Wavetek 193 Instruction Manual page 35

4-9
20KO
Figure 4·4. Simplified Output Amplifier
OV R256/R257
TO
ATIENUATOR
DC
CONTROL
R2221R224
R223 2KO
R254
FROM
PREAMPLIFIER
AMPLITUDE $_R...... 2-01-.-'\R2"'O"'3_-. _ _____,Rv2v18\r- __ O,V__J__C_10_1--1/C,f--1_03 __ -I
~ 12Hl
~ R261
DC ~
OFFSET
1
R262
Assume zero input voltage at the junction of R203 and
R218. The output at R222 and R224 is maintained at 0
volts by dc amplifier U2. U2 pin 3 is connected to a 0
volt reference. If the output drifts away from 0 volts,
this will be sensed at U2 pin 2 through R256, R257 and
R254. Amplifier U2 will sense a difference between its
inputs and produce an output voltage which adjusts
the bias in the ac coupled amplifier to return the out-
put to 0 volts. Because R218 and R223 form half of a
balanced bridge, and R253, R256 and R257 form the
second half, the amplifier node at the junction of R218
and R223 will be held at 0 volts as U2 has returned the
junction of R253 and R256 to 0 volts.
A dc input of
+
1 volt at R218 and R253 is sensed as a
positive increase at U2 pin 2, causing U2 pin 6 to go
negative. The ac amplifier output goes negative in
response to the dc control input. This continues until
the output becomes sufficiently negative to sink all
the input current, and return U2 pin 2 to 0 volts. The
bridge circuit causes the ac amplifier node to be 0
volts. If the input is
+
1 volt and the node at the junc-
tion of R218 and R223 is 0 volts then the input current
is 8.26 mA which flows through R223 to produce an
output voltage of - 16.52V. Therefore the amplifier
voltage gain is 16.52 (R223/R218).
Above about 16 kHz, the ac amplifier controls the
summing node directly, sinking or sourcing current
through R223 by adjusting the output voltage to hold
the node at 0 volts. The ac amplifier gain is also
R223/R218
=
16.25. This is divided by 2 at the output
terminal, due to the 500 source impedance resistors
R222 and R224, providing the output is also termi-
nated into 500.
Refer to sheet 7. The top half of the circuit amplifies
4.4.12 Preamplifier
Refer to sheet 7 of the schematic circuitry. For all
functions, full scale output voltage is produced when
2mA is injected into the input summing node U1 pin 8.
Transistor array U1 forms a cascaded differential
stage. Transistor 027 is a fixed current source. 028
and 029 form a high gain voltage follower. DC nega-
tive feedback is applied through R195 to U1 pin 8. The
closed loop voltage gain of the amplifier is determined
by the ratio of R195 to the input resistors, R152 for
square wave and R176 for triangle. The sine converter
output supplies the correct current directly from U3
pin 2 to U1 pin 8. The servo action of the preamplifier
holds this point at 0 volts, therefore no voltage can be
measured. U1 pin 4 is the non-inverting input and is
used both to adjust the offset to 0 volts at TP2 using
R185 and to inject the sine converter compensation
signal (SINCMP) described under paragraph 4.3.7,
Sine Converter. High frequency compensation is pro-
vided by R182, C81, C86 and C153. Zener diode CR29
provides increased collector voltages for U1 pins 11
and 12 and also allows these two points to be rela-
tively close in voltage.
4.4.13 Output Amplifier
The output amplifier consists of an ac coupled ampli-
fier for signals above about 16 kHz, and a dc coupled
amplifier for signals below about 16 kHz, and to main-
tain zero dc output offset within specified limits. Refer
to the simplified output amplifier schematic, figure
4-4.
other functions by turning on 026 and CR24 which
reverse bias CR22 and CR23 and prevents current
from flowing through R152.