magnitude and also has to track extremely well with temperature and common mode voltage changes. Some pretty good engineering!
http://www.analog.com/library/analog...versary/6.html

[MarcelvdG]

Fred,

Don't you mean that hFE would have to be 4000 (rather than 4E9) to get a 0.25uA typical input bias current with 1mA of collector current without base current compensation? I'm sure the AD797 is well-designed, but getting the base current matching between two transistors on an integrated circuit much better than 1% or so is nearly impossible, even with large devices and very careful design.

[john curl]

Looks like Fred slipped a digit ;-) No big thing. I analyzed, without actually proof from AD, that the quiescent current could be about .5mA per device. That would help the noise current a little, but it would assume very, very low Rbb'. I thought that the excess current noise could be from the bias compensation circuit. I could be wrong on this, and 1mA might be design center for each input device.

[Fred Dieckmann]

On most days I do know the difference between micro and pico. I guess today wasn't one of them. If one is going to make a mistake why not be off by a factor of a million..... Thanks for the heads up. The point is still valid that bias current is still much smaller than one would get without compensation. The link describes bias compensation and even a little bit about the effect of the bias compensation on current noise current .


For some of the lowest input bias current op amps:

The OP97 has bias currents of less the 250pA over the full itemperature range with bipolar inputs. One of the lowest bias current jfet op amps the AD549L has an input bias current of 60 fA !

http://www.analog.com/library/analog...versary/6.html


micro 10e-6
nano 10e-9
pico 10e-12
femto 10e-15
atto 10e-18

PS Mr. Curl is either a master of under statement or has exceedingly good manners rivaling those of Mr. Pass