DUALITY AS GUIDE IN TRANSISTOR CIRCUIT DESIGN 



417 



tube has an inherent phase reversal and the transistor has not, an extra 

 transformer is needed in the coupHng network. It is conveniently placed as 

 shown, where it can be combined with the inductor. The input circuits to the 

 amplifiers have self bias circuits which have already been described. 



The Colpitts oscillator is similar to the Hartley oscillator. The difference 

 lies in the coupling circuit. Figure 30 shows the coupling circuit for the 

 Colpitts oscillator and its dual. The tuned-plate tuned-grid oscillator can be 

 treated in exactly the same way. 



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(a) (b) 



Fig. 30 — Colpitts coupling network and dual. 



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1 



Ca) (b) 



Fig. 31 — Transistor oscillator coupling networks. 



The coupling circuits used in successful vacuum tube oscillators are char- 

 acterized by having a phase shift of 180°. This can be done with structures 

 like low- or high-pass filter sections, by R-C networks, and in other ways. 

 On the other hand, the coupling network required for a good transistor os- 

 cillator must have zero (or 360°) phase shift. It is therefore probably 

 most easily arrived at by designing a 180° phase shifting network and add- 

 ing a phase inverting transformer. Two simple coupling networks which 

 have zero phase shift are shown in Fig. 31. These are both band-pass struc- 

 tures which lead to oscillation in the pass band. Of the two, the network of 

 Fig. 31(b) gives a more rapid change of phase with frequency and hence leads 

 to a more stable oscillator. In addition, this circuit has the potential ad- 

 vantage of providing means for matching impedances. 



