HEWLETT PACKARD (HP) 11153A
Service Manual

Model 11153A

Section IV

SECTION IV THEORY O F OPERATION
4-1.
INTRO D UCTlON.

4-7.

CONVERTER AMPLIFIER.

A block diagram of the True RMS Converter is s h q m in Figure 4-1. In the VAC(AC) mode of operation, the input is capacitively coupled; while in the VAC(DC) mode the input is direct coupled, allowing the instrument to measure the total rms value of a signal containing both ac and dc.

4-2. ATTENUATOR AMPLl FIER.
4-3. The Attenuator Amplifier is a broadband inverting amplifier having a differential input stage and a push-pull output stage. Dual field effect transistors used in the input stage are connected as source-followers to maintain a high input impedance. Constant current sources are used at several points in the amplifier to maintain proper bias currents. The Attenuator Amplifier gain is equal to the feedback impedance divided by the input impedance. Range switching is accomplished by varying the attenuation of the Attenuator Amplifier and Post Attenuator as shown in the table given in Figure 4-1.

One half of the dual thermocouple converts the ac signal to dc, and the other half is used in the dc feedback loop of the Converter Amplifier. Since a thermocouple is a non-linear device, the feedback offsets the non-linearity of the amplifier input signal, resulting in a linear dc output. Using the dual unit in this manner also minimizes the effect of ambient temperature drift. An integrating ac feedback loop is employed to filter out the ac component of the thermocouple output. A non-linear amplifier in the ac feedback loop offsets the non-linearity of the thermocouple output to provide a linear integrating action. Integrating capacitance is increased in the VAC(DC) mode to permit measurement of signals down to 1 Hz. The gain of 1 indicated in Figure 4-1 for the Converter Amplifier is the gain from the rms value of the Converter Thermocouple input to the dc output of the amplifier.

4-8. THERMOCOUPLE PROTECTION.
To protect the thermocouple, a virtual short circuit is placed across the Post Amplifier output stage if operating conditions exist which might damage the thermocouple. An integrated circuit comparator amplifier, in conjunction with a bridge rectifier circuit, compares the Post Amplifier output to a predetermined level. If the output exceeds this level, the comparator causes a clamp circuit to shortcircuit the Post Amplifier output stage. A second comparator amplifier compares the output of the Converter Amplifier to another predetermined level, and causes the clamp circuit to cut off the Post Amplifier output stage if the level is exceeded.

4 4 . The Attenuator Guard (shown in Figure 7-1) is driven by an emitter-follower, maintaining the guard at nearly the same potential as the amplifier input. The Attenuator Guard is not connected to the input guard . terminal

4-5.

POST AMPLIFIER.

The Post Amplifier is a broadband non-inverting amplifier gaving a gain of 10. This amplifier also has a differential input stage and a push-pull output stage. The output of the Post Amplifier drives the heating element of the Thermocouple.

4-9.

OUTPUT ATTENUATOR AND FILTER.

4-6.

CONVERTER THERMOCOUPLE.

The Converter Thermocouple is one half of a specially designed dual thermocouple called a thermopile. Each half of the thermopile consists of 30 thermocouples in series, resulting in high sensitivity. The low thermal mass of the thin-fim construction permits rapid response to input signal changes.

The Output Attenuator is provided to adjust the True RMS Converter output to the specified accuracy. The Output Filter is bypassed in the VAC(AC) mode since ample filtering is provided by the 3484A Attenuator Assembly (Al). Because frequencies as low as 1 Hz may be measured in the VAC(DC) mode, the Output Filter is used. In addition, the Filter Amplifier output (from A2) is coupled into the filter to reduce 'response time. The True RMS Converter output is 0 to + 1.5 V on all ranges.

4-1