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TL082: output voltage related with load

Gene Qiu
Intellectual 1720 points
Part Number: TL082

Hi Team, 

Concering with the AMPs and load resistor, i had a few queries below:

1.The output terminal of op amp is connected with resistance load, the load resistance value is different, the output voltage range is different, but the open-loop output impedance of op amp tl082 is 125 Ω, the influence of internal resistance of op amp should not be so great. The picture on tl082 specification shows that the influence of load resistance is still very large (larger than the voltage difference caused by the internal resistance of OP AMP (I calculate it by 128 + 64 Ω) and the output load resistance). The output voltage of the op amp is related to the load resistance. What is the reason? 

2.DSP_ADC The input model of ADC is sample and hold type. There is no pull-down load resistor in DSP. I have seen most Voltage Feedback Op Amp outputs with load resistor, but it seems that some op amp output signals are directly sent to DSP_ ADC pin, no load resistance. So the question is: when does the load resistor of the op amp need to be added or not? Is the load resistor applied to provide a bias current path to the inverting input of the op amp?

  • +1
    Guru 140200 points

    Hi Gene,

    what appears to the outer world is not the open-loop output impedance but the closed-loop output impedance which is way smaller at low frequencies.

    And whether Rs is necessary depends on the charge kick-back filter recommended in the datasheet of ADC. There's no simple rule.

    Kai

  • Former Member
    +1 Former Member

    Hello Gene,

    The output impedance of the op amp is much smaller in the closed-loop than its open-loop specification.  If Zo is the open loop output impedance and Zout is the closed-loop output impedance, then Zout can be found using this equation.

    Since Aol is very big, much larger than Beta, then Zout is usually quite small.  So the output impedance of the op amp will be much smaller than 125Ω in your circuit.

    The output voltage is not normally related to the load resistor.  Under normal circumstances, they are independent and the output voltage is dependent on the input signal and feedback components.  The exception is when the load resistor needs a lot of output current.  If a lot of output current is needed, then the op amp's output cannot operate so close to the supply rails.  So, really the limitation is of the op amp's output current versus its output voltage.

    For your last question, an op amp does not need a load resistor to work.  It needs negative feedback to work as an op amp.  However, a load resistor is sometimes used to represent a load or perhaps it is needed to make another block later in the circuit work.

    I should not comment on the DSP ADC example, because it will depend on the ADC topology and a lot of factors.  As drawn in your image, RS is not a traditional load resistor but an isolation resistor for stability.  If you are unsure about whether or not an op amp needs an output resistor before going to an ADC, it is best to consult with the data sheet for that particular ADC.  You can also ask a question on e2e for a specific ADC and one of those teams would help you.

    There is also a good series on this topic for SAR ADCs (click here).  Hopefully this helps to clarify things.

    Regards,
    Daniel