A high input impedance buffer amplifier using bipolar transistors

Abstract

he input impedance of conventional emitter follower circuits is limited due to the finite value of the passive emitter resistance, shunting effect of biasing resistors and that of intrinsic collector to base feedback admittance and also to the fall in current amplification factor at low operating currents. Further, the input admittance is frequency dependent because the device parameters involved therein are themselves frequency dependent. However, the shunt positive feedback incorporated in such circuits minimizes the shunting effect of the biasing network and also that of the intrinsic feedback admittance. The simulation of negative capacitance across the input terminals nullifies the effect of the presence of the otherwise positive capacity. This technique extends the bandwidth over which the input impedance remains constant. A typical buffer amplifier circuit employing five conventional epitaxial planar bipolar silicon transistors has been described in the present communication. The input impedance of which is found to be constant over a frequency range of 10HZ to 2KHz and its magnitude is about 25MΩ. © 1973 Taylor & Francis Group, LLC.