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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



regulator and about 0.2 db at a dynamic unit. Thus an improved com- 

 pensation scheme was required which would connect only to an indirect 

 heater, the bead itself being already controlled by dc heating from the 

 regulators. 



The compensation circuit adopted is shown on Fig. 29. A second 

 thermistor called the compensating thermistor is mounted in the same 

 glass envelope with the regulating thermistor. The fixed resistor Ri 

 and the compensating thermistor together with transformer T form a 

 bridge which is made a feedback path for tuned amplifier A. The feed- 

 back is positive when the compensating unit is cold so oscillation begins 

 at the tuning frequency (4 kc). These oscillations heat the thermistor 

 and tend to bring the bridge into balance. The bridge stabilizes at a 



COMPENSATING 

 THERMISTOR 



Fig. 29 — Ambient temperature compensation oscillator. 



small unbalance just sufficient to yield a loop gain of unity. The level 

 of oscillation is forced to that value which will maintain this small un- 

 balance. Any changes in balance thereafter produce deviations of loop 

 gain from unity and the oscillation level increases or descreases until 

 the equilibrium is reestablished. Thus the level of oscillation changes 

 with temperature but the resistance of the compensating thermistor is 

 held constant. 



Because the circuit supplies nearly perfect temperature compensa- 

 tion to the unit in the bridge a suitable fraction of the oscillator power 

 may be fed to the heater of the regulating thermistor to achieve very 

 close compensation of it. Resistors R2 and Rz are adjusted in manu- 

 facture to correct for slight differences between the two thermistors. 

 Note that the compensating thermistor is provided with an unused 

 heater to match the thermal properties of the two units. 



The amplifici- consists of a single 403B tube with 20 db dc feedback 

 for current (and transconductance) stabilization. This feedback is vital 



