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。

Could anybody explain it？ thank you。