< Previous - Table of Contents - Next >

CATHODE DEGENERATIVE AMPLIFIER EQUATIONS

  The triode cathode degenerative amplifier is  an  R-C  amplifier  in  which
a portion of the cathode resistor R_k1 is left unbypassed ( Fig. 2-1 ).  Thus
the instantaneous signal voltage between grid and cathode is:
 

                          e_g = e_s - e_k = e_s - ( i_p* R_k1 )

and the plate-to-cathode voltage is:

                          e_p = ( -e_k - e_L ) = -i_p ( R_k1+ R_L )

  Substitution of these relations into  the  basic  equation  (4)  gives  the
amplification  equation:
 

      K = -e_L / e_s  =  ( -g_m* R_L ) /  [ 1 + ( g_m+ g_p ) R_k1 + ( g_p* R_L )]      (8)

This equation resembles that for the triode R-C amplifier    (Equation  5)
but has an added term in  the  denominator,  the  term  introduced  by  the
cathode degeneration, ( g_m+ g_p ) R_k1.

  For the pentode degenerative amplifier, the equations for  signal  volt-
ages are slightly different:

                   e_g1 =  e_s – ( i_k* R_k1 ) = e_s - ( i_p + i_g2 ) R_k1
and

                 e_p = ( -i_k * R_k1 ) - ( i_p * R_L ) = - ( i_p + i_g2 ) R_k1 - ( i_p* R_L )

  Now, if the screen grid is adequately bypassed to the cathode, the instan-
taneous value of the varying  component  of  the  screen  current  i_g2  may
be neglected.  Likewise,  for  properly  designed  pentode  amplifiers,  the
plate conductance term  may  be  neglected.  Under  these  conditions,  the
equation for amplification becomes:
 

                      K = -e_L / e_s = ( -g_m* R_L ) / [ 1 + ( g_m1* R_k1 ) ]           (9)

which, in terms of pentode parameters, becomes:

                      K = ( -G_m1* X_p* R_L ) / [ 1 + ( G_m1* X_p* R_k1 ) ]            (10)
 
 
 

8

< Previous - Table of Contents - Next >

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