1,213,110. Transistor amplifier. BECKMAN INSTRUMENTS Inc. 23 July, 1969 [5 Aug., 1968], No. 36921/69. Heading H3T. Three integrated circuit amplifiers 10, 11, 12 each having an inverting and a non-inverting input are interconnected by potential dividers 30, 31, and 32, 33 and 34, 35 substantially as shown, between two input terminals and an output terminal and a common terminal, 17. The power supply for amplifier 12 is derived from transistors 24, 25 having fixed base bias circuits; and the supplies X, Y for amplifiers 10, 11 may be derived from transistors 20, 21 so as to be modified in response to the output of amplifier 11, which is applied to the junction of Zener diodes 13, 14 connected in series with transistors 18, 19. The amplifier 11 has unity gain and operates at the common mode for lowlevel signals establishing a reference for the other amplifiers. Common mode rejection is thus provided without access to the internal circuits of the amplifiers 10, 11, 12. The use of the circuit using T18-21 to derive the supply voltages X, Y further improves rejection.
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