NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER 



If the valve is a pentode Rj^ <^ r„ and the output impedance is given with 

 sufficient accuracy merely by 1/^^. For both triode and pentode it is real 

 and positive, meaning that the output impedance is resistive. ^,„ is typically 

 5 mA/V, giving an output resistance of only 200 Q.. This is very low com- 

 pared with the output resistance of an amphfying stage, which is for a 

 pentode substantially equal to the load, say 100,000 Q. {Figure 11.19), and 



HT + 



9r 



,f>Vo 



Pentode 



output 

 impedance:/? 



HT- 

 Figure 11.18 



Figure 11.19 



for a triode {R . rJI(R + rj, where R is typically 30,000 Q. and r„ 10,000 Q, 

 giving an output resistance of 7,500 Q. {Figure 11.20). The cathode follower 

 is the nearest approach in electronics to a constant-voltage generator for 

 signals. 



Advantage of low output impedance — Many of the resistance-capacitance 

 networks we mentioned in Chapter 4, such as the twin T, were analysed on 

 the basis of their being driven from a constant-voltage generator and feeding 

 a load of infinite resistance. The nearest we can get to this state of affairs in 

 practice is to work the device between two cathode followers {Figure 11.21). 



HT+ 



Triode output 

 impedance 



R.ra 





W 



2C 



R-^ra 



-A/W — t 



&■' 



Figure 11.20 



Figure 11.21 



The input to the first cathode follower can then be derived from some 

 circuit having rather indifferent properties for constant-voltage generation. 

 Similarly the output of the second may be passed on to something having 

 quite a low resistance. Again in Figure 11.22, suppose device A is feeding 

 device B via a rather long piece of screened cable having a capacitance between 



174 



