Plot of Impedance Ratios

Impedance ratios between + and - inputs of the OPA627: Without compensating resistors the impedance ratio changes from 60 to 0.7 (red curve). The 1.5kOhm resistor at the + input reduces the slope. The impedance ratio across the audio band changes from 120 to 60 (green curve). The 2.0kOhm resistor at the - input shifts the curve parallel to the ordinate. Impedance ratio is now between 1.5 and 0.75 across the audio band (blue plot).

Plot of Output Impedances

Output impedances were determined in the following way. At different frequencies the unloaded output voltage is set to 3V_pp (U₀). Then the output is loaded with 27Ohm (R_load) and the voltage drop is measured. The output impedance is calculated according to following equation:

R_imp = ((U₀ / U_d) - 1) x R_load

R_imp : Output Impedance

U₀ : Output voltage unloaded

U_d : Output voltage loaded

R_load : Load resistance