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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 Vid, both laboratory measurements and PSPICE and LTspice simulation results for the output voltage Vo are considerably lower than one base emitter unit VBE(on) below the supply voltage VCC, as predicted by a textbook derivation. Modification of the derivation to include the p-n-p transistor base currents and the current gain βP and the supply voltage and specifications for the four transistors provide equations that predict results for Vo that are consistent with laboratory experience and computer simulations. The output voltage is in excess of three VBE(on) units below VCC when βP is 50. At larger values for βP , Vo is increased by a factor proportional to 1/βP, reaching one VBE(on) unit below VCC as βP approaches infinity. Larger values for VCC cause an additional modest difference between VCC and Vo at low values for βP. 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 Vid, while larger values for βP result in a modest increase in gain at all values of VCC.
Date of Publication: May 2012