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Example 3. Find the limit points  for  a  6J5  tube  used  with  a  supply
voltage of 250 volts and a load resistance of  25,000  ohms;  repeat  with
a load of 50,000 ohms.
                                                                                .
          Case    I                  Point I                   Point  2  .

  R_L = 25,000 ohms       e_b = 250 volts           e_b = 0
 E_bb = 250  volts             i_b =  0 ma                i_b = 10 ma
                                                                                  .

                                                                                  .
          Case   II                  Point I                   Point  2  .

   R_L = 50,000 ohms       e_b = 250 volts           e_b = 0
  E_bb = 250  volts            i_b = 0  ma                 i_b = 5 ma
                                                                                  .

These  lines  may  be  drawn  on  the   6J5   G-Curve.
 

AMPLIFICATION

Read  the  small-signal  parameters  g_m  and  g_p  at   several   desired   points
along  the  load  line  and  tabulate  for  calculation  of   the   amplification
using Equation 5.  These  values  of  K  are  true  small-signal  amplifications
and not average amplifications as are normally obtained.

Example  4.  Read   and   tabulate   the   small-signal   parameter   values   at
several bias values  for  Cases  I  and  II,  and  calculate  the  values  of  K.

                                                                                           .
Case I: 6J5 tube     E_bb = 250 volts and R_L = 25,000 ohms       .
e_c              0            -2           -4           -6            -8       volts
g_m           3800       3150       2500      1950        1350     umhos
g_p            165         150        130         110          80       umhos
K            -18.5       -16.6      -14.7       -13.0       -11.3
                                                                                          .

                                                                                          .
Case II: 6J5  tube   E_bb = 250 volts and R_L = 50,000 ohms      .
e_c             0             -2          -4            -6            -8        volts
g_m          3200        2400      1850       1350         900       umhos
g_p           140          120        105          80            60       umhos
K           -20.0        -17.1      -14.8       -13.5        -11.3
                                                                                          .
 

DISTORTION

The  second-harmonic  distortion  of  an  input  signal  as  generated   in   the
amplifier  may  be  determined   by   using   the   small-signal   amplifications
in  the  following   equation:

                        D = 25 ( K_p - K_n ) / ( K_p + K_n )               (15)

12

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