# How does unity gain frequency work in this case?

Discussion in 'Homework Help' started by justinjunghkim, May 9, 2015.

1. ### justinjunghkim Thread Starter New Member

Dec 23, 2014
7
0
The figure below.. is bode plot of second order band pass filter BPF of OTA
Let just focus on only light blue color

Way I understand is BPF covers 30hz to 300khz
which is almost and there is a bump around 500khz, but lets ignore for now.
In the pass band region, the gain is -92.

My first question is what does negative gain means in physical sense?
The unity gain equation is fu = gm/2piC
and it is 90kHz. but the graph doesn't meet the gain equal to 0.
then is that mean there no unity gain?
or unity gain is meaning less in this plot.

Thanks in advance

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2. ### crutschow Expert

Mar 14, 2008
12,991
3,227
Those gains are the relative gain in dB.
Voltage gain in dB = 20 x log (V2 / V1) where V2 and V1 are the two voltages.
0dB is a gain of 1 and negative dB means the gain is less than 1.
Thus a gain of -92dB equals a voltage gain of 25E-6 which is a high degree of attenuation. Something appears to be wrong with the circuit.