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。

In 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。
6 n: ~3 t; j/ h. w9 N1 j( C, E* E+ u
+ `$ g! J* ^5 Z. k: R  m) ~Could anybody explain it？ thank you。