Fluke 5720A service manual Output Stage, Phase Shifter, Power Amplifier Assembly A16

Theory of Operation 2

Analog Section Detailed Circuit Description

The mid stage is a common-emitter, Miller-compensated gain stage (Q5) that drives a common-base level shifter (Q13) on the Oscillator Output assembly. This stage is current limit protected by R12 and Q4. The dominant pole is set with the Miller capacitor C5 and the input stage transconductance. The mid stage is biased to 10 mA by Q6 and related components.

The output stage of the amplifier is a bootstrapped complementary Darlington pair. The only parts of the output stage on this A13A1 assembly are the input transistors Q7 and Q8. The output bias current is set by R17 and CR4-CR5 to be approximately 40 mA. This keeps the output stage class A for all normal output conditions.

2-137. Output Stage

The output stage circuit is a complementary Darlington emitter follower bootstrapped buffer amplifier. The input transistors are Q7 and Q8 on the A13A1 assembly. These transistors drive the output transistors Q8 and Q14 respectively. Transistors Q10 and Q11 in the positive side and Q16 and Q17 in the negative side are parallel transistors bootstrapped by VR3 and VR4. Current sources CR13, CR14, CR17, CR18, CR15, CR16, CR11, and CR20 provide the bias current for their respective bootstrapped transistors. Current limiting for the positive side is provided by Q9 and R91. During an overcurrent condition, the voltage drop across R91 turns on Q9, which draws current away from the base of Q11. Current limiting is done in the same manner for the negative side with Q15 and R99.

Switch S1 can be switched to pull the input of the A13A1 output stage low for troubleshooting the output stage. Refer to the Oscillator Output troubleshooting section for more information.

2-138. Phase Shifter

The phase shifter circuit provides a fixed amplitude variable phase auxiliary signal at the rear panel of the calibrator. This signal is the same frequency as the output, but can be phase shifted over a 360º range. The four phases (each 90º apart) for the oscillator circuit are divided by Z4 and Z5. These signals are connected to the dual four-channel multiplexer (U27), which is under the control of the PHASE control bus from the digital control circuit. This multiplexer selects any two adjacent oscillator phases (e.g. 0º and 90º) that are connected to the input of a dual monolithic DAC U28. These signals are then scaled by the dac (U28), also under the control of the digital control circuit. The two outputs of this dac are summed by op amp U29. Using this method, the output of U29 is a phase shifted signal between 0 and 360º, where the scaling of the signals phase shift within a 90º range.

2-139. Power Amplifier Assembly (A16)

The Power Amplifier assembly outputs dc voltages from ±22V to ± 219.99999V and ac voltages from 22V to 219.99999V rms. The frequency limit for 220V ac output is 100 kHz. Output voltage limits are derated at frequencies above 100 kHz. At 1 MHz, the maximum output voltage is 22V rms. The Power Amplifier drives the High Voltage assemblies (A14, A15) in all high voltage and high current functions.

This assembly also contains calibration circuitry that enables the internal calibration system to determine exact Power Amplifier ac and dc gain, offsets and frequency response.

The main sections of this assembly are the input stage, mid stage, output stage, sense- current cancellation circuit, the dc and ac gain calibration circuits, and the Power Amplifier Digital Control SIP assembly (A16A1), which is mounted on the Power Amplifier assembly.

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