NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER 



If the cathode current is still 2 mA, the voltage drop across Rj^ will be 20 V. 

 If the negative supply is made —18 V the cathode will still be 2 V positive 

 with respect to the grid and the bias is still negative even though it might 

 appear from Figure 11.7 that it is positive. The gain without feedback is 

 now 



50,000+10,000 _ ^. 10,000 



A = 50 



50,000 + 10,000 + 10,000 



= 43 



Bis 



10,000 + 50,000 



0-167 



and AB = 7-2. If it be allowed that 7-2 is ^1, then it may be stated straight 

 off that the gain is approximately 1/5 = 6. The exact gain expression is 

 Al(\ + AB) = 43/(1 + 7-2) = 5-25. If f^ falls to half the new gain is 

 21-5/(1 + 3-6) = 4-7. As a result of increasing Rj^ the change in gain with 

 feedback for a halving of valve [jl falls from 35 to 10-5 per cent — a desirable 

 improvement. 



CONCERTINA PHASE SPLITTER 



When R^ = R^^, dVji^ and dVn^ are equal and opposite, and the device is 

 called a phase splitter (Figure 11.8). When the grid is made positive, for 



HT* 





example, the valve current increases and so do the drops across R^ and i?^; 

 therefore the anode goes negative and the cathode positive. As the grid 

 potential alternates the anode and grid may be visualized as going 'in and 

 out' concertina-wise. The gain with feedback is 



dV 



in 



(5Fi 



an 



and is 



1 + AB 



where B 



and A 



julRj^ 

 ra + 2Rl 



It is about 2. 



The gain, taking the output between either output terminal and earth is 

 {^^Rj)l{^^in) and is therefore about unity. 



168 



