Tektronix 576 - Page 61

Ci r c u it Desc r i p tio n -Ty p e 576
t h e low impe d ance inp u ts . Eq uatio n 3-1
s h ows t h is relation-
s h i p .
It s hou l d
be note d t h at it is transistors Q3 and Q4 w h ic h
ca u
se Q5 and Q6 to con d uct more or less loa d current . As
i
n p
revious
exam p les, assume t h e n ormally constant d rai n
curre n t
ID
con d ucte d by Q1A is
cause d to increase eit h er
by increasing 1 s or
ΙΡ
1
. T
h
is
increase in
ID
causes t h e source
voltage of 01 Α to go
negative, causing
03
to con d uct more
current . T h is in turn causes
Ο
5 to
con d uct more current .
T
he
a dd itional
current con d ucted by
05
reduces t h e
current t h ro u g h Q2A and
causes t
h e drain
cu
r re
n t
Ι
ρ
to
be
re d uced back to its normal constant value .
T he gain of t
h
e
d is p lay amplifiers is a d j uste d in two
ways . T h e overall gain
is controlle d by varying t he loa d
resistance ( R L1 and R L2) . A d j
usting t h e loa d resistance
affects t h e gai n of t he h ig h im p ed ance
in p uts, as well as
t h at of t h e p ositioning cu rrent . R
L1
an d R
L2
are a d j uste d
so t h at t he p ositioni ng inp u ts p rovi
d
e
t he p ro p er deflection .
V arying t he source cou p ling
resistance (R s ) sets t he gai n of
t he h ig h impedance in p uts only .
R s is a d j uste d to matc h t he
h ig h imped a n ce gai n to t he p ositio
n
i
n
g in
p uts .
By switc h ing RM into t he circuit,
t h e overall dis p lay
am p lifier gain is increased by α
factor of 10 . L oad cur-
r ents I L1 an d I L2 flow t h roug h resistors
RN 1
an d R
N2 .
Wh e n RM is in t he circuit, any
c ha n ge in t h e curre n t
t h ro u gh R N1 an dR N2 ca u ses α voltage across
RM . T h is
voltage across RM causes additional
loa d curre n t to be con-
ducted
b y
05
and Q6, loa d current w h ic h is not felt by t he
emitters of Q2A and Q2 B . F or α given ch ange in c u rre n t at
t h e emitte r s of Q2A and Q2 B , t h erefore, α greater c
h
ange i
n
loa d current t h ro u gh Q5 an d Q6 occurs, causing a dd itio
n
al
gain of t he dis p lay am p lifier . T he gain of t he circ u it un d e r
mag n ified con d itions is con trolled by a d ju sting RM .
V ertical Dis p lay Am p lifier
T he Dis p lay Am p lifiers sc h ematic s h
ows
t
h
e
complete
sc h ematic of t h e vertical d is p lay am p lifier . T he table in
F ig .
3-12 relates t he t r ansistors an d FE Ts in t he simplifie d
sc h ematic wit h t h ose in t he actual sc h ematic of t h is circ u it .
T h e com p lete sc hematic s h ows t
h at t h
e
h ig h im p edance
in p uts of t he am
p
lifier h ave
t h ree se p arate gains (R s h as
th ree different val u es) . As h as been
mentione d p reviously in
the d iscussion of
t h
e signal
sensing an d d is p lay sensitivity,
the deflection factor of the vertical d isplay is
partially
d
e-
termi ned before the measurement is ap p lied to
t h
e
h ig h
impedan ce in p uts . T he t h ree gains of
t h e vertical d is p lay
am
p lifier allow t he vertical display to have t h ree d ifferent
deflection factors for each voltage signal
ap p lie d to t he h ig h
impe d ance in p uts in α 1-2-5 r elations h i p . 1'S GAI N a d j
ust-
ment 8541, 2'S GAI N ad ju stment R 538 an d 5'S
GAI N
a d j ustment R 536 d ete r mine t he t hr ee gains of
t
h e h
ig
h
im p ed
ance i np uts . R elays Κ 537 Α and Κ 541 Α d etermine
w h ic h resistors will control t he gain for t he various p
osi-
tions of t he VER TICA L switc h . VER
T
O U
T
PU
T GAI N
a d j ustment R 592A and Β d etermi
n es t he overall gain of t he
3- 20
vertical d is p lay am p lifier by allowing a d ju
stment to t he
load resistors R
L1
an d R
L2 .
T he overall balance of t he p ositioning
curre n ts of t h e
vertical d is p lay am p lifier is cont r olled
by VER T C EN T
ad j ustment R 581 . In add ition,
1'S
B
A L a d j ustme n t R 550
and 2'S B A L ad j ustment R 545 p rovi d e p
ositioning current
balance w h en t he VER TICA L
switc h is set to α p osition
wit h α one times or α two times
multiplier, res p ectively .
R elays Κ 537 Β and Κ 541 Β determine w
h
ic
h resistors con-
trol the p ositioning current balance for
various p ositions of
t he VER TICA L switc h .
Wh en t he DIS PL AY O FF SE T Selector
switc h is set to
VER T Χ 10, R 574 an d VER T M AG
GAI N a d j ustment
8573 are adde d to t he vertical d is p lay am p lifier
circuit .
T h
ese
resistors constitute RM an d i n crease t he sensitivity of
t he vertical d is p lay 10 times . R 580 is always in t he circuit
an d gives t he o
u t p ut stage an unmagnifie d gain of about
1 .8 .
H orizontal Dis p lay Am p lifier
T he Dis p lay Am p lifiers sc h ematic s h ows t he com
p
lete
sc h ematic of t he h orizo n tal d is p lay amplifier . T
h
e table
i n
F ig . 3-12 relates t h e
transistors and FE Ts in t he sim p lified
sc h ematic wit h t h ose in t he actual sc hematic of t h is circuit .
T he h orizo n tal d is p lay am p lifier op erates
basically t he
same as t he vertical display am p lifier .
1'S GAI N ad j ustment
R 638,
2'S GAI N ad j ustme n t R 636 and 5'S GAI N ad j ust-
ment
R
641
co
n t r ol t he t h ree gains of t he h orizontal h ig h
im p edance
in p uts . R elays Κ 637 Α and Κ 641 Α d etermine
w h
ic
h resistors will control the gain for t he various p osi-
tions of t he H O R IZO N TA L switc h . H OR IZ O U T PU
T
GAI N ad j ustment R 692A and Β controls t he loa d resis-
tance . OR T H OG ad j ustment R 685 i n teracts wit h t he verti-
cal d isplay am p lifier and allows ad j ustment of t he ort h o-
gonality of t he d is p lay on t he CR T . Wh en t he DIS PL AY
O FF SE T Selector switc h is set to H O R IZ Χ 10, R 674 an d
H
O
R
IZ
M
AG
GAI N ad ju stme n t R 673 are adde d to t he
circuit an
d form
RM .
R 680, li k eR 580, is always in t he
circuit and
gives t he out p ut stage an unmagnifie d gai n of
about 1 .8 .
T
h
e overall
balance of t he p osition currents of t he h ori-
zontal d is p lay
am p lifier is controlle d by H
O
R IZ C EN T
a d j ustment 8681 . In a dd ition, 1'S B A L ad j ustment R 650
an d 5'S B A L ad ju stment
R
645
provi d e p ositioning current
balance w h en t he H O R IZO N TA L switc
h
is
set
to α p
osition
wit h α one times or α five times multi p lier, res p ectively
.
R elays Κ 637 Β an d K641 B d etermine w h ic h resistors
con -
trol t he positioning current balance for various
p ositions of
t he H O R IZO N TA L switc h .
R
ea d out
Α dis p lay of t he vertical and h orizontal d eflection
fac-
tors, t he step am
p
litu
d
e
and
the
β or g m p er d ivisio n
(verti-
cal
d eflection factor d ivi d e d by
ste p am p litu d e) is given to
t he rig h t of t he CRT . T
h
is d is p lay of
n umbers an d units is