GATE EC 2016 Set 2 Q46 — Op-amp Circuits

In the feedback system shown below $G\left ( s \right )=\frac{1}{\left ( s+1 \right )\left (s+2 \right )\left ( s+3 \right )}.$

The positive value of $k$ for which the gain margin of the loop is exactly $0$ dB and the phase margin of the loop is exactly zero degree is _________