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LM321LV: i use LM321LV as voltage follower

user6326364
Prodigy 10 points
Part Number: LM321LV
Other Parts Discussed in Thread: LM321,

What is the input and output impedance of the LM321, and what is the output error used as a voltage follower?

  • 0
    TI__Expert 6625 points

    Hi User, 

    The input impedance is the impedance seen by the source driving the input of the amplifier. The output impedance is about 1200ohm for LM321LV. Please refer to Figure 17 of the datasheet. 

    In a buffer configuration the output error is directly correlated to the offset voltage. In this case it is +/-1mV for a 5V supply. 

    Regards,

    Bala Ravi

  • 0
    Guru 140200 points

    Hi,

    as the LM321LV is a CMOS OPAmp with ultra low input bias currents of only a few pA, the input impedance of LM321LV is in the TOhm range.

    An 1200 Ohm "Open-loop output impedance" is specified in the datasheet. The "Closed-loop output impedance" (the output impedance of the real circuit) is not given in the datasheet because it heavily depends on the set gain ("Closed-loop gain") and by this on the feedback factor ("Loop gain") of OPAmp circuit:

    "Closed-loop output impedance" = "Open-loop output impedance" / (1 + "Loop gain").

    With

    "Loop gain" = "Open-loop gain" / "Closed-loop gain"

    "Open-loop gain" at DC is 110dB according to the datasheet. So, the DC "Closed-loop output impedance" of LM321LV as voltage follower is in the milliOhm range.

    Keep in mind that the "Open-loop gain" is heavily frequency dependent. The "Open-loop gain" plot can be seen in figure 3. So, the "Closed-loop output impedance" will increase with increasing frequency.

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 6625 points

    Hi User, 

    Kai is correct. Please refer to his comments for detailed answer on your question. 

    Regards,

    Bala Ravi