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THE PENTODE CATHODE FOLLOWER

  Pentode-type tubes are used for cathode  followers  when  a  very  low  out-
put resistance, a very  high  input  resistance,  and  a  very  small  input
capacitance are required.  The equation for amplification is:

                      K = ( G_m1* X_p* R_k ) / [ 1 + ( G_m1* X_p* R_k )]

  The pentode G-Curves may  be  used  with  this  equation  to  determine  the
small-signal parameters and the  gain,  output  and  input  resistances  are
found just as  with  triodes. ( This  procedure  must  be  modified  if  the
screen is bypassed to ground instead of to the cathode. )
 
 

CALCULATING THE CATHODE BYPASS CAPACITOR

  When cathode degeneration is not desired, cathode resistor R_k  may   be
bypassed with a capacitor of sufficient size to ensure that the  alternating
voltage between the cathode and  ground  is  negligible  over  the  passband
of the amplifier.  The  amount  of  cathode  degeneration  is  given  by  the
term ( g_m + _gp ) R_k1 in Equation 8. If a  bypass  capacitor  C_k  is  connected
in parallel with R_k this degeneration term  becomes ( g_m+ g_p ) Z_k,  where
Z_k is given by R_k / (1 + j w*C_k*R_k ).  Sufficient  bypassing  is  obtained
when the  degeneration  term  is  small  compared  to  the  balance  of  the
denominator  of Equation 8.  The  approximate  conditions   required   for
a triode are given by:

                    C_k = 5 ( g_m + g_p ) / [ 2 pi * f₁ ( 1 + g_p*R_L ) ]         (27)
 

  For pentode tubes, this equation may be written:

                     C_k = 5 G_m1* X_p / ( 2 pi * f₁ )                                (28)

   These  equations  may  be  obtained  in  the  same  way  as  Equations 21
and 22.  The  actual  derivations  however,  are  published  elsewhere  (see
bibliography.)

  The fact that the designs considered  here  seem  only  to  apply  to  R-C
amplifiers, should not mislead the reader into  thinking  that  other  types
of amplifiers cannot he designed in similar manner.  As  a  matter  of  fact,
any amplifier in effect develops its output in some kind of a  load  resist-
ance or impedance.  For  example,  the  transformer-coupled  amplifier  may
be solved by drawing  a  static  load  line  corresponding  to  the  primary
resistance of the transformer, followed  by  a  dynamic  load  line  at  the
effective impedance of the load as seen at the  input  to  the  transformer.
Tuned amplifiers are handled similarly,  since  the  dynamic  load  line  is
established by determining the  effective  impedance  of  the  circuit,  and
then plotting the corresponding line. In  fact,  the  method  is  completely
general and can  be  used,  with  minor  modifications,  with  almost  every
circuit confronting the electronics man.
 
 

22

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