On Sun, 29 Jun 2008 03:24:38 -0700 (PDT), lionelgreenstreet@[EMAIL PROTECTED]
>If i consider an opamp in corrent-to-voltage configuration with:
>-ideal generetor current in inverting input
>-non inverting input to ground
>-feedback resistance equal to 100Mohm
>-open loop gain egual to A=128dB
>-capacitance common mode plus differential mode equal to 3pf
>i'm looking for pole frequency that shoud be equal to A0/(2*pi*C*Rf)
>120dB=20LogA0=>A0=2.512e6
>fp=2e6/(2*pi*3e-12*100e6)
>but this frequency is too high for this opamp (http://focus.ti.com/
>docs/prod/folders/print/opa128.html) , so i think that i've made some
>errors...what's wrong?
The pole caused by Rf and the capacitance at the inverting input, Ci,
has its corner at a frequency of 1/(2*pi*Rf*Ci), irrespective of the
opamp's gain. So rip A0 out of your numerator.
If that pole is below the amp's unity-gain frequency, it adds
(approaching) 90 degrees of feedback lag to the opamp's (nearly) 90
degrees of forward-gain lag, so the mess may go unstable.
John
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