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The  value  of  e_b at  maximum  plate current  may  be  checked  in  the  process
of design,  to verify  the  correctness  of  X_c2.  The  equation  for  this  calcu-
lation is:

                             e_b = E_bb - ( I_p * Xp * R_L )                             (19)

Example 9.  Determine the maximum plate and   screen  dissipations
for the amplifier of Example 7.

                                                                                         .
  Plate  Dissipation                       Screen Dissipation           .
 e_b = E_bz/ 2 = 100 volts                     I_pp = 7.0 ma
      I_bm = 5.26 ma               e_b = E_bb - ( I_p* X_p* R_L )= 74 volts
     P_pm = 0.53 watt                           X_c2 = 0.42
                                                        I_c2 = 2.94 ma
                                            P_c2 ( max ) = 0.294 watt
                                                                                          .

These dissipations are well within the prescribed ratings.
 

CALCULATIONS OF THE SERIES SCREEN RESISTANCE

A  series  resistance ( R_s  in Fig.3-4 )  must  be  used  between  the  screen   of
the  tube  and  the  voltage  source  to  limit  the  screen  voltage  and  current.
The screen voltage is  held  constant  by  a  bypass  capacitor ( C_s  in  Fig.3-4 )
of  sufficient  capacitance  to  keep  the  effect  of  screen   voltage   variation
negligible.  The  value  of  resistance  R_s  required   may   be   calculated   from
the equation:

                                R_s = ( E_bb - E_c2 ) / ( I_p* X_c2 )                    (20)

where the I_p and the X_c2 are the values at the static bias.

Example 10.  What  series  screen  resistance  is  required  for  Example 9?

                                                                                                        .
Data Given            Read from Curves           Equation          Solution     .
E_bb = 200 volts         I_p = 2.8 ma               Equation(20)
E_c2 = 100 volts         X_c2 = 0.39
E_c1 = -1.5 volts                                                         Rs = 90,000 ohms
 e_b ~ 150 volts
                                                                                                          .
 

THE SCREEN BYPASS CAPACITOR

The  screen  bypass  capacitor  may  be  found  by  the  use of  the  equation:
 

              C_s = ( 5 * G_m2* X_c2 ) / ( 2*pi* f ) = ( 5 * g_m22 ) / ( 2*pi* f )         (21)
 

where g_m22 is the screen conductance of the tube, or  by  the  equation:

              C_s = delta I_c2 / ( 2*pi* f *delta E_c2 ) = delta I_c2 / ( 2*pi * f *delta E_s )     (22)
 

18

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Copyright 2008 for Phyllis K. Pullen, M.D.,
by Robert J. Legg