High input impedance buffer with low feedback resistance

A high impedance, high frequency input circuit for an instrument suitable for measuring ac signals whose frequencies are within a predetermined frequency range, comprising:

• (a) an input terminal for receiving the ac electric signals that are to be measured and an output terminal;
• (b) an input resistor having a first end coupled to said input terminal;
• (c) an operational amplifier having an amplifier input coupled to a second end of said input resistor and an amplifier output coupled to said output terminal;
• and,
• (d) a feedback path coupled from said amplifier output to said amplifier input, said feedback path having a first resistor and a voltage divider, wherein said input resistor, first resistor and voltage divider determine the gain for said input circuit and both the resistance of said first resistor and the total resistance of said voltage divider are significantly less than the resistance of said input resistor such that when said ac electric signals are within said predetermined frequency range said gain of said input circuit is unaffected by stray capacitance associated with said first resistor and voltage divider.