Tektronix 2213 - Page 33

T h eory of Operation-2213 Se
r vice
Ste p B alance p oten tiometer R 138
(at pin 1 of R 139) is
ad j uste d to compensate
for in pu t offsets reac h ing U 120
p ins 2 an d 3 w
h
en
switc h ing b etween VO L TS/DI V switc
h
p ositions .
In
t h e fast path , t h e input signal is ac-co
u ple
d
to i np ut
FE
T
Q122 t hr oug h C121 . T he in p ut
FE
T
is arra n ge d i n α
source-follower config ur ation use d to
drive complementary
emitter followers 0133 an d Q134
. T he combi n ation of
Q125, R126, R131, R132,
V R 130, and R130 sets α
constant cu rre nt t h ro u g h R 125
i n t he source lea d of Q122 .
T h e voltage d rop across R 125
b iases Q133 and Q134 for
abo u t α 10-mA idle current .
Α
b ootstra p ci r c u it com p ose d of Q139, VR 122,
an d
R 122
con nects t he Q122 drai n to t he Q122 so
u rce .
T
h is
circuit
forces t h e b ias voltage across Q122 to r
emain
con-
stant,
w h ic h i n con ju nction wit h t
h
e
consta n t b ias c urr en t
s upp lie d b y R 125, k eeps Q122
op erating
at
α constant
power level to p revent d istortio
n due to ch anging sig n al
cu rrents .
Com p lementary emitter followers Q133
an
d
Q134
s upp ly drive current to t he =1, =2 .5, an
d
=5
voltage divi d ers
an d p rovi d e impedance matc
h ing b etween in pu t FE T Q122
and t h e d ivider
networ k . T he b ias levels of Q133 and Q134
are sta
b ilize d by emitte r resistors R 139A an d R 139
E
res
p
ectively . Average voltage c h anges occurring in t
h
e o
u t-
p
ut
of Q133 an d Q134 are sensed t
h
roug
h
R 139 B
and
R 139D w h ic h are con n ected to t
he p oint of lowest
impedance (t h e emitters of Q133 an
d Q134) . R esistor
R 139C p rovi d es α pat
h
t
h
at
completes t h e fee dback loop
to t he slow- p at h portion
of t h e
B
uffer Am p lifier .
V olts/Div
V ar Circ u it an d X1/X10 Am p lifier
T h e a p p
ro p riate voltage d ivi d er sig n al outp ut (=1, =2 .5,
or =5) is
selecte d by V
O
L TS/DI V switc h S105 B and ro u te d
to t he
V
olts/Div
V ar circ u it compose d of C141, R 141,
a n d R 143 . . C
h anges t h at occur in t he B uffer Amplifier
outp ut im
p e
d
ance due
to
setting R 141 or switc h ing t h e
d ivi d er ou t p u t are se
n
sed
via
R 139M
. T h ese c h anges mo d ify
t h e slow- p at h fee db ac k signal to
cause U 120 to again matc h
t h e gain of both p at
h
s .
F rom t he V olts/Div V ar
circ u it, t he signal is a pp lied to
t h e in p ut of t he X1/X10 Switc
h able-gain Amplifier U 145 .
Am p lifier U 145 p roduces α
d ifferential outp ut signal f r om
t h e si n gle-e nd ed
in p ut sig n al . T h e gain of t he am p lifier is
controlled by
t he setting of VO L TS/DI V switc h S105 .
Am p
lifier gain is c hanged b y switc h ing between two
p airs
of transistor am p lifiers containe d
in
U 145 .
Gain of t he
Χ 10 am p lifier pair is a
d j uste d by R 145 to obtai n t he
3-
6
cor r ect d
eflectio nfacto r for t h e 2m,5m, an d 10m V O LTS/
DI V switc
h positio n s . R esistors 8146, R 147, an
d R 148
act
to b alance any do offsets between t
h e Χ 1 and Χ 10
amplifiers . T r ace s h ift occu
r ri n g
w
h en t h e VO L TS/DI V
V aria b le con trol is rotate d
is minimized by resistor R 142
w h ic h stabilizes t he in
p
ut b ias current to U
145 .
VER TICA L PRE A M P S
T h e
C
hann el 1 an d C hann el 2 Pream p ci r cu itry, s
h own
in Diagram 2, incl ud es t
he
ve
r tical p reamp lifie r s, t he
inter n al trigger pic
k off
am
p lifie r s, and α com m on-base
output stage fo r
each
c
h annel . V ertical p ositioni ng of the
c h
annel
d isplay
is i nco r po r ate d in t he commo n -base
am
p lifier stage .
C h annel
1 V ertical P ream p lifier
T
h e
C
h annel 1 V ertical P reamplifier prod uces d iffer-
ential out p ut sig n als to dr ive t he V ertical Out p ut Am
p lifie r
and internal trigge r signals to
drive
t
he
T
r igger circuitry .
Differential signal current
from
t
he Attenu ator circuitry
is app lie d to common-base
transistors Q157 and Q167
t h roug h ca b le-te
r minati n g resistors R 151 and R 161
r es p ectively
. T h e collector currents of Q157 a n d 0167 will
flow t h ro
ug
h R 158
an d R 168 to pro du ce level-s h ifte d
signals w h ic
h drive U 170D a nd U 170 E . B alance p otenti-
ometer R 154
is ad j uste d to b alance t h e d o level of t he
C h annel 1 ou tput
wit h t he
C
h annel 2 outp ut by setting t he
b ias levels of Q157
an d Q167 . C h annel 1 fre q uency
r es p onse is matc
hed to Ch annel 2 res pon se b y a d j usti ng
C167 .
Transistors U 170D
and U 170 E form α common-emitter
amplifie r . T
h
e gain
of U 170D and U 170 E is set by R 180
(co
nn ecte d between the emitters), and t h e h ig h -fre q ue ncy
respo n se
is
com p ensate d by C180 . T h e emitte rs are also
co n necte
d to t he b ases of U 170C and U 170 B r es p
ectively
to p rovi de an internal trigger signal
p
ic
k off point . V ertical
signal output current
flows from t he collectors of U 170D
an d U 170 E to
t he emitters of commo n -base amplifiers
Q177 and Q187 . Α
s hunt resistor gain networ k ( R 176 a n d
R 186) sets
t h e gain of the common-base stage . C h annel 1
P OSITIO N control R 190 s up plies α va r iable
offset curre n t
to t he emitters of Q177 an d Q187
w h
ic
h allows t he trace
to b e vertically p
ositioned
on
t he crt . T he commo n -base
amplifier stage converts t
he
d
iffere n tial signal in p ut current
to α d ifferential sig
n
al
ou tput voltage t h at is applied to t he
Diode Gate
circ u itry (Diagram 3) .
C h annel 2 V ertical
P ream p lifier
T h e C h annel 2 V ertical
Preamp lifier fu n ctions t he same
as t he C h annel 1 V ertical
Preamp lifie r p reviously d escri bed ,