Tektronix PG 501 Instruction Manual page 12

Output Buffer
Output buffer U30A is the third section of integrated
circuit U30. As the name implies, U30A buffers and shapes
the output of duration generator U30C for the output
amplifiers . The output of U30A is a push-pull pulse which
is applied to the base of transistors Q110 and Q115 in the
+ amplifier and to the base of Q150 and Q155 in the
- amplifier.
Theory of Operation-PG 501
The 47 SZ series-parallel network along with the emitter
of Q140 looks like a source impedance of approximately
50 Sl during the pulse on condition. (During the pulse off
condition, the source impedance is higher .) Resistor R141
pulls the output of Q140 low during the pulse off
condition . Diode CR141 protects the Q140 output from
inductive loads. Resistor R138 pulls an offset voltage
through R136 and R137 which biases Q140 nearly on to
improve pulse response when the signal from Q136 steps
high .
Network R148/C148/C149 picks off a small amount of
Output Amplifiers
the output pulse, filters it, and applies it through Ampli-
tude Monitor switch S150 to rear connector pin 25A.
The two output amplifiers operate identically except for
output polarity . The +output amplifier uses PNP transis-
tors where the -amplifier uses NPN transistors . To
complete the voltage inversion, the diodes in the +amplifier
are electrically opposite of those in the - amplifier. Due to
External Trigger/Duration Mode
the similarity of the output amplifiers, the remainder of
Period generator U30B is utilized in a Schmitt bistable
this discussion concerns the +output amplifier with
multivibrator configuration in the EXT TRIG and EXT
differences mentioned where pertinent.
DURATION modes. Contact 15 on PERIOD selector cam
switch S30A connects feedback resistor R29 between pins
7 and 9 of U30B and, thus, enables U30B to operate as a
bistable switch .
Transistors Q110 and Q115 comprise a current mode
switch driven by output buffer U30A . This circuit also
shifts the input voltage from U30C to a positive level and
Diodes CR17 and CR20 with their associated RC
" provides a push-pull potential at R110 and R115 to drive networks shift and clamp the ground-referenced input
the bases of buffer emitter followers Q120 and Q124 . signal from the TRIG/DURATION IN connector down to
Emitter followers Q120 and Q124 allow lower source
the proper operating level for U30B, i .e ., between -2 V and
impedance drive for the bases of the output driving current
-0.8 V.
switch composed of Q130 and Q136 . The current through
Q130 and Q136 is variable with the front-panel + OUTPUT
AMPLITUDE control R134 . Maximum current through
Normally, the output from pins 6 and 7 of U30B is
Q130 and Q136, thus, the maximum output amplitude at
applied to trigger amplifier transistors Q40 and Q42.
the + output connector is adjusted by + Amplitude Cal
However, in the EXT DURATION mode, contact 1 is
potentiometer R114 . Potentiometer R114 adjusts the
closed and 2 is opened by DURATION cam switch S30B
emitter voltage at Q130 and Q136 which changes the
which bypasses the period generator output signal around
voltage drop across 8132 and R134 . At maximum output
the duration generator and applies it directly to pin 4 of
amplitude R134 is at minimum voltage drop so the current
output buffer U30A . Output buffer U30A and the output
is actually set by R132, the end resistance of potentiometer amplifiers function as described for normal pulse output
R114, and the voltage difference between the +18 V supply operation.
and the emitter voltages .
Inductor L132 , provides isolation from the effects of
Output LOCked On Mode
R132 on the emitters of Q130 and Q136 . The collector of
In the LOCKED ON mode, contact 13 of DURATION
' Q130 is tied to R130 and zener VR130 to reduce the
cam switch S30B closes while contacts 1 and 2 are open .
power dissipated in Q130 . The collector of Q136 drives the
The voltage at diode CR65 is then applied to pin 4 of
base of output transistor Q140 . Peaking inductor L136 and
output buffer U30A . Thus, pin 4 of U30A is high and pin 5
load resistors R136/R137 apply a peaked signal to the base is low, locking U30A on . Transistor Q115 in the +output
of Q140 providing flat response into 50 S2 . Output transis- amplifier and Q150 in the - output amplifier turn on .
for Q140 is an emitter follower driving the +output
Since both output amplifiers essentially follow the state of
connector through the 47 SZ series-parallel resistance net- the output buffer, the resulting output at the + and
work R 143/R 144/R 146/R 147 . Capacitor C 143 (+ Output - output connectors is a DC level which is variable with
~~
Comp Set) allows the first section of the 47 SZ network to
front-panel
+ and
- OUTPUT AMPLITUDE controls
compensate for the output reactance of Q140 .
R134/R174.