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LM2902: differential circuit questions

Max Han
Intellectual 2260 points
Part Number: LM2902

Hi

LM2902 TINA simulation: differential circuit.

1. resistor=10k, single 15V power, output is good.

2. resistor=1k, single 15V power, output is wrong.

3. resistor=1k, +15V and -15V power, output is good.

what's the cause? thanks.

  • 0
    Guru 140200 points

    Hi Max,

    can you post your simulation file?

    Kai

  • 0
    TI__Guru*** 149986 points

    Max,

    The output must be able to sink the current from the inverting feedback. With both input sides at 1.4V , the ideal output would be 0V.  With 10k resistors, the feedback current is 1.4V/20k = 70uA . The model supports this sink current at low voltage, however most  LM2902 will need to use the second sink driver, a PNP follower, so output will be near 600mV.  With 1k resistors, things get worse; now sink current is 700uA so an output near 0.7V is certain. See figure 1 in data sheet, output sinking characteristics. Also see application note section 3.1 , Design Guidelines for Devices with LM324/LM358 Cores

    Try 100k resistors, that is only 7uA sink current.