Every amplifying device has a rating for current noise and voltage noise.
Some (like BJTs) have high current noise and low voltage noise, and these give their best noise performance with low impedance sources. The higher the beta of a BJT, the lower the current noise and the higher the source impedance it suits.
Others (JFETs) have hardly any current noise but more voltage noise, so are better suited to high impedance sources.
It follows that "ultra low noise" is something you get from a well-designed system, not any particular component. Anyway, try the BC846, 2N4403, SSM2220, LSK389, and so on.
Every audio power amp is a discrete op-amp. If you don't want to drive a speaker, just replace the output stage with a smaller one.
Some (like BJTs) have high current noise and low voltage noise, and these give their best noise performance with low impedance sources. The higher the beta of a BJT, the lower the current noise and the higher the source impedance it suits.
Others (JFETs) have hardly any current noise but more voltage noise, so are better suited to high impedance sources.
It follows that "ultra low noise" is something you get from a well-designed system, not any particular component. Anyway, try the BC846, 2N4403, SSM2220, LSK389, and so on.
Every audio power amp is a discrete op-amp. If you don't want to drive a speaker, just replace the output stage with a smaller one.
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Hello Scopeboy,
Thanks a lot for your reply.
Well, I read carrefully this post (http://www.diyaudio.com/forums/parts/168561-some-transistor-voltage-noise-measurements.html), and I don't understand why some parts which are not specially designed for low noise application could have very low noise characteristic like the ZTX849...
Does the spice model of the ZTX849 could be used for noise simulation ? which parameter spice model need to do this simulation ?
I can understand that the board, the power supply needed for low noise preamp (for example) is very important but I suppose that, for a same preamp board, using a transistor with low noise characteristic should help to reduce the noise ...?
again, do you know a good book in order to learn about discrete op amp ?
Thanks again for your help.
Thanks a lot for your reply.
Well, I read carrefully this post (http://www.diyaudio.com/forums/parts/168561-some-transistor-voltage-noise-measurements.html), and I don't understand why some parts which are not specially designed for low noise application could have very low noise characteristic like the ZTX849...
Does the spice model of the ZTX849 could be used for noise simulation ? which parameter spice model need to do this simulation ?
I can understand that the board, the power supply needed for low noise preamp (for example) is very important but I suppose that, for a same preamp board, using a transistor with low noise characteristic should help to reduce the noise ...?
again, do you know a good book in order to learn about discrete op amp ?
Thanks again for your help.
High current transistors with a large die will often show low voltage noise (but not necessarily low current noise, unless they also have high beta at the collector current you plan to run)
The 2N4403 is a case in point: it was designed as a high current switching transistor, but turned out to be suitable as a low noise amplifier. I guess the ZTX849 is the same story.
Literature on discrete op-amps is limited because not many people want to design them when there are so many good IC ones available. I suggest you try the Self or Cordell books and think of the discrete op-amp as just an audio power amp with the power stage removed.
The 2N4403 is a case in point: it was designed as a high current switching transistor, but turned out to be suitable as a low noise amplifier. I guess the ZTX849 is the same story.
Literature on discrete op-amps is limited because not many people want to design them when there are so many good IC ones available.
I suggest you try the Self or Cordell books and think of the discrete op-amp as just an audio power amp with the power stage removed.Samuel Groener has a nice section on discrete opamps.
SG-Acoustics · Samuel Groner · Analogue Audio
SG-Acoustics · Samuel Groner · Analogue Audio
Some nice designs there, but look carefully at the input bias current specs. They are a good deal worse than IC opamps and this will limit the circuit configurations you can use them in without excessive DC offset.
The "HV" version with its 100nA bias current shows the basic method of reducing it. To a first order, the bias currents of the PNP and NPN input stages cancel.
Edit: I maybe overstated the case. The NE5532 has an input bias current of 1uA.
The "HV" version with its 100nA bias current shows the basic method of reducing it. To a first order, the bias currents of the PNP and NPN input stages cancel.
Edit: I maybe overstated the case. The NE5532 has an input bias current of 1uA.
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