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

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.