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LMC6081 & LMC6001 gain queries

Bob Lerwill
Prodigy 110 points
Other Parts Discussed in Thread: LMC6081, LMC6001

I am having trouble with a design which uses either the LMC6001 or the LMC6081 and I have a suspicion that the magnitude of the problem may relate to the value of the open loop gain.I have questions about the gain figure quoted in both the LMC6001 & LMC6081 data sheets.

The LMC6081 datasheet gives a headline voltage gain figure of 130dB in the "Features" section of the front page. The detailed specification gives a number of figures for large signal gain under different conditions, but the highest of these is 1400 V/mV; equivalent to 123dB. Looking at figure 8, I guess the discrepency is because the gain carries on increasing as the load impedance rises above 2k, but under what condition is the 130dB figure appropriate?

Both data sheets give a significantly  higher gain when sourcing current than when sinking current. The difference is around a factor of 4. The notes state that the tests were carried out with a single supply configuration (15V) with the "sourcing current" figure applying when Vout > 7.5V and "Sinking current" when Vout < 7.5V. In my circuit, the output is used to drive a PNP transistor gain stage, so it is always sinking current whatever the output voltage happens to be. Should I then apply the current sinking gain figure even when the output voltage is > 7.5V? Could I get a higher open loop gain if I included a high value dummy load down to 0V so that the output was always sourcing current? Is this change in open loop gain depending on the direction of current flow reflected in the SPICE models?

  • 0
    TI__Guru 55086 points

    Hi Bob,

    The LMC6xxx series does have asymmetrical sink/source gains. It is the nature of the beast.

    With light loads (>100K), the gains could easily surpass 130dB (I was measuring in the mid 140's at megohm loads at the time - device noise limits the ultimate resolution), but under heavy loads (<2K), the sinking gain drops quite a bit. The "Input voltage vs Output Voltage" graph in the LMC6001 datasheet shows this very clearly.

    Eyeballing that graph, the 500K load line looks like 7.5V/2uV = 3.75M  = 131dB. But at 2K load, it is 7.5V/10uV = 750K = 118dB.

    If your circuit tends to operate within a narrow area, you could "pre-load" the output to bypass some of the sinking current off of the amplifier with a pull-down resistor. But this could limit your positive swing.

    PNP transistors tend to have lower betas than NPN's - particularly "power" transistors. Betas in the teens are possible under load, which demands more drive from the amplifier. Do you have enough headroom to use a Darlington output transistor configuration? Or possibly a MOSFET? This would reduce the amount of base drive required and unload the amplifier.

    As far as the model, it looks like the model does NOT model the asymmetric gain at high loads.

    Regards,