TRANSISTOR OSCILLATOR FOR MULTIFREQUENCY PULSING l!^!!! 



mediate frequencies and a third for the last two frequencies. Data on 

 these coils are given in Appendix I. 



The frequency of oscillation for each coil is determined by the capaci- 

 tance used with it and is given approximately by the tntnnila: 



Sufficient inductance is used in these coils so that mica condensers can 

 be economically used with them. Mica condensers are used because of 

 their low temperature coefficient. 



Leakage reactance and capacitance between windings make the circuit 

 resonant at more than one frequency. To minimize the current fed back 

 to the emitter under such parasitic conditions, the 4-8 winding was 

 placed next to the core with terminal 8 next to terminal 1 . This placed 

 an ac ground next to the end of the \\'inding connected to the emitter. 

 As a further precaution against parasitic oscillations the resistance be- 

 tween emitter and transformer is kept high. 



TRANSISTOR OPERATION 



It is customary to consider the transistor as an amplifier working 

 into a load represented by the tuned circuit. However since the current 

 in the collector and that in the emitter are intimately related during that 

 part of the cycle when power gain is obtained, the collector circuit can 

 be considered equally well as a negative resistance of a value established 

 by the feedback used and the emitter circuit simply as a load. At best 

 either method is only an approximation due to the nonlinearity of the 

 transistor characteristics for large signals. Representation of the transis- 

 tor as a resistance puts the requirements in terms of values readily ob- 

 tained from the static characteristics of a transistor. This form of treat- 

 ment is therefore used. 



The regions in which positive or negative resistance is obtained is il- 

 lustrated in Fig. 3. The characteristics shown are those for an ideal tran- 

 sistor. That is, the ratio of an incremental change in collector current to 

 an incremental change in emitter current, a, with a constant collector 

 voltage is a uniform value in region 2. Also, that in regions 1 and 3 the 

 slope of the lines in each region is constant. That this is not very different 

 from that obtained from some transistors can be seen by comparison 

 with the actual characteristics shown in Fig. 4. 



The division line between regions 1 and 2 represents the magnitude of 

 the dc voltage applied between collector and base, Vc-Vi, . On the left 

 hand side of Fig. 3 the phase relationship and magnitude of ac voltage 



