Tektronix 576 - Page 53

Circuit
Description-Ty p e 576
1/2 volt/ste p
J
Device
Und e r
Test
3- 1 2
F ig . 3-7 . Bl oc
k diag
r am o f Ste p
O u tp u t Am p l if ier : ( Α ) Current M ode, ( Β ) Vo l tage M ode .
tive in p ut, t he voltage
setting resistors, is ch ange d , t h us
varying t he
feedbac k gai n of t h at si d e of t h e d ifferential
am p lifier . In t h is manner voltage ste p s of vario
u
s
am p li-
tud es are obtaine d .
Current
M
od e .
I nput
to
t he n egative si de of differential
com p arator 0150 (t he base of
Q150A) is always t h roug h
VO L TAG E S E TTI NG RE SISTO RS
8141 t h roug h R 145 . In
t he current mo d e,
t h is in put resistance is set at 3 .01 k Ω
(8141) for all current
p ositions of t h e A MPL IT U D E
switc
h . Wh en 1/2 volt ste p s are app lie d to t he base of
Q150A t h
roug
h
R 141, t h
ey
are inverte d , app lie d to t he
base of Q164 and
inverte
d
again . T he ste p s are t hen trans-
mitted t h roug h
emitter follower Q169 to t he bases of Q172
an d Q176 .
Dep en d ing on t he p osition of relay
contacts
Κ
102
Β an
d K102C, eit her Q172 an d Q180 or Q176 an
d
0184 are turned on . If, for exam p le, Κ 102 Β and K102C
are
i n t he + p ositions
signifying p ositive-going ste p s out, Q176
an d Q184 are on t h e
Q172
an
d Q180 are off . In t h is case
t he input to Q176 is negative-goi
ng ste p s . T hey are inverted
by 0176 an d t he resulting p
ositive-going ste p s are trans-
mitted t h roug h emitter follower
0184 to t h e negative si d e
of t he floating 50-volt supp
ly . E ac h time α p ositive ste p
occurs at t he negative si de of t he 50-volt su pp
ly, t he su pp ly
is push e d up by t h e amount of t
h
e
ste p . T he p ositive si de of
t
h
e 50-volt
su pp ly is connecte d to bot h t he feed bac k resis-
tors and t he in put
to t he current setti ng resistors, so
t h at
each time t
h e 50-volt supp ly is raised by α ste p , t h
e voltage
at t h is
connecting point is also raise d by t he amou n t of
t h e
ste p . Due to t h e p resence of t he 50-volt su p p ly, t he voltage
at t h e in put to t
h
e current
setting resistors is offset by 50
volts . To com p ensate for t
h
is
offset,
50
volts of o pposing
offset is a dd e d to t he in put of t he current setting resistors
t h roug h relay Κ 102 Α . If Κ 102 Β and K102C are in t
h
eir-
p
ositions, Q172 and Q180 are on and
Q176 and Q184 are
off . In t h is case
negative-going ste p s are a pp lie d to t he
p ositive si d e of t h
e 50-volt s upply and negative-going ste p s
a pp ear at
t h
e
in put to t he current setting resistors .
T he out put of t he negative si de of t he d ifferential am
p li-
fier at eit h er Κ 102 Β or K102C is fe d bac k to t he base of
0105 Α t h roug h feedbac k resistor R194 . Since R194 is 6.04
k Ω and t he in p ut resistance, R141,
is 3 .01 k Ω , t he feed -
bac k gain of t h
is circuit is 2 . F or α h alf volt p er ste p in p ut,
t he resulting out p ut of
t he negative side of t he d ifferential
amp lifier (as seen
by t h e in put to C URRE NT S E TTI NG
RE SISTO RS
R197 t h roug h R 216) is ste p s of o ne volt p er
ste p , t he
zero level bei n g at groun d . (If offset h as
been