My point is not to misinform, but to shed a little clarity on the subject. It is a good engineering practice to begin with a specification that defines your design criteria. Newbees especially can use such advice. And what is presented on the list is observed also by those who are just entering engineering and learning. What is considered an acceptable noise level in one application may be considered excessive in another. If a single stage single transistor or FET will work it is probably a better solution with respect to noise. I don't have anything against "rule of thumb" design, it has a valid place in engineering. But to say that the noise contribution does not present itself after the first two stages is a bit misleading to a novice or a student. You may say that the majority of the noise is in the first stage. When you amplify, you amplify both the signal and the noise which is passed on to additional stages. It would be wonderful to get a small signal out of the noise by amplifying it up but that just can't happen. Every stage that has active components will contribute to the overall noise. There is no active electronic component that I have ever been aware of that does not contribute to noise. If electrons move there will be noise. Even resistors contribute to noise. That is apparent when one looks at the physics of the devices. In the final analysis the formal design begins with a design specification, and that specification includes an error budget which also accounts for the noise; stage by stage. I have not been convinced that such an approach is unnecessary or an unimportant consideration in engineering. Perhaps at the hobby level one can dispense with the specification but not when you are designing product for some company. ----- Original Message ----- From: "Mark Rages" To: "Microcontroller discussion list - Public." Sent: Saturday, July 26, 2008 1:02 PM Subject: Re: Low Noise, High Gain Amplifier > On Sat, Jul 26, 2008 at 12:31 AM, Rich wrote: >> The more stages you add the greater will be the noise problem. > > No, noise performance is determined by the first stage or two. > > Since noise performance can sometimes be traded off against gain, > sometimes adding stages is helpful to build a low-noise amplifier. > > Regards, > Mark > markrages@gmail > -- > Mark Rages, Engineer > Midwest Telecine LLC > markrages@midwesttelecine.com > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist