Heater Transistor Balance; Figure 8-11. Turn-On Current Limit Circuit; Figure 8-12. Heater Transistor Balance Circuit - HP 10811A Operating & Service Manual

8-39. HEATER TRANSISTOR BALANCE

8-40. Because heater transistors Q7 and Q8 are not equally spaced from the crystal, it is necessary to
offset the power dissipation between the two transistors.
schematic of this circuit. Amplifier U1A references a voltage divider across Vcc (R25, R26) and a
divider referenced to the mid-point between the heater transistors. From this U1A controls the
base current of Q7 to insure the Voltage at the mid-point between the heater transistors is a
constant percentage of Vcc (V4 ≈ 0.57 Vcc ± 2%).
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Figure 8-11. Turn-on Current Limit Circuit

Figure 8-12. Heater Transistor Balance Circuit.

Model 10811A/B
Figure 8-12 shows a simplified