Transistor various frequencies

  1. Good day ,

    Transistor characteristics tend to change under load and at various frequencies and teperatures etc, but when we have a simple linear class ab amplifier with two bjt output devices , I wonder how does a single transistor output various frequencies at once? I understand it's controlled by the input signal which is amplified itself and then drives the base of the output transistor but still.

    Music is various frequencies which differ also in the loudness which translates to voltage/current , so how does the transistor manage to output more than one voltage/current waveform at once or simultaneously?
  2. jcsd
  3. Clearly it can't output "more than one voltage" because there is only one output pin. It just outputs one very complex waveform that represents the sum of all the individual components.
  4. sophiecentaur

    sophiecentaur 14,151
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    Gold Member

    Transistors are inherently very non linear but that really doesn't matter.
    The whole point about 'modern' (i.e. since they started building them) amplifier design is that negative feedback is used to eliminate distortions and variations in characteristics from device to device. You start with enough gain to blow your hat off and then you use feedback to make the output voltage (or current or whatever) as near a match to the input signal as you need. Transistors are inherently very non linear but that really doesn't matter.
  5. Hmm, I;m not sure I understood , I did understand however that the single output pin of a transistor cant have many frequencies and voltages/currents at once but when there is a simple record of say a heavy bass line and a drum dish being hit at the same instant , the high pitch is a low voltage high frequency sound while the bassline is a low frequency much bigger voltage and current demanding sound for the speaker , so how does the transistor output these two different ones at once? does the little high pitch waveform is " put on" the lower longer wavelenght sound ?Like smaller waves on top of a big wave in sea?
  6. sophiecentaur

    sophiecentaur 14,151
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    Gold Member

    There can be only one value of voltage at any one time. The value will vary over time and this is the 'time domain' description of the signal. You can also describe the signal in terms of a number of frequency components (frequency domain). These all vary at different frequencies and, at any one particular instant, they add up to give the single voltage value.

    You should think in terms of one domain at a time - at least whilst you are learning about this stuff or you can fall over yourself. True Fourier Analysis operates over an infinite time and gives you a continuous 'spectrum'. You can, however, do almost as well with the Discrete Fourier Transform which looks at a finite length of a signal. This will describe the signal as a series of discrete harmonics.
  7. Exactly.

    Plot the two voltage waveforms vs time.
    Add up the voltages.

    The current takes care of itself thanks to ohms law (speaker modelled as a constant resistance).

    PS In case not obvious..The output transistor never actually sees two separate waveforms as you describe. They were merged into one complex waveform when the original sound was produced/recorded.
  8. sophiecentaur

    sophiecentaur 14,151
    Science Advisor
    Gold Member

    Another way to convince yourself is to think that the loudspeaker cone can only be in one place at once and it is 'carrying' all those sounds.
  9. Oops duplicate post deleted

  10. Guess this is also the reason for high range , mid range and low range speakers and dividing in frequencies for better audio reproduction.
    Seems like the average speaker suffers more from frequency overlap than the transistors because a speaker is a physically moving device ?
  11. sophiecentaur

    sophiecentaur 14,151
    Science Advisor
    Gold Member

    I am not sure what you mean by the "frequency overlap". The only reason for using more than one mechanical driver unit is because of the large number of octaves involved in audio and the difficulty of matching a single drive unit to the air impedance. It can be done with impracticably large horn loudspeakers, though. The distortions involved with multiple crossover filters introduces additional problems and it's all a matter of compromise.
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