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Homework Help: Unity gain in lowpass STC circuit

  1. Feb 7, 2009 #1
    One of the question asks to find the frequency at which the gain becomes 0dB for a low pass Single Time Constant circuit. In the solution manual after finding the 3dB frequency = 10^6, it states that since the gain falls off at a rate of -20dB/dec (see attatchment for graph) starting at w0(Omega-knot) , which is 10^6, the gain will reach 0dB in 2 decades (a factor of 100), therefore unity gain frequency = (10^6)(10^2)= 10^8. Now i dont understand how the gain will reach 0dB in 2 decades, because if you look at the graph of a low pass stc circuit it starts off at 0dB then falls by -20dB/dec. Am i reading the log scale graphs incorrectly? Please clarify. Thank You.

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  2. jcsd
  3. Feb 7, 2009 #2
    Sorry, the dc gain (K) is also given which is 100 V/V = 40dB, so it seems like the gain should be 40dB until w=w0 (omega = omega-knot), then the gain starts to attenuate at -20dB/dec. I'm not really good at reading the log graphs so does this mean depending on what your dc gain value is, it will start there and then start decreasing at -20dB/dec after 3dB frequency? Thanks Again.
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