DUALITY AS GUIDE IN TRANSISTOR CIRCUIT DESIGN 



409 



voltage, the plate potential falls, and the positive feedback accelerates the 

 process so the first grid rises to a positive potential and the other grid falls 

 to a large negative potential. The process is then repeated. 



The dual behavior of the transistor multivibrator is as follows: Suppose 

 that the emitter current of one transistor is very large, and of the other 

 about zero. The large emitter current passes through the emitter, an in- 

 ductance, and a small resistor, and will decay at a rate controlled by the in- 

 ductance and the effective resistance of the resistor and emitter. As it decays, 

 no effect will result in the collector circuit until the emitter current falls 

 below the collector voltage cutoff point, after which the collector current 

 will decrease. The emitter current of the other transistor will increase^ as a 

 consequence of the phase inversion built into the circuit, and the collector 

 current of the second transistor will increase. As a consequence of positive 

 feedback, the whole process will accelerate suddenly and proceed until the 



(a) (b) 



Fig. 18 — Cathode follower and dual. 



emitter current of the second transistor is large and that of the first is zero 

 or negative. The process will then repeat. 



Figure 18 shows a simple cathode follower and its dual. It has been ex- 

 plained in the text that, in circuits where the cathode current or the grid- 

 to-plate voltage play an important part, the dual circuit will usually require 

 a transformer. Alternatively it may be said that, in any circuit in which 

 feedback in a single stage plays an important role, a transformer may be a 

 necessity. In fact, we have found it impossible to avoid the use of a trans- 

 former in the dual of the cathode follower. 



The transistor circuit shown will need another power supply for emitter 

 bias, and a blocking condenser to prevent the bias current from flowing 

 through one winding of the transformer. The power supply is required be- 

 cause the transformer will not transmit d-c. signals, and the condenser is 

 necessary because the d-c. impedance of a transformer winding is nearly 

 zero. Extra blocking condensers will appear in association with transformers 

 in many circuits, especially in cases where the transformer is being used as 

 the dual of another transformer. 



A vacuum tube cathode follower ordinarily has a high input impedance 



