I recently came across a block diagram of a state variable filter on one of my reference books which looks like this:

enter image description here

I am not sure about the negative sign going into the summing point from the side of Vin. In my limited experience with control systems I have not encountered this and do not understand what it implies.

Without taking into consideration the negative sign of Vin, the transfer function I came up with is:

enter image description here

My question is not whether the transfer function is correct, but simply what effect does the negative sign next to Vin mean and how it affects the transfer function above. Thanks!


Vin could be a positive voltage or a negative voltage. The sign of the input signal is, largely, irrelevant; the negative sign simply inverts the input signal and has no impact on the form of the CLTF. The minus sign is just there to make the student think.

Also, the negative sign in the CLTF is wrong, the -K1/s block makes it negative f/b.

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  • \$\begingroup\$ Thanks for the clarification! The negative sign cropped up when working G/1+G instead of G/1-G with G being -K1/s. \$\endgroup\$ – rrz0 Dec 16 '17 at 10:14
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    \$\begingroup\$ Cyber - please note that the number of minus signs within EACH loop must be odd (mostly "1", sometimes "3"). Only in this case, you have negative feedback. The denominator of the general expression for feedback D(s)=1-LG(s). For negative feedback, the loop gain LG(s) is always NEGATIVE (because of the odd number of minus signs). \$\endgroup\$ – LvW Dec 16 '17 at 15:26

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