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Role of pre-amplifier

  1. Sep 14, 2008 #1
    Hi all, while studying nuclear electronic system, for conversion of electrical pulses into no. of counts i had came across a term "PRE-AMPLIFIER", for which it is written that it is used for impedence matching between pre-amplifier input of low capacitance and its output of high capacitance.
    What i dont understand is that, the impedence matching is used in case of maximum power transfer theorum,but here we are matching two capacitances???.Its slightly confusing.
  2. jcsd
  3. Sep 14, 2008 #2
    I'm in a non-mathematical mood so I'm going to answer without invoking any engineering equations or theorems, if that's okay.

    Capacitance is just how easily something stores charge. So a really big capacitance just keeps slowly filling up with charge as you push a current into it; the voltage rises slowly. For a tiny capacitance, you only have to nudge a little bit of current in there before it fills up and the voltage jumps up. What this means for voltage-controlled circuits (which most circuits are) is that you have to do more work (i.e. source or sink more current) to change the voltage of a large capacitance in a given amount of time than a small capacitance.

    So, if you have a tiny little detector that can only supply a little bit of current, but you want the results to register on a big machine with a big capacitance on its input, then to get the voltage on the big machine's input to change fast enough to convey information, you need to somehow push and pull a lot of current there. The job of a preamp is to do exactly that. The preamp has a big, muscular output that can provide a lot of current to shift around plenty of charge fast -- perfect for quickly changing the voltage on your big machine's high-capacitance input as new information arrives. However, the preamp also has a sensitive ear: a low-capacitance input whose voltage will change dramatically at the slightest touch of current from the delicate little detector.

    The slightly more jargoney way to say this is that capacitance is impedance. A capacitance C has an impedance of exactly 1/(j w C), where w is the angular frequency of the signal and j is the square root of -1.
  4. Sep 17, 2008 #3


    Staff: Mentor

    I don't know if your book was talking about this, but in most systems I have seen the usual role of a pre-amplifier is to provide a small gain with very little noise. If you have several amplifiers in series the noise figure of the whole chain is dominated by the first amplifier. So the usual approach is to have one small preamplifier that gives a small amplification with a very low noise figure, followed by the main amplifier that gives a large amplification with a normal noise figure. The total amplification then is a very large amplification with a low noise figure, which is much better than if you had simply used the main amplifier directly.
  5. Sep 17, 2008 #4
    Pre-amps are used for various different purposes....impedance matching maybe with a certain frequency/gain curve.. equalisation....or low noise amps used to improve a system's noise figure.
  6. Sep 21, 2008 #5

    Im guessing from your description the circuit is in relation to a piece of test equipment? If that is the case, there are 2 factors that have a fairly high priority, the first being noise as you want the amplified signal to be as noise free, or accurate, as possible, the second is really to do with responsiveness and I think that is where the capacitive matching comes in you will want your test kit to alert you to the fact you have a change in dose rate above background as soon as possible.

    I used to use a standard mk 10 nuclear radiation monitor on a regular basis in a former job but in certain situations I would also have a mini-mon to hand as it was extremely responsive, though not as accurate compared to the mk 10.

  7. Sep 26, 2008 #6
    @DaleSpam's reply
    does this imply that the pre-amplifier just acts as a buffer?

    Also, I was thinking of how would the role of a pre-amp change in case we need a current amplification, instead of voltage amplification?
  8. Sep 26, 2008 #7


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    Gold Member

    As Pumblechook has already stated: pre-amps are used for a variety of reasons and there are many types of pre-amps, so the there is no single answer to your question.
    Anyway, a pre-amp is generally speaking NOT the same thing as a buffer: by buffer we usually mean a device that make it possible to drive difficult loads by e.g. making it possible do supply more current; they usually do not amplify the signal at all.

    A current pre-amp is different from a voltage pre-amp in that the former is connected in series with the signal and therefore also act as a "sink" for the current; they also tend to have much higher gains that voltage amplifiers (gains of 10^9-10^12 are not unusual). Current amplifiers used in measurement setups also tend to have a voltage output (meaning most current amplifiers are actually transimpedance amplifiers); e.g. giving 1V out for every nA in.
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