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LM324: Pspice bode plot analysis

Bruce Pan
Prodigy 40 points

Part Number: LM324

Tool/software:

The first image shows my simulation circuit. The second image presents the gain response when the AC voltage source is set to 1mV, while the third image shows the response with a 10 V AC source.

Theoretically, the amplitude of the AC source should not affect the gain results in a Bode plot, as AC sweep analysis is based on small-signal linearization. However, my simulation results differ depending on the input amplitude. Why is this happening?

Additionally, the simulated gain appears negative (indicating attenuation) rather than showing the expected amplification. What could be causing this behavior?

  • 0
    TI__Guru* 77501 points

    Hi Bruce, 

    Bruce Pan said:
    Additionally, the simulated gain appears negative (indicating attenuation) rather than showing the expected amplification. What could be causing this behavior?

    You need to establish the proper input Vcm for the op amp, so it can be operated in a linear range. The op amp has Vcm_min of 0 (for single supply rail), and you need to have a least 200mV above to the minimum in order to have a design margin. DC Vbias voltage of 0Vdc is inadequate, unless you have dual power rails. 

    If you have other questions, please let me know. 

    Best,

    Raymond