input -->

***(+-)***---> 2/(s-1) ---

***--> output**

There is a feedback connection from -*- to

There is a feedback connection from -*

***(+-)***

--------------------------------------------------------------------

Here's my question:

1. Is this system stable?

The output/input transfer function is 2/(s+1)

( H(s) = 2/(s-1), transfer function = H/(1+H) = 2/(s+1) )

The only pole of the transfer function is at -1, LHP,===> Stable

2. On the otherhand, if the input is a constant dc

then 2/(s-1) becomes just "-2" (s=jw, w=0, 2/(s-1) = -2)

Therefore, the total feedback is (-1)

***(-2) = 2 (negative feedback)**

The feedback is 2*input.

The feedback is 2*

A positive feedback! ===> Not stable.

The bode plot shows the same thing: open loop transfer function

= 2/(s-1) ==> At w=0, phase = -180 deg and gain = positve =>

Not stable!

Where is the mistake?