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Original question:

OPA855-Q1: OPA855-Q1

OPA855: Pspice Bias point Problem

JHENG JYUN LIAN
Prodigy 10 points
Part Number: OPA855
Other Parts Discussed in Thread: OPA858

Hi Team, I'm experiencing Bias issues when using Models from the OPA855 Unencrypted PSpice Model Package (Rev. D).

As shown in Fig. 1, it is an inverting amplifier with a gain of -10 V/V and an input voltage of 100mV, the expected output voltage should be -1V, but the output voltage of the Pspice Model is 5.977V and there is no virtual short, I'm not sure if it is a problem with the model or the circuit.

Therefore, I use OPA858 instead of OPA855 to test the circuit, as shown in Fig. 2, which is also an inverting amplifier, with a gain of -10 V/V, an input voltage of 100mV, and an expected output voltage of -1V, but the actual output voltage is -0.998V, and there is a virtual short circuit on the negative input of the OPA.

Can you give me some comments or ideas? Thank you!

OPA858_Inverting Amplifier

Figure1. OPA855 Inverting Amplfier

Figure 2. OPA858 Inverting Amplfier 

  • 0
    TI__Genius 9270 points

    Hi Jheng,

    I was able to run the model on Pspice and I am getting the appropriate value. The reason it is not -1V exactly is due to the bias current interacting with your feedback network. For this device we recommend using a feedback resistor around 453-Ohms and adjusting Rg accordingly. Is there a connection that might be missing, or did you maybe change the .lib file?

    As for your other question, the model accounts for non-idealities of the device like offset voltage and bias current. An ideal amplifier which has no offset voltage or bias current would give you the exact value of -1V. 

    Best Regards,

    Ignacio

    OPA855_Spice.zip