Tektronix 7A16P Instruction Manual page 105

Transistors (FET's) ar e
u
sed in pl ace o f r ee d s
wi tc hes .
L
ar ge
r
es i sta n ces
in se ri es wi t h t he con t r o l lin es
i
so l ate t h e
l
og i c c ir c ui ts fr om t h e
F
ET
gate ca pac i ta n ce . S in ce the
F
ΕΤ 's a r e fl oate d on
l
arge
r
es i sta nces, t h e y
can p ass ve ry li tt l e c urr e n t to g r o und, a nd t he c ir c ui t can wi t h sta nd
l
ar ge ov e rl oad vo l tages . The d i od es in
t
h e co n t r o l lin es (C R 116 an d
C R 106) limi t t h e c urr ent in to the
F
ET c ir c ui t ry .
Wh en ΑΤ 212
i
s unse l ected, th e d rai n-to-so ur ce ca p ac i ta n ce o f Q200
co ul d coupl e ΑΤ 212 into t he c ircui t, deg r a di ng hi g h fr e qu enc y r es po n se .
Q110 s hun ts t h e
i
n pu t o f
t
h e attenuato r w i t h α ca p ac i to r wh e n ΑΤ 212
i
s
unselected . T hi s pr e vents t h e capac i ta n ce o f Q200 fr om co upling ΑΤ 212
into t h e c ir c ui t .
Buff er Amp l ifier . The o u t pu t s i gna l fr o m
t
h e Χ 10 Atte nuato r n etwo rk
a pp ea r s at t h e to p o f R 204 . Hi g h fre qu e n c i es a r e co upl ed thr o u g h C207 to
α uni t y ga i n a mplifi e r, Q302 .
V
R303 pr o vi des α bootst ra pp ed, pos i t iv e
b i as vo l tage fo r t h e gate o f Q302 . Q303 protects Q302 fr om ove rl oa d s .
7 Α 16 Ρ IN S TRU C T ION
L
ow fr e qu e n c i es, in c luding co rr ect i o n fo r Q302's drift, a re fed to
t he inv e r t i ng inpu t of U 310 . U
310's ou t put
i
s inv e r te d aga in by Q300 so
t he s i g na l at
i
ts collecto r i s in ph ase w i t h t h e ou t put o f Q302 . Th e s um
o f
t
h e ou t pu t s i g n a l s
i
s co up led t hr ou g h em itte r- f o llo w e r Q304 to t h e
l
ow
i m peda n ce atte nu ato r . Α p
a r t o f
t
h e ou tp u t s i g n a l is
f
ed bac k t hr o ug h
R 307 to
U
310 . C202 sets t h e
r
o ll-o ff
c
h a r acte ri st i c of U 310 . R315 a nd
R
412 pr ovi de nulling fo r U 310's inpu t c urr ent a n d o ff set vo l tage .
Low Impedance
Atte nu ato r . Q306, Q308, Q400,
Q402 an d t h e assoc i ated
comp o n e nts at t h e bottom o f sc h emat i c 1 fo rm α se l ectabl e Χ 2 o r Χ 5
atte nuato r . W h e n t h e Χ5 atte nuat i o n f acto r i s selected (see
T
ab l e 5-1),
Q306 a n d Q402 a r e on a nd Q308 a nd Q400 a r e o ff .
A
s α
r
es ult, R404, R406,
R407 an d
R
400 fo rm α Χ 5 vo l tage d ivi de r .
L
408
i
s α " p eaking" co il to
imp r o v e hi g h fr e qu e ncy r es p o n se . W h e n Q308 a n d Q400 a r e on, Q306 a n d Q402
a r e o ff a n d t h e d ivi de r i s co nfi g ur e d to pr o vi de α Χ2 atte nuat i o n f acto r .
R
E
V
Α
JUN
1979
T
h e o u t pu t o f
t
he attenu ato r i s co upl ed to t h e a mplifi e r stages
t hro u g h Τ 416 . Thi s t ran s f o rm er kee p s s hif ts in
t
h e attenu ato r g
r o un d
l
e ve l from be ing co upl ed in to t he amplifi e r b y m a kin g t h e s hif ts a pp ea r
as α
common- m ode s i gn al at t he in p u ts o f t h e amplifi e r . R414 pr o vi des
DC ba l a nce fo r
t
h e a mplifi e r inpu ts .
5-5