Fluke 5720A Complementary Drive Circuit, Transconductance Amplifiers, Shunt Resistors

5700A/5720A Series II Calibrator

Service Manual

The input signal return paths are selected for ac or dc operation by control lines FET1 and FET2, and NMOSFETs Q20-Q23. With a dc or 13V reference input, control line FET1 goes high, turning on Q21 and Q22, connecting RCOM and DAC OUT LO to the respective circuit. For ac current input, control line FET2 goes high, turning on Q20 and Q23, connecting OSC SENSE LO and OSCOM to the respective circuit.

2-189. Complementary Drive Circuit

The circuitry containing transistors Q2, Q3, Q18 and Q19 provides the complementary drive to the output transconductance amplifiers from the single-ended output dc amplifier on hybrid HR2. Transistors Q2 and Q18 provide the drive during a positive input, while Q3 and Q19 provide the drive during a negative input. Relay K2 switches the drive to the 220 μA/2.2 mA transconductance amplifiers when reset, or to the 22 mA/220 mA transconductance amplifier when set.

The HR2 assembly consists of an op amp mounted on a heated-substrate hybrid, bonded to the shunt resistor network. The HR2 assembly gives this circuit excellent dc characteristics of low offset, noise and drift. The hybrid heater control circuit, outlined on sheet 2 of the schematic, adjusts the base voltage of Q1 to deliver the proper power to the heater resistor. This maintains the HR2 assembly at a constant temperature in spite of environmental temperature variations. Transistor Q14 protects the hybrid in case Q1 fails.

2-190. Transconductance Amplifiers

The transconductance amplifiers are the 220 μA/2.2 mA range amplifier the 22 mA/220 mA range amplifier circuits. The 220 μA and 2.2 mA ranges are provided by the 220 μA/2.2 mA range transconductance amplifier, containing transistors Q4, Q5, and relay K3. With relay K3 reset as shown on schematic, the amplifier is in the 220 μA range. To place the amplifier in the 2.2 mA range, relay K3 is set so emitter resistors R10 and R12 are shunted by R11 and R13 respectively.

The 22 mA/220 mA range transconductance amplifier contains transistors Q6-Q13 and relay K4. It provides the 22 mA and 220 mA ranges. In the 22 mA range, the transconductance amplifier is composed of Q6-Q9 with K4 in the reset position. In the 220 mA range, the transconductance amplifier is composed of Q6-Q13 with K4 in the set position. In this range, Q10-Q13 are in the Darlington configuration with Q6-Q9 respectively. This provides the additional current gain needed for the 220 mA range.

2-191. Shunt Resistors

A resistor network is used to sense the output current in each of the four current ranges. This network is composed of four four-terminal resistors attached to the heated substrate of HR2. The shunt resistors are 10 kΩ for the 220 μA range, 1 kΩ for the 2.2 mA range, 100Ω for the 22 mA range, and the 10Ω for the 220 mA range. Relays K5-K9 select the INPUT, OUTPUT, and SENSE binding posts for each of the four ranges as Table 2-15 shows.

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