406 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



SO that the resultant emitter current is positive most of the time, becoming 

 negative only long enough to keep the inductor "charged." 



The output circuit is easier to explain. The resistance R2 provides a path 

 through the battery which is not a short circuit. The resistance R2 provides 

 a path around the collector current supply which does not have infinite im- 

 pedance to the signal. The loads Zl andZz, are the ultimate receivers of the 

 amplified signal. The duality of the loads may be emphasized by pointing 

 out that the condition corresponding to Zl = 00 is Zl = 0. 



If the circuits are analyzed with the aid of the equivalent circuits dis- 

 cussed in the text, the voltage amphfication of the vacuum tube circuit will 

 be found to be 



fp R2 Zl 



while the current amplification of the transistor circuit is found to be 



rm 

 t. + R'. + Zl' 



These expressions are obviously duals. 



Transistor amphfiers like the one shown in Fig. 14 can be connected in 

 cascade. Three examples are shown in Fig. 15 (b) and (c) and (d). Figure 

 15(b) is the most obvious connection, and 15(c) and 15(d) have provisions 

 for correcting the relative phase inversion that occurs in the transistor cir- 

 cuit. If the circuit equations for the three examples are written out, it will be 

 discovered that only 15(c) and 15(d) are duals (in the sense defined in the 

 text) of the vacuum tube circuit 15(a). The remaining example, 15(b), is the 

 dual of a pecuHar looking circuit with one vacuum tube inverted. 



In the range where operation is nearly linear, the three cascaded amplifiers 

 behave much alike; and 15(c) and 15(d) can be regarded as pedantic at- 

 tempts to make the signs come out ''right." As soon as non-hnear operation 

 is encountered, however, the differences between the circuits become pro- 

 nounced. This will be clearer when multivibrators are considered. 



Figure 16 shows a variation of one of the circuits of Fig. 15 designed to 

 operate on a single power supply. A circuit like this with four cascaded stages 

 has been built and tested, and was found to work satisfactorily with selected 

 matched transistors. The two extra resistors in each stage, Ri and R2 , 

 are voltage dropping resistors, chosen to balance the voltage drops in the 

 emitter and collector circuits respectively. 



Figure 17 shows a multivibrator, conveniently illustrated as a two-stage 

 RC coupled amplifier with its output connected to its input. Below are shown 

 three circuits, of which the first is almost a dual and the other two are duals 



