Power supply vs Function generator

Discussion in 'General Electronics Chat' started by sotinam, Jun 3, 2016.

  1. sotinam

    Thread Starter New Member

    Apr 20, 2016
    Hi all,

    We know that a Function Generator circuit has Thevenin equivalent and the whole circuit is equivalent to a Voltage source in series with a 50 ohm resistor (internal impedance). That is why the output of the Function Generator depends on the load resistor we use (1Mohm load yields a 2 times bigger voltage compared to a 50 ohm) and this is because of the voltage that is divided by two Load and Internal resistors. However, we do not see such a variation in the output voltage of a power supply! In other words the output of the power supply is always 12 V (or whatever) no matter if the load resistance is 1Mohm or 50ohm. This is a little confusing for me. What is the main difference between FG and PS circuits?

    Why can’t we find a Thevenin equivalent for a PS and do the same math (voltage divider) as for FG?

    I read on the web that PS is a POWER SOURCE (fits the name actually) and FG is VOLTAGE SOURCE but I do not know what it means. Can anyone help me to understand this?

    Thank you,
  2. MrChips


    Oct 2, 2009
    You have already answered your question. Your Function Generator has a source impedance of 50Ω.
    Now make the source impedance of your Power Supply very low such as 0.01Ω. Take it from there.
    sotinam likes this.
  3. RichardO

    Well-Known Member

    May 4, 2013
    A little more about what MrChips said...

    The output amplifier of a function generator does have very low equivalent resistance. There is a 50 ohm resistor added to purposely increase the output impedance at the connector. This resistor has two purposes. The first is to help protect the output amplifier from external shorts or voltages. The second is to make it easy to feed a 50 ohm coax cable.

    Note that both the function generator and power supply also have a maximum output current that is independent of the output resistance. A function generator is commonly limited to 100 ma. A power supply often has an adjustable current limit.
    sotinam likes this.
  4. sotinam

    Thread Starter New Member

    Apr 20, 2016
    Thank you guys.

    It makes sense to me.