DIFFICULTIES WITH SINGLE-SIDED AMPLIFIERS 



The HT battery inevitably has some internal resistance r. The last valve 

 is a power valve, or a high level amplifier, and passes more standing anode 

 current than the other two. This anode current flows through r and develops 

 a voltage drop across it which will fluctuate as V^ anode current fluctuates. 

 This varying voltage drop appears as inconstancy of the supposedly fixed 

 HT. A Httle thought shows that with three triodes (or more) these HT 

 variations are transmitted back to V^ anode, are then amplified by V^ and 

 Kg, and re-appear as variations in V^ anode current in such a sense as to 

 reinforce themselves. This is an example of undesired positive feedback, 

 and in the early days of radio was called 'motor boating' because of the 

 noise it made when the load is a loudspeaker. 



Or again, consider the two valve amplifier of Figure 10.3. The first valve 



1 



Input 



Figure 10.3 



is a pentode and the screen is fed from an appropriate tap on the HT battery. 

 The battery resistance is represented as being spht into halves at each end. 

 An increase in load current will produce a reduction in HT which is fed back 

 to Fi anode; but there is also a reduction of screen potential which wiH 

 reduce V-^ anode current, and hence the drop in F^ anode load, so that this 

 tends to increase V^ anode potential. It is Hkely that the screen effect will 

 be the greater, producing a net positive-ward movement of V^ grid and further 

 increase in V^, anode current. This arrangement can therefore also motor 

 boat. 



Decoupling 



With a.c. coupled amplifiers motor boating can be prevented. Since the 

 amplifier has little gain at low frequencies below the pass-band, if the offending 

 inconstant supply potential is passed through a low-pass filter which also 

 cuts off at a frequency below the pass-band, then motor boating cannot occur. 

 This process is called 'decouphng'. A practical version of Figure 10.1 is 

 Figure 10.4, where the decoupling components are R^ and Q. Some loss of 

 HT voltage occurs across R^, and the first stage must be designed with this 

 in mind. It is reasonable to lose between 50 and 100 V across R^. 



Having fixed R^ we find Q. The higher Q the more eff"ective the decoupling. 

 The quickest way to find how much is required is by experiment. Similarly, 

 Figure 10.5 shows a practical version of Figure 10.3. Ra is worked out first 

 to give the correct screen voltage and current, then Q is made large enough 



160 



