POWER PACKS 



Another important improved form of voltage stabilizer is due to Attree*, 

 who points out a basic difficulty in the design of stabilizers of the classical 

 type; this is that since the optimum mean bias in the series control valve 

 determines the permissible standing voltage drop across the load of the 

 amplifier valve, and since this drop is not as large as one would like, one is 

 obliged to make shift with either a low value of load resistance for the 

 amplifier valve or a low anode current. With valves the mutual conduc- 

 tance falls at low anode currents. In either case the gain is poor. Attree 

 goes on to show how a slightly modified cascode circuit overcomes this 

 difficulty, enabling him to quote an output impedance for his circuit of 

 only 0-2 ohms, and a ripple and noise content of less than 500 /^V. The 



12 El 



+ 300V 

 0-1 50 mA 



Stabilized 



supply 



out 



Figure 37.8 



modification consists merely in by-passing the upper cascode triode by a 

 resistor, so that enough current passes through the lower triode to give 

 satisfactory g,„, but only a small proportion passes through the upper 

 triode; thus the volts dropped across the load are not excessive. Since the 

 cascode gain depends on the product of the load resistance and the g^ of 

 the lower valve, high gain and therefore excellent stabilization result. 

 Attree's circuit in one form appears in Figure 37.8. The 220k resistor is 

 the one responsible for the excellent performance. 



When a stabilized negative as well as positive supply is required, two 

 separate stabihzing systems, each with its own transformer and rectifier, 

 may be used, one with its positive terminal and one with its negative terminal 

 earthed, as shown in Figure 37.9. In general two mains transformers are 

 necessary (since components possessing two HT secondaries are not stock 

 items) and two voltage reference sources, giving a somewhat expensive 

 arrangement. Using series stabilization it is not possible to derive both 

 positive and negative supplies from a single HT secondary winding, but it 

 can be done using shunt stabilization; the circuit of Figure 37.10 was de- 

 vised by K. E. Machin and, besides having only one HT secondary, only 

 employs a single voltage reference source as well. The mode of operation 

 is as follows: 



Whereas the action of the series stabilizer is to absorb fluctuations in 

 supply voltage across the series valve, leaving load voltage constant, the shunt 

 stabilizer works by absorbing fluctuations in the load current, so that a 



590 



