1094; THE BELL SYSTEM TECHNICAL JOvJRNAL, SEPTEMBER 1956 



3. The necessity of a feedback circuit time constant equal to or shorter 

 than the output pulse length results in a relatively low output power 

 efficiency. 



Due to the above considerations, capacitor feedback appears to be the 

 least attractive type of feedback. 



3.3 Transformer Coupled Feedback 



A transformer appears to be the most convenient and versatile com- 

 ponent for feedback coupHng in a regenerative amplifier. The pertienent 

 features* of a transformer are: 



1. Current or voltage gain (impedance matching.) This feature per- 

 mits full use of the power gain of the transistor, even if such gain be in 

 the form of voltage or current gain only. 



2. Bias isolation between circuit parts and the possibility of supplying 

 dc voltage bias without the use of additional elements. 



3. Phase inversion, if desired. 



All of these featvu-es, conveniently combined in a transformer, provide 

 great design freedom to meet specified circuit objectives. Since positive 

 feedback is possible with any type of transistor (with power gain, of 

 course), the choices of transistor and connection are determined by other 

 circuit requirements. 



The use of transformer coupled feedback yields the familiar blocking 

 oscillator circuit. An important feature of this circuit is the fast rise time 

 that is obtainable. Linvill and Mattson^ have shown that a junction - 

 transistor with an alpha cutoff frequency of two megacycles may exhibit 

 a rise time of 0.1 microsecond in an unloaded blocking oscillator with 

 collector to emitter coupling. Fig. 6 (a). It can be shown that the same 

 response may be expected with collector to base or base to emitter 

 coupling, provided that the transformer turns ratio is modified. Figs. 

 6 (b) and 6 (c) . When the circuit is providing useful output power into a 

 load, a slightly different turns ratio would be used for optimum rise ■ 

 time, which may be appreciably slower than in the unloaded case. How- 

 ever, it should be noted that the foregoing gives no information about the 

 initial response of the circuit from the time that the input trigger is 

 applied until the output reaches ten per cent of its final value. In some 

 instances this initial time, which is a complicated function of the trans- 

 istor non-linearities, may be comparable to the output rise time. 



In a blocking oscillator circuit with a fixed output load, the degree of 

 stability in the "on" state decreases with time. The reason is that the 



* The operation of a transformer over the non-linear portion of its magnetiza- 

 tion characteristic is outside the scope of this paper. 



