I'm having trouble calculating the 3db freq of a noninverting op amp circuit.
basically, i'm looking at:
in \
 \_____ out
 / 
/ R1
______

R2

C

GND
(hopefully my ascii art makes sense)
i've calculated that at high frequencies, the impedance of C is negligible, so the gain is equal to 1 + R1/R2  no problem there.
now, according to the textbook i'm referencing, the 3db point is the freq at which the impedance of C equals R2
however, when i do the calculations with real values, it doesn't make sense to me. i'm using R1=18k, R2=2k, and C=4.7uF, for a high freq gain of 10. this is the example my textbook gives
at lower frequencies, i'm figuring the gain is equal to 1 + impedance(R1)/(impedance(R2)+impedance(C)). according to the book, the 3db point is at 17Hz. however, at this frequency i'm calculating a gain of 5.5 whereas i would expect a gain of 7.07 (1/sqrt(2)*10)
what am i missing?????
basically, i'm looking at:
in \
 \_____ out
 / 
/ R1
______

R2

C

GND
(hopefully my ascii art makes sense)
i've calculated that at high frequencies, the impedance of C is negligible, so the gain is equal to 1 + R1/R2  no problem there.
now, according to the textbook i'm referencing, the 3db point is the freq at which the impedance of C equals R2
however, when i do the calculations with real values, it doesn't make sense to me. i'm using R1=18k, R2=2k, and C=4.7uF, for a high freq gain of 10. this is the example my textbook gives
at lower frequencies, i'm figuring the gain is equal to 1 + impedance(R1)/(impedance(R2)+impedance(C)). according to the book, the 3db point is at 17Hz. however, at this frequency i'm calculating a gain of 5.5 whereas i would expect a gain of 7.07 (1/sqrt(2)*10)
what am i missing?????
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