High speed buffers for op-amp characterization

Techniques for testing the frequency response of an operational amplifier (op-amp) sometimes require a buffer in the feedback path. This buffer is needed to prevent an inadvertent forward signal path from the input of the op-amp to the output of the op-amp through the feedback circuitry. The buffer must have very good linearity and a higher bandwidth than the op-amp under test. Three CMOS buffer designs are presented. Simulation results show total harmonic distortion of about 1% at frequencies as high as 10 MHz into a load capacitance at 5 pF. The buffers settle to within 1% of the final voltage in fewer than 20 ns.<>