Circuit Design of a Fast, Low Input Impedance Operational Amplifier for Current Follower Use

An operational amplifier circuit with a common base input stage is described. The advantage of this circuit is that the inverting input does not deviate from virtual ground voltage by more than a few tenths of a millivolt at any time after an input transient, even when large feedback resistors (100 kΩ) are used. Equations are derived for the input impedance-time function of current followers based on conventional operational amplifiers and on this design. Comparison of calculated and observed transient response show that this new design allows a reduction of three orders of magnitude in input impedance of a current follower at short times (0.1 μusec) after an input transient.