Output System - HP 202A Operating And Service Manual

Sect.
1 1 1
Page 6
The sinewave is approximated by varying the shunt
resistance across R93B is steps determined by the
diode synthesizing network. The waveform slope,
at first,
is
just that determined by R94, R93B and
the input waveform. When the first diode conducts
R93
is
shunted by a predetermined amount, decreas-
ing the slope. Each diode in turn decreases the
slope until a l l the diodes a r e conducting and the
triangular wave has reached its crest. The triangular
wave starts down, the diodes stop conducting in turn
until the triangular wave has reached its crest. The
triangular wave starts down, the diodes stop conduct-
ing in turn until the triangular wave reaches the
average level. The other half-cycle i s formed in
the same manner, but by the diodes that are biased
to shape the negative excursion.
It can be shown that using seven segments to approx-
imate one half cycle of the sinewave results in ap-
proximately 11 6% rms distortion. However, variations
in the diodes limit the practical result to about 1%
r m s distortion.
In the triangular wave position of the FUNCTION
selector switch the non-linear load consisting of
the diode network is replaced by R95 s o that the
combination R94 and R95 is a simple linear divider
for all voltage levels. It is adjusted to give equal
sine and triangular wave peak magnitude. The
squarewave is connected to the FUNCTION selector
switch through the divider R59 and R22 which adjusts
the average voltage of the squarewave to the voltage
a t the cathode of V4. In the squarewave position
of the selector switch, R63 parallels R93B to adjust
the amplitude of the squarewave to be equal to the
amplitude of the sinewave and the triangular wave.
3-5 OUTPUT SYSTEM
The output system consists of three stages a s shown
in Figure 3-7. The first Stage V18 is a dual triode
acting as a pair of separate cathode followers. These
cathode followers isolate the signal input from the
output stage. Any dc unbalance a t the output ter-
minals can be corrected by varying R65.
The second stage V19 is a differential amplifier.
The difference between the two signals at its grids
appears a t both plates in nearly equal magnitudes
and 180" out of phase. This effect is due to the large
common cathode resistance. In this stage ampli-
fication takes place and also the signal difference
E minus F is converted to push-pull voltages. The
third stage V20 is another pair of cathode followers.
The signals appearing a t the plates of V19 a r e
Figure 3-7. Output Amplifier System of Model 202A