What is the purpose of the RG10MEG Resistor in this circuit?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Without the 1 M&Omega; RG resistor the bias circuit becomes a high-pass filter with a cut-off frequency of 10.3 Hz.

The effective impedance at the junction of R1 - R2 is R1 || R2 = 19k || 11k = 7 k&Omega;.

Adding the 10 M&Omega; resistor drops the high-pass cut-off to 7 mHz.