Abstract
A requirement exists for a simple high performance buffer amplifier for use with capacitive loads. A suitable circuit uses a bootstrapped emitter follower in conjunction with a near unity gain feedback amplifier. Tha a.c. input impedance of the emitter follower is determined by the isolation resistance of the input terminal, the base recombination currents, and the incremental changes in base charge. Operation at low current levels permits operation at near unity gain (∼0.999) while with capacitive feedback the input impedance is equivalent to a capacitance in the ±0.1 pF range at frequencies above 1 kHz. The amplifier's voltage gain is 0.9980 ± 0.0005 (adjustable to 1.0000 ± 0.0005) and its output impedance is less than 0.1 Ω between 700 Hz and 70 kHz, for capacitive loads up to 0.01 µF. Minor alterations permit increased capacitive loading and operation at lower frequencies.
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