< Previous - Table of Contents - Next >

THE TRIODE CATHODE FOLLOWER

  The equations for the cathode follower  are  given on  page 8. They  show
that it can handle a  much  larger  input  voltage  than  can  an  ordinary
amplifier because most of the input signal is offset by the signal  voltage
developed in the cathode circuit.  This leaves only a small  grid-to-cathode
voltage.  The load line for it is usually dependent  on  the  value  of  the
cathode resistance alone since normally no  plate  load  resistor  is  used
with it.  As with ordinary amplifiers, a static  and  a  dynamic  load  line
should be used  if  the  coupled  loading  has  sufficient  magnitude.  The
equation for amplification is:

                        K = g_m* R_k [ 1 + ( g_m+ g_p ) R_k ]

  The  dynamic  output  impedance ( not  to be  confused with  the  dynamic
load  impedance, which  should  be  large  compared  to  R_k ) is  given  by
the   equation:

                                   R_o = 1 / ( g_m+ g_p )                         (24)
 

  The input impedance, with the grid returned to ground, is:

                                          R_i = R_g                                     (25)

It may, however,  be  made  much  higher  by  returning  grid  resistor  R_g
to a tap on the cathode resistor ( between R_k, and  R_k2 ).  Usually  suffi-
cient resistance R_k1  is placed between the cathode and the tap  point  to
provide the necessary bias. In this case the input resistance is:

                                         R_i = R_g / ( 1 - K )                      (26)
 

The output impedance is also higher in this arrangement.

Example 14.  A cathode follower is required using a 6J5, E_bb = 250
volts, and R_k = 25,000 ohms.  What are its characteristics?

The small-signal data may be tabulated in the usual manner:

                                                                               .
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        0.950     0.942   0.936   0.926   0.918
R_o        252       303       380      485      700     ohms
                                                                                .

  If the static bias point is -4 volts and the grid  swing  is  8 volts  peak
to peak, the distortion  is 0.43%, the  output  voltage  is  115  volts,  and
the input signal 123  volts  peak to  peak. The output resistances  are  very
easily  obtained  at  each  bias point   by   the   G-Curve  technique.  When
matching  is  critical, therefore, the additional information  can be invalu-
able. The example  shows that the output  resistance  varies  rather  widely,
with the result that the selection  of  the  proper  values  of  R_k, and  the
static bias point E_c can  easily  provide  the  required matching.

                                                                                                                21

< Previous - Table of Contents - Next >

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