Fluke 5720A PA Common Circuitry, 2-142. +PA and -PA Supplies, PA Input Stage, ∙ ±185V ∙ ±365V

Theory of Operation 2

Analog Section Detailed Circuit Description

2-141. PA Common Circuitry

Common circuitry consists of the +PA and -PA supplies, input stage, mid stage, and the output stage. These four circuits are described under the next four headings.

Power Amplifier input node, gain, and feedback are different for dc and ac operation. Power Amplifier gain is -20 in the dc function, determined by the ratio of resistor network bonded to the HR8 assembly (500 kΩ/25 kΩ). Gain in the ac function is -10, which is determined by the ratio resistors [(R11 + R12 + R13)/R17]. This is described in more detail under "DC Voltage Function" and "AC Voltage Function".

2-142. +PA and -PA Supplies

The ±PA supplies are high voltage supplies generated by the Filter/PA Supply assembly (A18). These supplies can be controlled by the Digital Control SIP assembly (A16A1) and are switched between the two modes shown in Table 2-13.

∙±185V

∙±365V

Theory of operation for the Filter/PA Supply assembly (A18) describes how these voltages are generated and selected.

Table 2-13. PA and -PA Supply Settings at Different Outputs

Calibrator Output

Less than 22V ac or dc 22 to 110V dc

110 to 220V dc -220 to -110V dc

22 to 101V ac (freq < 120 kHz)

22 to 85V ac (freq > 120 kHz) Other voltages

220 to 550V dc or ac

550 to 1100V dc or ac

220 mA to 2.2A

+PA

+185V

+185V

+365V

+185V

+185V

+185V

+365V

+185V

+365V

+365V

-PA

-185V -185V -185V -365V

-185V -185V -365V

-185V -365V

-365V

2-143. PA Input Stage

The input stage consists of a heater-controlled hybrid HR8, op amp U7, transistor Q6, and JFET Q2. The HR8 assembly consists of an op amp mounted on a heated-substrate hybrid, with a resistor network bonded to it. Hybrid HR8 provides the input stage with excellent dc characteristics of low offset, noise and drift. The hybrid heater-control circuit (on sheet 3 of the schematic) adjusts the base voltage of Q38 to deliver the correct current to the heater resistor. This maintains the hybrid assembly at a constant temperature in spite of environmental temperature variations. Transistor Q35 protects the hybrid in case Q38 fails. Input of the hybrid op amp is protected by CR13 and CR14. Output of the hybrid op amp is connected to the input of a faster op amp (U7), which provides additional dc gain and a higher slew rate. JFET Q2 and transistor Q6 combined with these two op amps complete the input stage. Q2 is a very low-bias-current, high- frequency JFET.

In mid to high-frequency operation, Q2 is effectively the only path for the input stage signal. HR8 and the U7 op amps are bypassed at these frequencies by R89, C42, R24, and C12. As a result, the base of Q6 is at ac ground. In dc to mid-frequency operation,

2-87