Circuit Description - Tektronix TYPE Z PLIG-IN Instruction Manual

Circuit Description — Type Z
drop across V7689 and the zener diodes is slightly more
than 100 volts, R7684 and R7685 (if needed) reduce the
potential to the exact amount required.
The precision voltage dividers consisting of R7687A and
R7687B (X I0), and R7687C and R7687D (X I00), reduce the
comparison voltage from 100 volts to either 10 volts or
1 volt by means of the COMPARISON VO LTAG E Range
switch.
Input Cathode Follower Stage
Signals applied to input connectors A and B of the Type
Z Unit go to the AC-DC switches which either include,
or short across, the coupling capacitors. The signals then
pass through the PUSH TO DISCON N ECT SIGN AL switches,
the VOLTS/CM switches, and the Mode switch, SW7611.
The signals are then impressed upon the grids of the input
cathode followers, V7613 and V8613.
The wide dynamic range of the Type Z Unit requires
constant-current operation of both the Input Cathode Fol­
lower stage and the Differential Amplifier stage. Another
requirement is that screen-to-cathode voltages remain con­
stant in both stages. Transistor Q7618 is the constant-current
source for both V7618A and input cathode follower V7613.
This transistor operates as a common-base amplifier.
base is held approximately 6 volts above the — 150-volt
supply by zener diode D8679 (see Note, below).
N O TE
W hen reference to the — 150-volt supply is m ade,
the actual typ ical vo ltag e measurement is — 144
volts a t pin 9 of the interconnecting plug in a ll
oscilloscopes using a decoupling network in the
main oscilloscope.
The base-to-emitter voltage is essentially constant, the
base being a few tenths of a volt more positive. With base
and emitter voltages fixed, Q7618 operates at a constant
current. Note that a variation in the — 150-volt supply has
little or no effect on the transistor bias. Thus no change oc­
curs in either the base-to-supply drop or the base-to-emitter
drop. O nly the base-to-collector drop varies.
The voltage drop across zener diode D8679 sets the
voltage drop between emitter and the — 150-volt supply.
A decrease in emitter resistance would require a greater
current to establish the same fixed drop.
the current control for Q7618, the constant-current source.
The collector current of Q7618 is slightly less than the
emitter current, very nearly constant, and independent of
the base-to-collector voltage. Such a circuit is very stable
with respect to transistor parameters and temperature.
To describe the constant-current circuit of the
Cathode Follower stage during peak operation, assume
that an input voltage swing from — 100 to +100 volts is
applied to the grid of V7613. The cathode of V7613 and
plate of V7618A follow the 200-volt swing.
of V7618A varies -------- r—
li +
I
3-2
Its
Hence R7619 is
Input
The cathode
times the plate swing. In this
circuit, the cathode swings about 1 /30 of the plate swing.
Therefore the grid-cathode swing of V7618A is approxi­
mately 6.6 volts. The voltage swing is now low enough
for direct coupling to the collector of Q7618 whose func­
tion is to provide the constant-current source for this half
of the input stage. The voltage swing of 6.6 volts is easily
handled by the transistor at its operating current of ap­
proximately 8 ma.
The effect of a transistor ''long-tail" in the cathode circuit
of V7618A is shown in Fig. 3-2.
the constant-current characteristics of a 2N1302 transistor
when connected similar to Q7618. The lower curve displays
the constant-current characteristics of V7618A with Q7618
connected in its cathode circuit to control the current. For
practical purposes, no measurable change in current occurs
during these voltage excursions.
The grid of V7618A returns to the zener diode D8679
through a temperature-compensating diode-connected tran-
•*
OP
'=
1 ma/div
L
>PEI[AT NG REGIC
- c
< P
1 ma/div
Fig. 3 - 2 . T o p : co n stan t-cu rren t co lle cto r ch a ra c te ristic s of 2 N 1 3 0 2
tra n sisto r h a v in g a 1-k resisto r in the em itter circu it; bottom :
stan t-cu rre n t p la te ch a ra c te ristic s of 6 D J8 trio de connected s im ila r to
V 7 6 1 8 A . (T ra n s is to r d is p la y o b ta in e d
T ra n sisto r-C u rv e T ra c e r; trio d e
C h a ra cte ristic -C u rve T ra c e r.)
The top curve displays
ERA TIN G REG ON - 1
V
cb
1 vo lt/div
>N
E
pk
20 vo ts/div
con­
from T e k tro n ix T yp e 5 7 5
d is p la y o b ta in e d from T yp e 5 7 0