NOISE IN RESISTANCES 



173 



Here Vg is the grid voltage and Vp the plate voltage of the triode. If the 

 plate voltage is held constant and n is taken as constant 



dVo = dV, 



Hence, under these conditions 



a/o/dFo = G = dlQ/dVg. 



(67) 



(68) 



Here G is the conductance of the equivalent diode, the reciprocal of R 

 which appears in (65), and is also the transconductance of the triode. 



As we wish to calculate the noise with no a-c grid or plate voltage, and 

 as these through (64) specify the plate voltage Vq of the equivalent diode, 



Ci GRID-CATHODE CAPACITANCE 

 C2 GRID -PLATE CAPACITANCE 



Fig. 6 — Low-frequency noise in a triode can be ascribed to a fictitious noise resistance 

 Rn , acting into an open circuit to cause voltage fluctuations on the grid. 



the equivalent diode may be regarded as short-circuited. Hence, the noise 

 current will be 



= {.6U)4:kTaGB. 

 If we express this as shot noise reduced by a factor T^ we obtain 



i^ = 2eIoT^B 



2kTM 



(69) 



r = (.644) 



(70) 



elo 



Often, the noise expressed by (69) is ascribed as a fictitious noise resistance 

 Rn , at room Temperature T, connected between the grid-cathode capaci- 

 tance and the ''controlling action" of the grid as shown in Fig. 6. This 

 fictitious resistance looks into a complete open circuit; hence, it has a noise 

 voltage 



v^ = ^kTRnB 



(71) 



