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OPA2188: GBW questions

Eric Liu(SZ-FAE)
Intellectual 1590 points
Part Number: OPA2188

Tool/software:

Hi Expert，

for amplifier GBW, I have some questions，

for VF amplifier，why the GBW is  a constant value？

if we set one amplifer as a close loop mode，we use the negative feedback to stablize its gain  and increase the close loop bandwidth， is this right？

do we have any mathmatical derivation？

  • 0
    TI__Mastermind 20437 points

    Eric,

    1. The gain-bandwidth-product (GBP) is the closed loop gain multiplied by the closed loop bandwidth.  GBP = G_closed_loop x Bandwidth_Closed_loop.  For example, assume GBP = 1MHz.  For closed loop gain of G=1, the closed loop bandwidth would be 1MHz.  For G=10, the closed loop bandwidth would be 100kHz.  For G= 100, the closed loop bandwidth would be 10kHz.
    2. For voltage-feedback amplifiers (VFB), the GBW is a constant value because it relates to the AOL being a first-order frequency response (1 dominant pole).  The closed loop gain is set by the the reciprocal of feedback factor 1/Beta.  This is easiest to see graphically.  The 1/Beta curve will intersect the Aol curve to set both the closed loop gain and bandwidth.  Derivations and examples are given in TI Precision Labs - Op Amps Video series bandwidth videos 2 and 3.

    I hope this helps.  Please review the videos referenced above for details.

    Art