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BJT: common-base amp; moving-coil microphones/preamps

  1. Mar 27, 2007 #1
    I was researching a bit about the common-base configuration of a BJT transistor on wikipedia, and I found this quote:

    "This arrangement is not used as much as other configurations in low frequency circuits but is often employed for amplifiers that require an unusually low input impedance, for example to act as a preamplifier for moving-coil microphones. "

    Can someone explain an example of WHY a low input impedance would be needed? Would this not have a nasty source loading effect? Does it have to do with very small input voltage and not wanting an input impedance that is going to treat the input like an open circuit ie keeping current flow?
  2. jcsd
  3. Mar 27, 2007 #2
    Low input impedance is required when source resistance of a signal gen, mic.. is low, it's done in order to satisfy maximum power transfer theorem. When designing amplifiers, you're taking in account the source impedance and from impedance you'll normally choose the amplifier configuration that works best.

    If I see that the source impedance is low, i would go for the common base configuration, so I would not need to use a hi-Z configuration and then use my time on impedance matching sections.
  4. Mar 27, 2007 #3


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    Impedance matching isn't really a huge concern at audio frequencies. A low impedance input when working with very low signal levels is always desirable due to imunity of noise. Obviously we can't drive a low input impedance very well with a high impedance microphone though so we need to get an input stage that doesn't load the mic. It may seem like impedance matching is a concern because typically low impedance mics generate a much lower signal voltage than high impedance mics. So when trying to substitute a low impedance mic in place of a high impedance mic the signal level is often too low right off the bat.
  5. Mar 28, 2007 #4
    I'm aware of the points you stated, but I just wanted to explain that you don't need to use hi-Z configuration having a Lo-Z input, when you already have a nice low-Z config to use. "hmm.. still thinking of impedance matching :biggrin: "
  6. Mar 28, 2007 #5


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    that depends on what the input noise current is compared to the input noise voltage. or, equivalently, what the output noise is when the input is short circuited compared to what the output noise is when the input is open circuited. if the latter is quieter than the former, then a low input impedance is not as desirable.

    usually, for amplifiers at low frequencies, i think that a high input impedance is useful so you can connect your low output impedance source to as many inputs as you want (within reason) and the devices as still pretty much decoupled.
  7. Mar 28, 2007 #6


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    rbj, what I mean by imunity to noise is that a low impedance circuit often does not pick up noise as easily as a high impedance circuit. The noise is more loosely coupled to it compared to a high impedance circuit. Noise voltage vs noise current is irrelevant. That is determined by the impedance, not the other way around. Short circuiting an input should ALWAYS reduce noise.
  8. Mar 30, 2007 #7
    Okay, I think I've pieced it together now.

    ~impedance matching = highest power delivered (Power transfer theorem)
    ~results in a source loading effect losing 1/2 the input voltage (may need another amp, thus the 'preamp')
    ~Also makes it more immune to noise.

    Thanks everyone
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