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LM4876: How is gain measured for differential op amps?

Peter Borenstein
Mastermind 8695 points
Part Number: LM4876

How is gain defined for a differential op amp?

Using the LM4876 and the datasheet, I made the circuit in figure 1 with Rf=42K and Ri = 20K. According to equation 1, my gain should be ~4.2. I am actually only seeing a gain of ~2.1. When the input signal is 0.8 Vpp, the two differential signal are both 1.6Vpp. 

Do the differential signals "combine" to form the expected 4.2 gain? This explanation is naive. I am unsure about the circuit and my understanding.  

  • 0
    Mastermind 8695 points

    Could you also clarify how power is measure for a differential signal? Does equation 3 from the datasheet describe the power dissipated by the op amp or the power delivered to the speaker?

    How is the power to the speaker measured?

  • 0 in reply to Peter Borenstein
    Mastermind 8695 points

    I think equation 10 in the datasheet is wrong. I believe this should read sqrt(Po*Rl/(Vinrms^2)) = Vorms/Vinrms. In the datasheet, the Vin on the left side of the equation is not in RMS and not squared. My reasoning is shown below.

    Also, Figure 12 seems to be the same as figure 13.

  • 0 in reply to Peter Borenstein
    TI__Mastermind 20085 points
    Hi Peter,

    The output must be taken differentially in order to determine the gain. You can refer the Bridge Configuration Explanation section in order to find further information about differential gain. The equation 3 describes the power dissipation.
    Please let me know your thoughts.

    Best Regards
    José Luis Figueroa
    Audio Applications Engineer