Input Amplifier - Tektronix 7a11 Instruction Manual

voltage at the
d
rain . R 28
p
rovides α small amou nt of cur-
rent (about 0.1 mA) to offset that re quired by the
b
ase cir-
cuit of Q33.
The signal is coupled through Emitter
F ollowe r s Q33
and Q43 and
t
hen ap plied throug h
t he C44- R44-R 45- R 46-
R47 networ k to α 93 Ω coaxial ca ble wh ic h
a
pp
lies th e
signal to the Input Amplifier. C44, R44, R45, R 46 and R 47
provide reverse terminatio n for th e 93 Ω ca ble .
The O FF SET control causes α current to flow th roug h
t
he R 20, R45, R 46, R47 reverse termi natio n networ k with-
out changing t h e o
p
erating voltage of Q43.
The 7- V su
pp
ly w hic h is use
d
to
p
ower the probe cir-
cuitry is
b
oot-strappe d
to ensure that the active devices
d
o
not change their c haracteristics as α result of DC signal com-
p
o n e n ts
b
ei ng a
p
plie
d
to the
p
ro be ti
p
. See the Power
Supply ex p lanation for
d
etails .
The ID EN TI F Y switc h , when actuated, interrupts α
grou nd pat h to an identifyi ng signal inj ection circuit. De-
tails regarding th e circuit are co ntaine
d
i
n
t
he
V
O LTS/DIV
Switc h a nd R ea dout description .
General Descri
p
tion
I
NPUT AMPLI F I ER
Refer to the I n
p
ut Am plifier sc hematic in the
b
ack of
t his ma nu al . The
p
ri ncipal
p
arts of the I nput Am
p
lifier are
the
P
ara
p
hase Am
p
lifier, the atte nuation network, th e va r i-
able gain control circuit, the ban
d
widt h a nd polarity cir-
cuits. Signals received from the
P ro be are app lie
d
to t he
base of Q124A, where they cause equal and
o
pp osite signals
to be
d
evelo
p
e
d
in
t
he collector circuits of Q124A and
Q124 B . Sig nal current
p
asses through
t
he
b
an
d
width
limiting filter, the Χ2 attenuators, the X2 .5 attenuator and
throug h
t
he
p
olarity switch circuits, after which
t
hey are
ap
p
lied to the Output Am
p
lifier . Wh en the CA L IN
b
utton
is in
t
he out
p
osition, the
V
AR IA BLE co ntrol can cause
signal current to be shu nted throug h field effect transistor
Q198 to attenuate the signal
b
ei ng applied to the Output
Am plifier .
Α ST EP ΑΤΤΕΝ BA L circuit
p
ermits
b
alancing the cur-
rent in the two si des of the amplifier un
d
er no-signal con
d
i-
tions. This ensures that no trace sh ift will occur wh en atten-
uators are switched into or out of the circuit during no-
signal conditions .
An offset voltage can be applied to the base of Q124A .
In normal
u
sage, the amount of offset voltage injected at
this
p
oi nt is only sufficient to ca ncel α DC voltage level
existing on
t
he incoming signal .
Circuit
Description-7A11
An ad
d
itional function occurring at the Q124A base is
the sampling of the DC voltage com
p
onent of the incomi ng
signal for use in
b oot-strappi ng the
P
robe
p
ower su
pp
lies. Α
d
etaile
d
description of t his boot stra p ping feature is con-
tained in the power su
p
ply ci rcuit
d
escription .
Detaile
d
Descri
p
tion
T he base of Q124B is connecte d to grou n
d
in the
P
ro be,
thus
p
roviding α reference
p oint for t he para
p
h ase ampli-
fier .
U nder no-signal conditions, th e
b
ase of Q1 24A is also
at zero
p otential . This places equal voltages at the emitters
of Q124A an d Q124 B. T he curre nt th at is available throug h
R 148 an d R 149 is evenly
d
ivided between R 127 and R 147,
an d passes throug h transistors Q1 24A an
d
Q1 24B.
The
p
rinci
p
al loa
d
for Q1 24A consists of R 126 in paral-
lel wit h R 175, p roviding approximately 40 Ω im
p
edance as
seen from the top of R 124. 1f any or all of the I nput
Am
p
lifier attenuators are switched into the circu it, they
a
pp
ear in series-
p
arallel combination with R175 in α ma n-
ner whic h maintains α 40 Ω im
p
e
d
ance as viewed from the
top of R 124. Changes in attenuators therefore
d
o
n
ot affect
the parameters associated with Paraph ase Am
p
lifier Q124A-
Q124 B.
The gain of the circ uit is a
p
proximately equal to t h e sum
of the load resistances of Q1 24A an
d
Q1 24B divi de d by th e
resistance se
p
arating the two emitters . Th us, 40 Ω
p
lus
40 Ω
d
ivi
d
ed by the effective impe
d
ance
b
etween
t
he two
emitters (R 127
p lus R 147 in parallel wit h R 137, R 139 an
d
t he effective resista n ce of R 138) is equal to a
p
proximately
one.
H
alf of this gain of 1 can
b
e
d
etecte
d
at
L
126 an
d
h
alf
at
L
146. As previously mentione
d
, this gain remains one
r
e-
gar
d
less of the attenuators inserte
d
into the circuit. How-
ever, the attenuators combine with R175 and R176 to
reduce the signal available to the Outp ut Am plifier to one-
half, one-fift h or one-tenth of the signal available at
L
126
and
L
146. The relays are energized to
p
rovi
d
e the straigh t-
throug h conditio n . The signal is re duce
d
to one
h alf by
d
e-
energizing Κ405 and Κ407 . It is reduced to o n e fo u rt h by
d
e-energizing Κ411 an
d
Κ413. All
t
hree
p ai rs of Yel : ;ys are
d
e-e n ergize d to re
d
uce the signal to one te nth .
Gain an
d
freq uency compensation adj ustments are pro-
vi
d
ed between t h e emitters of Q124A and Q124 B . R124
and R 144
p
rovi d
e thermal com
p
ensation, with C124 and
C144 providing
h
ig h-frequency signal by-
p
ass. VR 124 is
normally non-conducting . It limits the voltage wh ic h can
b
e
dro
p
ped across R124 to ap
p
roximately 3
V,
maintaini ng
good circuit res
p
onse under large signal co n
d
itions .
In FULL ban
d
wi
d
th mo de, relays K401 and K403 are
activated. Wh en 20 MHz ban
d
width is selecte
d
at the front
p
anel,
L
152,
L
155, C153 and C154 cause α ban
d
widt h
3-3