Fluke Car Amplifier 5220A User Manual
Theory of Operation
Circuit Analysis
3
3-9
3-10. A7 Output
The A7 Output is a discrete-component, complementary-output, high-current amplifier
which serves as the final output stage of the transconductance amplifier. See Figure 3-6.
It consists of a pair of driver transistors, Q101 and Q102; two parallel pair(s) of output
transistors, Q103, Q104 and Q105, Q106; a thermal sensing transistor, Q107; and a
thermal overload switch S101. The differential output from the A6 Driver is applied to
the bases of Q101 and Q102. These transistors are configured as emitter followers and
provide the high current drive required by the output transistors. Collector resistors, R114
and R115, for Q101 and Q102 provide current limit protection under transient or fault
conditions. Emitter resistors in the parallel output pairs prevent current hogging which
would otherwise occur due to unequal VBE characteristics.
which serves as the final output stage of the transconductance amplifier. See Figure 3-6.
It consists of a pair of driver transistors, Q101 and Q102; two parallel pair(s) of output
transistors, Q103, Q104 and Q105, Q106; a thermal sensing transistor, Q107; and a
thermal overload switch S101. The differential output from the A6 Driver is applied to
the bases of Q101 and Q102. These transistors are configured as emitter followers and
provide the high current drive required by the output transistors. Collector resistors, R114
and R115, for Q101 and Q102 provide current limit protection under transient or fault
conditions. Emitter resistors in the parallel output pairs prevent current hogging which
would otherwise occur due to unequal VBE characteristics.
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Figure 3-6. A7 Output Functional Block Diagram
All transistors in the A7 Output as well as the thermostatic switch, S101, are physically
mounted on a common heat sink. The temperature of the heat sink is sensed by Q107
(part of the A7 Driver’s bias network) and returned to the A7 Driver to provide bias
compensation for the change in VBE with output-transistor temperature. (See A7 Driver
description given earlier in this section of the manual.) Thermostatic switch S101 senses
the temperature of the heat sink and provides an open contact output when an over-
temperature condition is detected. The open output is sent to the A10 Logic which reacts
by switching the 5220A to standby (over temperature may result from a loss or restriction
of cooling airflow).
mounted on a common heat sink. The temperature of the heat sink is sensed by Q107
(part of the A7 Driver’s bias network) and returned to the A7 Driver to provide bias
compensation for the change in VBE with output-transistor temperature. (See A7 Driver
description given earlier in this section of the manual.) Thermostatic switch S101 senses
the temperature of the heat sink and provides an open contact output when an over-
temperature condition is detected. The open output is sent to the A10 Logic which reacts
by switching the 5220A to standby (over temperature may result from a loss or restriction
of cooling airflow).