Tektronix 1A5 Instruction Manual page 32

Purpose Of
Adjustment
Input Capacitance
H.F. Attenuation
L.F. CMRR
H.F. CMRR
The output of the attenuator circuit develops across R108,
C 1 08, R121, C 1 22 and R122. C 1 08 adjusts the input capaci ­
tance to 20 pF. C 1 22 also affects the input capacitance, but
is adjusted primarily to improve the high frequency com ­
mon-mode rejection ratio. R108 and R1 21 form a suppres ­
sion network to improve circuit stability.
are inserted in the signal path to improve transient response.
Source Follower and Bootstrap Circuitry
Identical source follower and bootstrap circuits are con ­
tained in the A and B channels.
of the A channel is presented in Fig. 3-1.
Signals passing through the input circuitry are applied
to the gate of FET Q123A. (An operational analogy exists
between a field effect transistor and a vacuum tube, with
the gate comparing to a grid, the source to a cathode and
the drain to a plate.) In typical cathode-follower manner,
the signal is developed at the source of Q123A and applied
to the base of emitter-follower Q143. The signal from the
emitter of Q143 is applied to relay contact K295A.
The source of Q123A operates quiescently at approxi ­
mately +0.6 V. This voltage is established by a relatively
constant current through the high impedance source resistors
R123 and R1 27 and constant current transistor Q128. R129
is adjusted to balance this voltage between the A and B
channels during quiescence. When R129 is adjusted prop ­
erly, there will be no trace shift when switching from one
gain setting to another with no signal applied.
Part of the Q123A source impedance is made up of the
Q143 base-emitter circuit. Resistor R141 and potentiometer
R140 are placed in parallel with this circuit to allow adjust ­
ment of differences between the A and the B channel load ­
ing effects. With R140 (COMMON-MODE BAL) properly
adjusted, no trace shift will result when DC or low-frequency
common-mode signals within the input signal range are
applied simultaneously to the A and B channels.
The bootstrap (positive feedback) circuitry consists of
R148, R149 and Q133. A majority of the s i gnal occurring
at the emitter of Q143 is felt across R149 and is applied to
the base of Q133. The polarity of this signal is the same
as that applied to the gate of Q123A. Q133 and Q123A
therefore act in unison and form a high impedance input
circuit relatively independent of the gain of the active
devices, and provide a gain of approximately 1 from the
FET gate to the emitter of Q143.
Input Protection Circuitry
Protection from positive input signals is provided by the
circuit design and D133. The high impedance source load
TABLE 3-2
Input Attenuator Adjustments
" A " Input Attenuators
× 1 ×10
×100
Cl 08 C105A Cl 06A
C105B C106B
R105D R106D
C 1 09 and R109
A simplified s chemati c
Circuit Description — Type 1A5
Adjustments
" B " Input Attenuators
×1000
×1 ×10
C107A C208 C205A
C107B C205B
C205D
of Q123A allows the s o urce voltage to follow the gate sig ­
nal up to +100 V without component damage. This voltage
dicta t es the value to which the emitter of Q143 and the
base of Q133 will go. As the input signal rises and the
base of Q133 approaches +
tion of Q133 becomes forward biased; D133 becomes back
biased. All connections to Q123A, Q133 and Q143 are
allowed to rise to the vicinity of th e voltage at the ga t e
of Q123A and no stress approaching breakdown voltage
exists across any componen t
Negative s i gnal input protection up to
tained through B122, R138, R
operating conditions, Q137 is s
dropped across it. The voltage at its base is approximately
+ 21 V. When the s i gnal on the gate of Q123A reaches
about — 60 V, B122 ionizes and maintains approximately
55 V across itself . This places the base of Q137 ne
forcing its em i tter to follow. Any further change in gate
vo ltage will be transferred directly to Q 137, keeping the
diffe r ence in voltage between Q123A source and Q137
em i tter within component breakdown values.
R e fer back to the Input Amplifier s
the discuss i on of the remaining components in the A chan ­
nel s o u r c e-fol lower circuit.
Input cross ta lk (inter-action) at high frequencies through
the capacitance of de-ionized neon tubes B122 and B222
is kept to a minimum by decoupling action of C139. C126
(in the base biasing n et work of Q128 and Q228) minimizes
cross -ta lk between the cons tant current circuits. C 1 24 and
C148 improve high frequency response.
bination of C 1 32 and R1 3 2 provides high frequency damp ­
ing. The s o urce and emitter circuit capaci
and Q143 are s e t for optimum CMRR in the 1 to 10 MHz
range by adjusting C 1 23. R e sistor
to improve the gain of Q143 at high frequencies.
R 2 59 and PR O BE STEP ATTEN BAL R
of the A and B channels during active probe operation.
B channel operation can be followed by relating the
components in. the B channel to their counterparts in the
A channel.
Comparator Circuit
The comparator is made up of two cascaded, emitter ­
coupled amplifiers with a gain of about 4.
a push-pull output in response to any unbalance of inputs
at TP160 and TP260. This unbalance can be the result of
a signal input from one channel or the difference between
the signals from the two channe l s.
×100 ×1000
C206A C207A
C206B C207B
C206B
2 0 V, the collector-base junc ­
s.
— 100 V is ob ­
1 39 and Q137. During normal
a turated and has about 0.2 V
a r — 5 V,
che m a tic to follow
The series com ­
t ances of Q123A
R 1 45 is shunted by C 1 44
R159,
1 58 allow balancing
It provides
3-3