Transistor amplifier electromagnetism

Discussion in 'The Projects Forum' started by I have no education!, Sep 19, 2012.

  1. I have no education!

    Thread Starter Member

    Sep 19, 2012
    Hi all. New to this forum!! I have a reasonable understanding of electronics and so will be able to separate the wheat from the chaff on any replies to this post. If anyone has any ideas please feel free to reply. I can assure you if your advice is good then I will no doubt be able to repay the favour with regards to multiple other areas of electronics!

    why is there never a mention with regards to electromagnetism when designing transistor amplifiers? The methodology is to bias the transistor using dc analysis then superimpose the ac signal! Now a time changing ac signal will cause a magnetic field, this field could cause the direction of current flow to change. Now even if the 'ac' signal does not cross into the negative half of the cycle it will still produce a magnetic field and could cause a change in the amount of current flowing. My first thought was that the magnetic field aspects are ignored as the input to the amplifier is extremely small, but the gain can be extremely large if one has a multi stage amplifier so the magnetic field could change the current flow in the circuit. Also, when using operational amplifiers there is never any mention of electromagnetism either. Am I missing something here?
  2. wayneh


    Sep 9, 2010
    Yes! The magnetic permeability of air is very low, so the small magnetic fields around current-carrying wires are not well "conducted" to where they might cause problems. And, magnetic field strength is proportional to amp-turns. Most circuits inside an amplifier don't have all that much of either amps or turns, so there are not big fields present normally. Where there ARE fields, such as in the power transformer, you find shielding to contain them.

    Another way to think about this is energy transfer. At any point in a circuit, the amount of energy introduced by nearby magnetic fields is going to be tiny (for the reasons above) compared to the "normal" energy in that circuit.
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