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Chapter 3
 

AMPLIFICATION TECHNIQUES

  The resistance-coupled amplifier, although one of the simplest to design,
demonstrates  many  of  the  important  techniques  used  with  G-Curves.
Separate consideration of the design of  triode  and  pentode  amplifiers
is required as the design techniques differ appreciably.  Design  features
which are not directly  dependent  on  the  G-Curve  technique,  such  as
bandwidth, cutoff frequency limits, etc., are not discussed here as  they
can be established adequately by standard techniques.
 
 

THE TRIODE R-C AMPLIFIER

  As the important specifications on an amplifier are the input and  output
voltages, the impedance levels, the amplification and the distortion, the
purpose of the design procedure is to provide  a  circuit  that  conforms
with the specifications.  The following  basic  steps  may  be  used  with
G-Curves to provide the required design:

  1 - Select a trial tube.

  2 - Select a tentative supply voltage E_bb and  load  resistance  R_L,  and
      draw a load line.

  3 - Read the small-signal parameters, g_m and g_p, at several points  along
      the load line.

  4 - Calculate the small-signal amplifications.

  5 - Calculate the distortion.

  6 - Calculate the dissipations.

  The selection of a trial tube initially is  an educated  guess.  If  an
amplifier for handling small voltages is required, a tube may be selected
from among the top third ( low g_p or G_m2 ) of the  appropriate  Table  of
Power-Handling Ability.  For moderate  voltages  select  from  the  middle
third, and so forth.  After a tube has been selected  the  design  may  be
prepared as described, and if desired, a re-design made with a tube  hav-
ing a lower or higher nominal g_p or  G_m2  rating,  as  indicated  by  the
results from the initial design.
 

10
 
 

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