DOUBLE-SIDED AMPLIFIERS 



where A = gjR^ + Rk) 



^-Rk' + Rl 

 and R^' is the effective resistance in the cathode circuit of each half 



\Ry . 2R^. 



Rk = 



Of this the useful fraction is 



\Ry + 2R^ 



Rr 



Rl + Rr 



so the usable gain is 



Rl graiRL + RK) _ gm - Rl 



Rl + Rk'\ lW^ , D 'J ^'k \ ^+gmRl 



^+U^i^ + Rk)(-^^}j 



This is plotted in Graph 29 for a typical stage having Rj^ = Rj^ = 100 kQ, 

 and g^ = ImA/V. It is clear that the control obtained is very non-hnear if a 

 wide range of gain control is required. However, if a threefold reduction is 

 sufficient, a Hnear law 5 kQ potentiometer may be cahbrated in dB's without 

 producing an immoderately non-hnear scale. 



Notice that the presence of R^ does not affect the operation of the stage to 

 in-phase signals, for then both cathodes move together, no current flows in 

 Ry and therefore its value is irrelevant. Notice also that the reduction in 

 gain secured by this method must not be too great, otherwise, in-so-far as the 

 anti-phase gain/in-phase gain ratio is being reduced, the overall amplifier 

 rejection is adversely affected. 



D.c. amplifiers 



Potentiometric control of the gain of direct coupled amphfiers is compli- 

 cated by the requirement that alteration of the gain control when the input 

 to the amplifier is zero ought to produce no effect at the output, since zero 

 times any number is still zero. Thus some kind of control based on Figure 

 12.31 is no use because there is a potential gradient along the gain potentio- 

 meter, and manipulation of the gain control alters the bias on the second valve. 



It is possible to devise several networks in which no standing current flows 

 along the gain potentiometer; probably the simplest is Figure 12.32. The 

 control is necessarily in discrete steps in order to preserve the accuracy of gain 

 balance. R^ and i?2 ^re compensated coupling potential dividers, and when 

 the amphfier is in the resting state points A and B are at the same potential. 

 Thus altering the stepped gain setting has no effect on the potentials supplied 

 to the following grids. When an anti-phase signal is applied to the amplifier, 

 current flows between A and B and a greater or lesser potential difference is 

 tapped off according to the setting of the switches. 



Remember that the effective anode load for each of the first stage valves 

 in Figure 12.32 is now (RlRz)I(Rl + Rz) ^^^ therefore that the maximum 

 gain wiU be reduced if this gain circuit is added to an existing design. If 

 starting a design from scratch the effect is offset by choosing Rj^ about double 



188 



