374 



BELL SYSTEM TECH SIC A I. JOURJ^AL 



In the grounded base circuit the property of low base resistance is im- 

 portant, since the backward transmission depends directly on this property. 

 In circuit terms, the base impedance is the feedback impedance in 

 the grounded base circuit, and its value helps to set a limit on the stable 

 gain which can be realized. 



TUBE ANALOGY -GROUNDED GRID 



f 



EQUIVALENT CIRCUIT 



Equations: ii{Ro + r,+ ;■(,) + 12 rb = vg 



h{n + r,„) + i2(rb + ro+RL) =0 



Circuit determinant A = (i?G + »-£ + rb){RL + ^c + rb) - rb{rb + r„^) 

 > for stability 



Input impedance i?n = r^ -\- Tb — 



Output impedance Rn = fe -{- n — 



bin + rj 



RL + rc + /-fc 



fbin + rm) 

 RG + r, + n 



Operating power gain Go = ■^Rg 



-(--^•y 



Typical values: For Rg = oOO", Rt = 20,000' 

 Then Rn = 280'", /eo. = 9600- 

 Go = 17'^'' 

 Fig. 7 — Synopsis of grounded base ami)lilier. 



The grounded base circuit has [)roperties which are strongly reminiscent 

 of the grounded grid electron triode amplifier in that both have low input 

 impedance, high output impedance, and no change of signal polarity in 

 transmission. The analogy was pointed out by Shockley. That this similarity 

 is no coincidence can be seen by comparing the third equivalent circuit 



