INPUT CHANGEOVER CIRCUIT

PURPOSE:To realize the high speed and highly accurate input changeover circuit by configurating a source constant current supply circuit with a high speed IC comprising resistors and NPN transistors (TRs). CONSTITUTION:A differential amplifier consists of TRs Q14, Q15 and an operating current is respectively Ix/2 according to an output current Ix of a constant current source 19. TRs Q16, Q17 form an output buffer and the entire gain of an output amplifier 5 is almost the unity by a feedback resistor 21 and an output voltage Vout is nearly equal to an output voltage V1 of diode bridges 2, 4. A TR Q18 and resistors 25,26 output a level shift voltage VS in following to an output voltage V1' of the diode bridges 2,4 and a TR Q28 outputs a bootstrap voltage VBS. When a base-emitter voltage of the TRs Q16,17,18,28 is identical to each other, the VBS follows to the input signal. Then the resistance of resistors 12,13 is selected to attain matching with a current source 11 and then the source constant current source with an excellent linearity with respect to the input signal is realized.