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I'm trying to understand how a preamplifier works.

  • Is the noise not amplified the same amount as the signal?

  • If so, how is that done in the preamplifier?

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There's two things to consider with a preamp which explains why it improves SNR.

The first, and most important, is that the pre-amp is closer to the source (such as a microphone). There's fewer noise-generating components between the microphone and the pre-amp than there would be if you waited all the way to the main amplifier. For example, quite often there are treble/bass knobs between the pre-amp an the amplifier to adjust the tone of the sound. These devices add noise, but because they occur after the pre-amp, they're adding noise to a 1V signal from a low-impedance source instead of adding noise to the tiny signal that comes from the high-impedance microphone.

The second advantage is that it's easier to do clean amplification at low currents/voltages. It's much easier to do the amplification in two steps: one step raises it from the tiny signal from the microphone, and the other takes that signal and powers the speakers. In fact, it's likely that if you didn't have a pre-amp before your tone controls, you would likely have a 2 stage amplifier (which is a pre-amp and main amp slapped together).

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    \$\begingroup\$ In the context of experimental physics we rarely have treble and bass knobs, but we do have significant cable runs and get a lot of advantage from the two stage business. \$\endgroup\$ – dmckee --- ex-moderator kitten May 13 '17 at 0:21
  • \$\begingroup\$ @dmckee chuckles I showed my biases =) I didn't even think to address it in terms of anything other than audio! \$\endgroup\$ – Cort Ammon May 13 '17 at 0:41
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Is the noise not amplified the same amount as the signal?

From a different but related perspective than Cort Ammon's answer, in a low-noise multi-stage design, one puts most of the overall gain in the 1st stage which might be called a preamplifier.

It isn't so much that the preamp amplifies the noise along with the signal, it's that any noise added by the preamp (all amplification stages add noise) is amplified by any following amplification stages.

So, for example, if your 2 stage system has an overall gain of a 1000, assuming equal added noise in each stage, it would be better from a SNR perspective to have a gain of 100 in the first stage and a gain of 10 in the second stage rather than the other way around.

As an example of further reading, consider Put Gain Up Front

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    \$\begingroup\$ In particle physics the traditional reasons for not doing this seriously were the lack of room, money, and capacity to handle the thermal load. \$\endgroup\$ – dmckee --- ex-moderator kitten May 13 '17 at 1:19
  • \$\begingroup\$ ah right okay, so there is the signal going into an amplifier and then after the signal is amplified the noise (low compared to amplified signal) is added by the amplifier? what about the noise before the signal is amplified? would that be typically lower than the signal? \$\endgroup\$ – physicsnoob1000 May 13 '17 at 12:29
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    \$\begingroup\$ @physicsnoob1000 Any noise which is introduced before the pre-amp will get amplified by the pre-amp just like the signal does. Thus, if all of your noise sources come before the pre-amp, you'll find no difference between having a pre-amp vs just one high-gain amplifier. Practically speaking, however, this is rarely the case. Usually there are other components which can cause noise after the pre-amp \$\endgroup\$ – Cort Ammon May 13 '17 at 20:26
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The most important thing about a preamp is to have a low noise factor. I'm coming at it from the point of view of radios and microwaves. Yes, it amplifies signal and noise, anything coming in. The input from the antenna may have some atmospheric noise and interference, and that is amplified, but the signal is typically larger in radios/microwave (if not you do pre filtering, or noise cancellation or interference cancellation. Nothing is free). More importantly, the noise inserted by the preamp goes in on top of the S+N amplified. Still, the outside noise, in band, if you designed your system right, is very small, mostly it's only the preamp noise factor that you have to worry much about. The formulas for adding noise factors and signal levels along the chain are easy and well known by RF designers.

A good radio/microwave preamp could have noise factors like a few dB, meaning dB above 290K thermal noise, and one worries about harmonics, 3rd order intercept points, and so on, which one usually has to filter or keep out off band by design, or have the parameter be good enough within the dynamic range wanted. The preamp is the biggest factor in setting dynamic range. Cell phone preamps are less than 5 dB noise factor. For microwave radars it is a little higher, and worse when you go to mmw. Preamps are also worse if too wideband, so there's always design constraints. Of course miniaturization is important for cell phones and consumer electronics, and satellite payloads. Less so for large radars except that a complex and sophisticated radar that must be used say in an airplane also needs to be relatively small.

That then also sets the noise value biggest contribution to the whole RF chain, and you can do otHer RF processing in the rest of the chain.

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The PreAmp allows another degree of freedom in handing random noise. Here is 3-stage system, 100uV input. The noise plot shows each stage contributes equally to output noise. Why?

Rnoise stage 1 = 50 ohms

Rnoise stage 2 = 5,000 ohms (gain1^2 * Rnoise stage 1)

Rnoise stage 3 = 500,000 ohms (gain2^2 * Rnoise stage 2)

Simply stated, even a moderate gain of stage 1 (preamplifier) greatly eases random noise design of later stages, by the SQUARE of stage1 gain. Thus 10:1 preamp gain allows 100X more Rnoise in the next stage, for equal contribution.

enter image description here

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