Tektronix 576 - Page 54

add ed in t
h
e ste p generator circuit, t he zero ste p level may
range from 0 to 10 volts .)
T he out p ut end of t he current setti n g
resistors is con-
nected t h roug h t he device un
d
er test to groun
d . Wh en volt-
age ste p s of 1 volt p er ste
p
are
app
lie d
between t he in put
en d of t h e current setting resistors
an
d
groun d , current
ste p s of variable am p litu
d e
flow t h roug h
t
h
e d evice un d er
test . T he cu r rent am p litu de
of
t h
e
steps is
d etermine d by
A MPL IT UDE switch S W 195 (see Ste p Generator Switc h ing
sc hematic), w h ic h c h
ooses
various
combinations of resistors
R 197 t h roug h R216 .
In
or d er to obtain calibrate d cu rrent
ste p s, t he voltage
across t he cu rrent setting resistors must
be h el d at 1 volt
p er ste p . T h e voltage at t h
e
out p ut, h
owever, may vary by
t h
e
amo u nt
of
t h
e turn-on voltage of t he device und er test
and alter t he current p
er ste p out put of t he ste p generator .
To
com p ensate for t
h
is
turn-on voltage, any variation from
groun d of
voltage at t he in p ut to t he d evice und er test is
transmitte d t h roug h
t
h
e
+1
am p lifier to t h e p ositive si de of
t he d ifferential am p
lifier . T h is starts α regulating p rocess
w h ic h
causes t h
e
voltage at t he in p ut to t he c u rrent setting
resistors
to move in t he same d irection as t he turn-on volt-
age
at t
h
e out p ut, t h us nullifying its effect .
T
h
e
+1
am p lifier is ma de up of p ara p h ase am p lifier
Q229A an d
Β , constant current sources Q233 and Q226,
an d emitter followers
Q235
an
d
Ο 241 .
In t he c u rrent mo d e,
a n y voltage at t h
e
in p
ut of t h e d evice un d er test is trans-
mitted t h roug h 8220 to t h
e h
ig h im p
e d ance gate in put to
0229 Β . If, for exam p le, t h is variation is α
rise
in
voltage at
t he gate i n p ut, it will be accom p anie d by α
rise in voltage at
t he d rain of Q229A, d ue to t he para p h ase op
eration of
Q229A an d Β . Raising t h e voltage at t he Q226A
d rain raises
t h e base of emitter follower
Q235, an
d
t hus t h e base of
emitter follower Q241 .
As
t
he emitter of 0241 follows its
base
up,
it p ulls t h
e voltage at t he gate of Q266A up so t h at
it is eq u al to t h
e voltage at t he gate of Q266 B . T h is rise in
voltage at t h e gate of Q266A is t h en transmitte
d to t he base
of Q150 B ( p ositive si d
e of t h e d ifferential am p lifier)
t h
roug
h
feedbac k resistors R243 and R244 . T he +1 am p li-
fier, t h erefore, transmits any voltage variation from
t
h
e
in-
p ut to t he d evice un d er test to t
he
in p
ut to t
h
e base of
Q150 B with no ch a nge in am p litu de or p
olarity . In p er-
forming t h is tas k , t h e +1 am p lifier provides
t h e voltage vari-
ation with α h ig h im p
e d ance in p ut an d α low im ped ance
out pu t . Wh en t h e rise in
voltage at t he base of Q150 B h as
been transmitted to t he in p ut
to
t he
curren t setting resis-
tors, it com pensates for voltage variations at t h
e in
p
ut to
t he d evice un d er test h ol d ing t he voltage across
t h
e c
u
rrent
setting
resistors at 1
volt per ste p . AMP BA L R224
a d ju sts t h e DC balance of p a r aph ase amplifier 0229, and
also com p ensates for unbalance in 0150 .
O U
T
PU T Ζ
ad j ustme n t R243 ad j usts t he out p ut im pe
d
ance
of t he ste p
am p l if ier .
Relay Κ 101 Β an d Q248 or Q250 are used to limit t he
voltage w h ic h may be a pp lie
d to α device und er test in t he
reverse d irection
using o pposing offset . If, for exam p le,
Ci r cuit Desc r iption-Ty p
e 576
p ositive going ste p s are to be a pp lie d to t h
e
d
evice u
n
d er
test, Κ 101 Β is in t he
+
p osition . If negative offset is
a pp
lie d
to t he device un d er test by pus h ing t he O PP
OS
E butto n
and turn i ng t he O FF S ET MUL T control clock
wise, t h e step
generator will attem p t to con duct negative
current at t he
in p ut to t he d evice u n der test .
I n d oing
t
h
is, t
h
e voltage at
t he in p ut an d t hu s t h e voltage
at t h e Q229 B gate input is
d riven d ow n . Wh en t he voltage
goes app roximately 2 volts
below groun d , Q248 turns
on .
Wit h
Q248 on, t he n egative-
going voltage ste p s at t he base of 0150 Α
are limite d , t hu s
limiti n g t he out p u t of t he out p
ut am p lifier (t he in put to
t he d evice un d er test) to
about 2 volts . T h is amount of
voltage s houl d not
d amage α device un d er test .
V
oltage
M
o d e .
V
oltage ste p s are obtaine d from t he ou t-
p ut am p lifier in α
manner similar to t h at use d to obtain
current ste
p
s . F
or voltage ste p s, h owever, t he VO L TAG E
S E TTI NG RE SISTO RS are c h ange d to obtain t he vario u s
voltage am p litu d es, rat h er t h an t he C URREN T S E TTI N G
RE SISTO R S (w h ic h are h el d constant in t he voltage mo d e) .
Also since it is not desirable
to regulate t h e voltage at t h e
input to
t h e C
URREN
T
S E TTI NG RESISTORS in t he volt-
age mo d e, t he fee d bac k to t h
e p ositive si de of t h e d ifferen-
tial am p lifier t h roug h t he +1 am p
lifier is d isconnecte d an d
t he in pu t
to
t h
e +1 am
p
lifier is connecte d to groun d . T h e
base of 0150 Β is, t h erefore, h
el d at essentially groun d .
Since t he out p u t of t h e +1
am
p
lifier is at groun d , reverse
voltage limiting transistors Q248 an
d Q250 are d isable d in
t he voltage mo de .
In t he voltage mo de w h en ste p s of 1/2 volt p er ste p are
ap p lie
d to t he ste p out put am p lifier, t hey are transmitte d
t h roug h V O L TAG E S E TTI NG RE SISTA N C E R 141
t h roug h R145, t he in p u t resistance . By varying t h is in pu t
resistance with res p ect to constant fee d bac k resistance
R194, t he fee d bac k gain of t h e negative si
d
e
of
t h
e d iffer-
ential am p lifier is ch ange
d , t hus varyi ng t he am p litu de of
t h e voltage ste p s . After
being con d ucte d t h roug h t h e volt-
age setting resistors,
t h e steps are am p lifie d and transmitted
t h roug h t h e negative si d e of t he d ifferential am p lifier
in t he
same manner as d escribed in
t he current mo d e section .
Wh en t he voltage ste
p
s
reac h t he C URREN T S E TTI N G
RE SISTO R S, t hey are transmitte d t h roug h α nominal resis-
tance
(
R 215and 8216) of 5 Ω , for all voltage p ositions
of
t he A MPL IT U D E switch ,
before being a
pp
lie d to t he d evice
under test . V oltage ste
p
s
of varying am p litu d es, as d eter-
mine d by t he A MPL IT UD E switc h , are t hen
develope d
across t h
e
in p
ut im
p
ed ance of t he d evice un der test . F eed-
bac k to t he in p ut to t he d ifferential am p lifier
occurs at t h e
out put of t h e current setting resistors, t h
erefore, mini-
mizing t h e effect of R215 and 8216 .
Wh en using voltage ste p s, t he current con
d ucted at t he
step generator in put
to
t h
e d evice un d er test may increase
q uite
ra
p
i d ly a
n
d
p
ossibly d
amage
t h e d evice un d er test
(es p ecially
w
h
en testi n g transistors) . As α means of limiting
t
h is
current in t he voltage mo d e, c u rrent limiting resistors
R 185, R 186 an d R
187 are added to t h e output am p lifier
circ u it by t he AM PL IT U D E switch . T h
ese resistors limit
current at t he Ste p
Generator O
u
t put by limiting current
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