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Input bias current specification

Discussion in 'Electronic Design' started by ioannis, Mar 20, 2007.

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  1. ioannis

    ioannis Guest


    i want to specify the charakteristics of an amplifier!

    Therefore ai have writen a script in hspice which is specifying the
    Iput Offset Voltage Vos and the Input Bias Current Ios. I know, that
    for specifiyng the Vos, ihave to put two exactly same Voltage sources
    (as it is shown bellow) on the invering and non-inverting inputs of
    the amplifier and measure the output Voltage.

    ****Voltage Sources******
    VDD VDD 0 0
    VSS VSS 0 0

    VinnDut_Vos 0 innDut_Vos dc .1 *sin(0, 0.1, 5Hz,0,0,0)
    VinpDut_Vos 0 inpDut_Vos dc .1 *sin(0, 0.1, 5Hz,0,0,0)
    vbiasDut_Vos vbiasDut_Vos 0 5

    What shall i have to do when measure the input bias current? Shall i
    do it in the same way?

    I have distinguished two cases:
    a) very low Input bias curernt
    b) normal low input bias current?

    For the first case i have writen for the operatin circuit this

    **** Input Bias Offset (Ios) specification
    **** Ios=I_B(+)-IB(-)
    **** I_B=C*(\delta(out)/(\delta(t))

    **** a) Extremly low Ios*****

    VinnDut_Ios_low 0 innDut_Ios_low dc 1 *sin(0, 0.1, 5Hz,0,0,0)
    VinpDut_Ios_low 0 inpDut_Ios_low dc 1 *sin(0, 0.1, 5Hz,0,0,0)
    VbiasDut_Ios_low vbiasDut_Ios_low 0 5

    *Circuit for measuring extremely low Ios*

    *Rsp 0 inpDut_Ios_low 1 *short out Capacitor C1 to test
    Rsm innDut_Ios_low outDut_Ios_low 1 *short out Capacitor C2
    to test I_B(+)
    C2 innDut_Ios_low outDut_Ios_low 1u
    C1 0 inpDut_Ios_low 1u

    and for the second case this one:

    **** b)Normal Ios *****

    *Input Sources*

    VinnDut_IosNorm 0 innDut_Ios_Norm dc .1
    *sin(0, 0.1, 5Hz,0,0,0)
    VinpDut_IosNorm 0 inpDut_Ios_Norm dc .1
    *sin(0, 0.1, 5Hz,0,0,0)
    vbiasDut_IosNorm vbiasDut_Ios_Norm 0 5
    vbiasGen_IosNorm vbiasGen_Ios_Norm 0 5

    *Device Under Test(DUT)*

    XDutIosNorm VDD VSS vbiasDut_Ios_Norm inpDut_Ios_Norm innDut_Ios_Norm
    outDut_Ios_Norm DA

    *General OpAmp Type*

    XGenIosNorm VDD VSS vbiasGen_IosNorm 0 innGen_Ios_Norm
    outGen_Ios_Norm DA

    *Circuit for measuring normal Ios*

    RoutDut_Ios_Norm outDut_Ios_Norm innGen_Ios_Norm 10k
    Rm 0 1 100
    Rp 0 3 100
    Rbn 1 innDut_Ios_Norm 1 *Rbn short out to test IB(+)
    Rbp 3 inpDut_Ios_Norm 1000K *Rbp short out to test IB(-)
    R2 3 outGen_Ios_Norm 100
    CinGen innGen_Ios_Norm outGen_Ios_Norm 1u

    Can anyone tell me if i have to put the same input voltages, in the
    same way with i did for the Vos specification?
    Or shall i have to test the Vos without input Voltages?



    P.S. Do anyone know a hspice forum?
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