HP_6289A.p21-30

remaining components of the feedback loop.Re-4-25 VOLTAGE CLAMP CIRCUITsistor Rl,in series with the base of Q1A,limitsthe current through the programming resistor during4-26 During constant current operation the con-rapid voltage turn-down.Diodes CRl and CR2stant voltage programming resistors are a shuntform a limiting network which prevent excessiveload across the output terminals of the power sup-voltage excursions from over driving stage Q1A..ply.If the output voltage changed,the currentCapacitors Cl and C2,shunting the programmingthrough these resistors would tend to change re-resistors,increase the high frequency gain of thesulting in an output current change.The clampinput amplifier.Resistor R13,shunting pulloutcircuit is a return path for the voltage programmingresistor R12,serves as a trimming adjustment forcurrent,the current that normally flows through thethe programming current.programming resistors.The circuit maintains thecurrent into the constant voltage summing point4-20 CONSTANT CURRENT INPUT CIRCUIT(A6)constant,thus eliminating the error due toshunting effects of the constant voltage program-4-21 This circuit is similar in appearance and op-ming resistors.eration to the constant voltage input circuit.Itconsists basically of the current programming re-4-27 The voltage divider,R51,R52,and VR3,sistors R16A and B,and a differential amplifierback biases CR30 and Q10 during constant voltagestage (Q2 and associated components).Likeoperation.When the power supply goes into con-transistor Q1 in the voltage input circuit,Q2 con-stant current operation,CR30 becomes forwardsists of two transistors,having matched charac-biased by the collector voltage of Q1A.This re-teristics,that are housed in a single package.sults in conduction of Q10 and the clamping of thesumming point at a potential only slightly more4-22 The constant current input circuit continu-negative than the normal constant voltage potentialously compares a fixed reference voltage with theClamping this voltage at approximately the samevoltage drop across the current sampling resistorspotential that exists in constant voltage operation,R54 and R55.If a difference exists,the differen-results in a constant voltage across,and conse-tial amplifier produces an error voltage which isquently a constant current through the pullout re-proportional to this difference.The remainingsistor (R12).components in the feedback loop (amplifiers andseries regulator)function to maintain the drop4-28 MIXER AND ERROR AMPLIFIERSacross the current sampling resistors,and.conse-quently the output current,at a constant value.4-29 The mixer and error amplifiers amplify theerror signal from the constant voltage or constant4-23 Stage Q2B is connected to a common (+S)po-current input circuit to a level sufficient to drivetential through impedance equalizing resistor R26.the series regulator transistors.The emitter biasResistors R25 and R28 are used to zero bias the in-potential for mixer amplifier Q3 is establis