VCA821： Refer to the VCA data manual (Figure 1), and design the automatic gain control circuit (Figure 2) in Multisim for simulation experiments. Experiments result shows tha this circuit does not achieve the function of gain control . Can anyone help exp

Hello,
         Refer to the VCA data manual (Figure 1), and design the automatic gain control circuit  (Figure 2) in Multisim for simulation experiments. Constantly adjust the size of the input signal (from 0.1mV to 20mV), but the simulation results show that the circuit gain has never changed. Moreover, the output voltage of the OPA820 op amp has always been 4.182v, so the gain control voltage of VCA821 has never changed. This circuit does not achieve the function of gain control . Can anyone help explain the reason?

Thank you!

参考VCA数据手册(图1)，在Multisim中绘制了自动增益控制电路仿真电路图（图2），进行仿真实验。不断调整输入信号的大小（from 0.1mV to 20mV），但是仿真结果表明电路增益大小从未变过。而且OPA820运放的输出电压一直是4.182v，该电路并没有起到增益控制的作用，有人可以帮忙解释一下原因吗？谢谢！