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TL084 Op Amp

Cavanough Rankins
Prodigy 20 points
Other Parts Discussed in Thread: TL084, TINA-TI

I have a question concerning the quiescent state of the TL084 op amp.  I have a quad TL084 op amp and I want to know what is the quiescent state voltages of the op amp?  Should there be a voltage at the output if I don't have anything connected to the chip except for the Vcc and -Vcc?

I have the op amp connected to +/-12V power supply and nothing connected to either the inverting input, non-inverting input, or the output, but I am getting a voltage of -10.34V on the output.

Also when I do configure the op amp as an inverting op amp with a gain of 10, and a Load Resistance of 2k Ohms, I am getting no voltage swing on the output when I feed the amplifier a signal of 0.5Vp-p.  If I load down the output with a Load Resistance of 100 Ohms or less I will get a signal swing but the chip does get rather warm with this kind of load.  Any guess as to why the chip acts this way?

  • 0
    TI__Genius 12335 points

    Cavanough,

    In regards to your first question, opamps do not have a guaranteed "quiescent state voltage" with the inputs simply floating, especially if feedback is not applied to the amplifier. In this state, the amplifier's output will always saturate to one of its power supply rails for multiple reasons. To start, without feedback, the output voltage of the amplifier will be (without considering the effects of input bias current to start) the input offset voltage multiplied by the open loop gain of the amplifier which will cause the output of the amplifier to saturate. Also, all operational amplifiers have input bias currents that flow into or out of the input terminals of the device. Without any connection to the inputs, these currents will charge on-chip capacitances, and cause the output of the device to go to one of the supply rails. 

    Your second question has me a bit perplexed. Normally, lower impedances limit the output swing of an amplifier, not higher impedances. To start, I recommend that you verify the assembly of your circuit is correct and that your 2k Ohm load resistor does indeed have this value of resistance. If the output of the amplifier has been accidentally shorted to ground this would cause the behavior that you are seeing. In the future, I highly recommend that you download TI's free circuit simulation tool Tina-TI, which will allow you to simulate your circuit and verify correct functionality before you build it.

  • 0 in reply to John Caldwell
    Prodigy 20 points

    Thanks for the help John, I believe I had the pin configuration wrong and I was using the non-inverting pin of the op-amp to create my feedback circuit and putting the signal into the op-amp on the inverting input.  This was a rookie mistake, but I appreciate you taking the time to diagnose my circuits problem.