I have question regarding examples from book art of electronics - i can't figure it out how to calculate and what happens in this circuits. Are points A and B on ground voltage because of point B or are they on point A because of voltage divider ? figures are in attachment

 

Try read this
http://forum.allaboutcircuits.com/showpost.php?p=532863&postcount=4

 

Try read this
http://forum.allaboutcircuits.com/showpost.php?p=532863&postcount=4

OK, thank you! I get it now how to calculate, but how can I know in some sort of intuitive way what does this circuit do ?

 

Op amp always do whatever he can at the output to force Va = Vb.
http://forum.allaboutcircuits.com/showpost.php?p=444315&postcount=9
The first circuit is quite easy to understand, we have simply "non inverting amplifier".
So as long as the input voltage Va is constant the VB voltage also will be constant.

Therefore ILoad = VB/R
The but the second circuit is not so intuitive.
http://www.circuit-fantasia.com/cir...and_current_source/howland_current_source.htm
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For the Howland current source, you can analyze it by naming the output voltage of the opamp Vo. Then, given Vo and Vin (plus R1 and R4), find the voltage at the inverting input (Point A). After than, given Vo and Iload (plus R2, R3, and Rload), find the voltage at the non-inverting input (Point B). Now apply the assumption that the opamp is operating in its linear region and therefore the voltage difference between the two inputs is negligible. You can then solve for the load current and should find that it is a function of the input voltage and the four resistors, but that it is NOT a function of the load resistance.

 

Thank you, I understand now much better!

also http://www.circuit-fantasia.com site is very good