How to design Vref circuit in picture?thanks

[viyag]

there is a vga dB-linear control block. attached in the picture
Vin,op=(Vexp1-Vexp2)*AVvga-Vref
and Vin,op≈0 because of the OP

in the paper ,it says that the constant voltage Vref is generated by an
external current source and then subtracted from the output of the VGA cell.

and i do not know how to generate this constant voltage Vref.
anyone can tell me ?
thanks.

 

[umery2k75]

You can use potentiometer for providing the reference voltage.You may can also use op-amp with current-to-voltage configuration and place rheostat in series with current source, to vary the current source.

You can make Vref and it will be tunable too, you can vary it accroding to your need.It's easy.