Differential Amplifier With Current-Mirror Load: Influence of Current Gain, Early Voltage, and Supply Voltage on the DC Output Voltage

George F Paulik,Raymond P Mayer

doi:10.1109/te.2011.2164542

Abstract

A differential amplifier composed of an emitter-coupled pair is useful as an example in lecture presentations and laboratory experiments in electronic circuit analysis courses. However, in an active circuit with zero input load V id , both laboratory measurements and PSPICE and LTspice simulation results for the output voltage V o are considerably lower than one base emitter unit V BE(on) below the supply voltage V CC , as predicted by a textbook derivation. Modification of the derivation to include the p-n-p transistor base currents and the current gain β P and the supply voltage and specifications for the four transistors provide equations that predict results for V o that are consistent with laboratory experience and computer simulations. The output voltage is in excess of three V BE(on) units below V CC when β P is 50. At larger values for β P , V o is increased by a factor proportional to 1/β P , reaching one V BE(on) unit below V CC as β P approaches infinity. Larger values for V CC cause an additional modest difference between V CC and V o at low values for β P . The derived equations and LTspice values are used to develop an empirical equation that predicts output voltages for the given Early voltages. The technique is adaptable to student data evaluation assignments. The computer simulations also showed that increased supply voltage results in increased gain for a given input value for V id , while larger values for β P result in a modest increase in gain at all values of V CC .

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