Tektronix PG 508 User Manual

T h eo r y of Operation--PG 508
variatio ns and do offset curren ts are
summed at t he
collectors of Q945
an d Q954, t he out p ut p ulse h ig h level
ch anges to maximu r n voltage a nd t he low level re m ai ns at
0 V .
It follows
that any
p ositive increase in eit herthe h ig h or
low
level co n trol voltages causes an i n crease in t he do
offset cu rrent . Α d ecrease i n eit h er control voltage causes
α d ecrease in offset current
. However, sig n al cu rr ent
variations res p o nd only to
d ifferences b etwee n t h ese
control voltages . 'T he la r ger t he voltage differe n ce, t he
larger t he signal curren t am p lit u de i n to t he out p ut
amplifier .
Since t h e dy
n
amic
range of t h e p ulse ge n erator output
is 4 - or
- 20 V, a nd t he maximum am p lit ud e is only 20 V
pea k
to
pea k , clam p ing circuits areprovided to preven t t h e
differe nce b etween t he h ig h and low level control
voltages
f r om excee d ing +2 .6 V, w h ic h corresp on d s to
maximum
outpu t . Α clampi
n g
circ
u it also p revents t he h ig h level
from becoming less t h an t h e low level co n trol voltage .
"T he level control unity gai n am p lifiers U
780A an d
U 780 B con tai n α precision
d io d e clam p com p osed of
C
R 782,
C
R 790, R 780, an dR 782 . Si nce t h e feed back for
U
780A
is
ta ken from the cath od e of C R 790, t he voltage at
t he ju nctio n of C R 782 a nd C R 790 is eq ual to
t h e
low
level
control voltage plus o ne d iode dro p (C
R 790) . If t h e h ig h
level attempts to
d ecrease below t h e low level or attempts
to
i n crease ab ove t h e h ig h level co n trol voltage, C R 782
co n ducts
clampi ng t he h ig h level at t h e low level voltage .
Cur r en t limiting for U 780 B , du ri n g clam p ing, is p
rovi
d
e
d
by R 780 .
U 800A is also
configured as α p recisio n clamping
circuit . T he o u tput at pi n 1 is eq ual to t he low
level control
voltage pl us 2 .6 Vp l u s o ne d iode dro p .
T h e r efore, if t he
h ig h level voltage
atte m pts to excee d t h e low level plus 2 .6
volts, or
t he
low
level is d ecreased m ore t h a n 2 .6 Vb elow
t h e h ig
h
level, C
R 805 cond ucts an d t h e h ig h level is
clamped at t he low level co n trol voltage p lus 2 .6
volts . Th e
low level co n t r ol always overri des t he
h ig h level control .
Cu r re n t limiti ng for U 780
B , duri n g cla m ping, is provi de d
by R 780 .
Output Am plifier
Th e p ositive and n egative d o voltages
for t h is am p lifie r
are p rovided by dual
p ower supplies w h ic h trac k t h e h ig h
a n d low level co n trol voltages . T h ese trac king su p
plies
ensure t h at t he p ositive pote n tial is at least 6 V above t
he
outp u t pu lse h ig h level a nd t he n egative p
ote n tial is at
least 6
volts b elow t he out p ut p ulse low level . 'T h is
ar r angeme n t e nables t h e p ulse ge ne r ator to provi de α40
V
dy n amic range, wit h α 20 V p ea k to p ea k
maxim u m output
pu lse, w h ile maintaining min imum p owe r d
issipation an d
2- 6
voltage re q uirements for
t h
e
amplifier tra n sistors . T h e
ra nge of t he p
ositive trac k i ng supply is - Ι -6 to - Ι -26 V, wit h α
-6 to ---26 V ra nge
for t h e negative supply .
Th e outp ut circuit functio n s as α
linear
cu
r re n t drive n
operational am p lifier wit h α closed loop transresistance
gai n of 2 V /mA . N egative feedbac k t hroug
h R 1042 to t he
i n p u t no de (collectors of Q945
and Q954) causes α virt u ral
g r ou
n
d at
t he i np ut . Α 10 mA p eak to p ea k input signal
creates α 20 V pea k to pea k outp ut p ulse (o pen circuit) .
Si nce t he out p ut a
m plifier
is
α co m plementary circuit,
only t he op eratio n of one side will b e explaine d .
F or α p
ositive-going ou tput pulse, cur r e n t is drive n i n to
t he in p u t no d e . "This action causes an i ncrease i n base
curren t to Q975 w h ic h d ecreases t h e b ase d rive to Q980
. Α
casco d e circuit is formed by Q980 an d Q990,
wit h R 985
an d L 980 servi ng as α co
n
sta
n t current so urce to t he
commo n collector-emitter
connection . 'T h erefore, w hen
t he base d rive to 0980
is decreased, t h e current in Q990
i
n
creases
. (T
he constan t curren t source su pplies ap-
proximately
60 mA w h ic h , wit h no signal, is e qu
ally
d ivi de d between Q980 an d Q990 .) Anot
he r constan t
cur r e n t source, Q1000, causes α 2 .0 V d ro p across
R 1002 .
T he emitter follower, Q995,
buffers t h
is p
otential and
p rovides α voltage source
for t he base of t he cascode
transistor, Q990 . An i n creasi ng curren t t h roug h 0990
i n creases t he b ase curren t from t he out
p
ut
parallel emitter
followers, Q1010 an d Q1015 . T h e emitters of
t h ese
tra n sistors drive t h e out p ut
positive t hroug h α networ k of
resisto
r
s a
n
d ca
p acitors . Compone n ts R 1020, R 1024, and
C1024, i n t h is n etwor k , are ad j ustable to provi d
e
an
i n ter n al resistive 50 Ω te r mi n atio
n for t he out p ut . N etwor k
C1048 an d R 1048, wit h p ote ntiometer
R 1050, p rovides
tra n sie n t res p o nse p ea k i n g for
t he am p lifier .
Eq uivale n t biasi ng for
t h e complementary outp ut
emitter followers is p rovided by C
R
990,
R 990, and C R 992 .
W it h zero out p ut,
t h
e q
uiesce n t current in eac h output
follower is a pp roximately 20 mA . T he o u tput potential is
availa b le at t he rea r i n terface connector, pin 25A, for
mo n itori n g t he out p ut am p litude .
P ower Supply
- The -1-11 .5 Vd c from t he main frame p rovi des
t h e raw
su pp ly voltage for t he series
regulate d +5 .2 Vd c su pp ly .
T h
e
precisio n voltage regulator, U 1210, i n clu des α
temperature compe n sate d voltage refe r ence su p ply at p i n
6 . T he +5 .2 V p oten tiometer, R 1210, wit h t h e
voltage
divider R 1209 and R 1211 p rovides t he reference
in p ut to
t h e non-inverting
i np ut, pi n 5 . T h e output voltage is
applie d to t he i n verting i np ut, pi n 4, t h roug h R 1217 .
Se n si n g d ifferences i n t he two i nput pote n tials, t he
regulator amplifier p rovides b ase cu rrent drive f
r
om p
i n
10

Tektronix PG 508 Specifications

General

Brand	Tektronix
Model	PG 508
Category	Pulse Generator
Language	English