Voltage-to-Current Converter

wwm101

In the paper 'A Wideband CMOS Variable Gain Amplifier With an Exponential Gain Control'，a Voltage-to-Current Converter is realized and the structure is shown  in  the following figure。

1 U& U# ^" D& W7 e- j) QIn the figure，VC** denotes the voltage copier and current generation，Here ，i don't understand why VC** can copy voltage from node A to node B。
! J+ d8 }. N: F( k6 o4 _4 A3 B
/ q- E4 y. n1 c6 T$ B1 xCould anybody explain it？ thank you。