SOME CJRCl IT ASPKCrS Ui THE TR.WSISTOU Mi 



standable from stability considerations, since the base lead is now one of 

 the signal terminals and, as before mentioned, putting resistance in the 

 base lead tends toward instability if a is enough greater than unity. The 

 etTect is so severe that often it is worth while to add resistance in the collector 

 lead, thereby reducing (/ to the neighborhood of unity, and simultaneously 

 reducing the amplifier to a state of greater tractability. 



Another feature of the grounded emitter amplifier is that the base re- 

 sistance rb is usually negligible, in contrast to its pronounced effect on the 

 reverse transmission of the grounded base amplifier. The role of feedback 

 element is taken over here by the emitter resistance r,. These considerations 

 have important effects on the properties of cascaded amplifiers and will be 

 reverted to later. 



For numerical comparison we might work the grounded emitter ampliticr 

 between the same two terminations as the grounded base amplifier above, 

 namely from 500 into 20,000 ohms. It would then have a gain of about 

 24 db, an improvement of 7 db over the grounded base, with about the 

 same power output and noise figure. This improvement is obtained at 

 greater risk of oscillation; in fact the output impedance of this amplifier 

 is negative. 



The remaining tube connection — the cathode follower or grounded plate 

 — is analogous to the grounded collector connection (Fig. 11); again, when 

 a = 1 the analogy is fairly close, in that the transistor has high-input im- 

 jiedance, low-output impedance, and no change of polarity in transmission. 

 In fact when a = 1 the device is usable in very much the same manner as 

 the cathode follower. The power output is lower than the other connections 

 because the output electrode (the emitter) does not carry much direct 

 current. 



However, when we make a greater than 1 the effect is even more pro- 

 nounced than it was in the grounded emitter case. As a increases from 1 , 

 the grounded collector amplifier rapidly loses its resemblance to the cathode 

 follower and begins to transmit in both directions as a bilateral element. 

 When (2 = 2, the operating gains in the two directions are the same; and 

 for a > 2 the transmission is actually greater in the "backward" direction. 

 Another curious feature is that, while the "forward" transmission is still 

 without change in signal polarity, the "reverse" transmission inverts the 

 signal polarity. 



In any device which is supposed to give gain in both directions, naturally 

 stability must be a controlling consideration. This amplifier is of course 

 still subject to the aforementioned stability condition (1) and it is found 

 that with care one can actually get power gains in both directions of trans- 

 mission without instability, i.e. a simi)le bilateral amplifier is present. One 

 numerical example may suffice. Assume a transistor liaving the properties 



