TRANSISTOR OSCILLATOR FOR MULTIFREQUENCY PULSING 1321 



teristics so connected (push-pull circuit) that a negative resistance would 

 be obtained from tt to 27r of the ac wave, the effect of a negative resistance 

 of constant value would be obtained. In order to reduce the transistors 

 to the terms of a two-pole device having a negative resistance the posi- 

 tive resistances represented by the emitter circuits and regions 1 of the 

 transistors would also have to be included. 



It is more convenient however to combine all elements that produce 

 a loss when using the static characteristics of a transistor to determine 

 if the transistor will satisfy the circuit requirements for oscillation. This 

 method is therefore used when considering the circuit operation. 



There is one important difference between the characteristics of a 

 transistor and the ideal characteristics shown. That is, for very small 

 values of emitter current bias and a constant E^ , the ratio A/c/A/e, or, 

 a, drops rapidly from three or more to approximately one as the emitter 

 current approaches zero. To eliminate this change in the dynamic nega- 

 tive resistance for very low voltage changes on the collector, a small 

 dc current is supplied in the emitter circuit. The need for this will be 

 discussed later. 



CIRCUIT OPERATION 



The negative resistance of the transistor is effectively connected in 

 parallel with the resistances representing the various elements that make 

 up the total loss by transformer action. The requirements for oscillation 

 are therefore met when —R2 for a complete cycle is less than the positive 

 resistance representing all losses. If the length of time that — R2 is ef- 

 fective is only one half cycle the value of —R2 must of course be cut in 

 half.* Since a single transistor could meet the requirement for —R2 only 

 one was used in the working circuit as is shown in Fig. 2. 



The transistor adopted for this use was the 1729 type, now in produc- 

 tion, which has been given the RTMA designation 2N25. The 1729 type 

 was used because its characteristics are least affected by changes in 

 temperature, and in addition the allowable power dissipation was ap- 

 proximately twice that of other comparable transistors. 



The various factors that when combined make up the load and their 

 normal variation are given in Table I. All losses are in terms of power 

 into the 1-2 winding. 



The corresponding value of load resistance is, 



J? - (^c - n - Fc.)^10()0 

 ^''^ ~ 2X2XWt 



* See Appendix II. 



