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Basic question: What is amplifier saturation?

  1. Apr 29, 2012 #1
    Hi all,

    I'm confused about saturation and cut-off. I've heard it described a few ways. Here is a very basic description of them.
    Explanation 1. Saturation occurs above the maximum input voltage within the linear range of the amplifier. Cut-off occurs at below the minimim input voltage that is in the linear range of the amplifier.
    Explanation 2. Saturation is where the current through the collector is at a maximum. Cut-off occurs when this current is at a minimum.

    These definitions seem to conflict. It seems to depend on whether the amplifier is inverting the signal or not... For an inverting amplifier the maximum input voltage (saturation in explanation 1 above) results in the minimum output voltage (and current) (this is cut-off if we use explanation 2).

    I guess I'm only thinking about this on a very basic level (I'm not sure I can understand it in terms of the semiconductor junctions and depletion regions yet). But any clarification would be greatly appreciated.

    Thanks in anticipation.
  2. jcsd
  3. Apr 29, 2012 #2
    You're confusing amplifier characteristics with device characteristics.
  4. Apr 29, 2012 #3


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    Saturation occurs when the device ( like an NPN transistor) draws as much current as possible through the collector load.

    For example a 1 K resistor as a load resistor can only conduct 12 mA from a 12 volt supply.

    So, a transistor using this resistor will draw 12 mA at most and always slightly less. Slightly less because the transistor's saturation voltage is small but not zero, so there will be less than 12 volts across the load resistor.

    There will then be no way of making the transistor draw more current even by greatly increasing the base current.

    Note that it is a function of the circuit and not of the transistor properties.

    Cut off happens when the transistor collector current is reduced to near zero by a removal of base current.

    In this case, the collector voltage will be close to the supply voltage as there cannot be any voltage across the collector resistor.

    It can happen if the transistor's base-emitter voltage drops below 0.6 volts.
  5. Apr 30, 2012 #4
    Thanks for the replies.

    From your description vk6kro, it seems that explanation 2 is correct (if greatly simplified)...
    So is explanation 1 incorrect? Or should we consider amplifier saturation and device/transistor saturation to be two different things? Is this what you are suggesting Kholdstare?
  6. Apr 30, 2012 #5


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    Neither explanation really describes saturation

    All waveforms produce a maximum current and a minimum current, but this does not mean the transistor could not produce more or less current with a different input waveform.
    So, you would have to rewrite the definition to include "maximum possible current", not just "maximum current".

    The saturation current is not related to the input. It is mostly a function of the supply voltage and the load resistance. You achieve saturation with a suitable input, but the saturation current is the same whether you achieve it or not.

    Power transistors have less gain as the collector current increases, so they get less and less sensitive as the drive increases. So, if they have no load, the collector current does not have a defined saturation point. It is just that extra current becomes harder to achieve.
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