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。
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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。

5 j, d  n# j9 S8 y- o% V& H. G5 u- tCould anybody explain it？ thank you。