Tektronix TYPE Z PLIG-IN Instruction Manual page 15

BLOCK DIAGRAM DESCRIPTION
Fig. 3-1 shows the block diagram for the Type Z Unit.
Signals applied to A Input and B Input connectors pass
through the VOLTS/CM Attenuator switches to the grids
of the tubes in the Input Cathode Follower stage.
VOLTS/CM switches insert frequency-compensated attenu­
ators into the circuit. When properly adjusted the input
resistance and capacitance of the unit remains unchanged
as the attenuators are inserted.
Accurate ± D C comparison voltages are obtained from
the Comparison Voltage Regulator. These can be applied to
the grid of either Input Cathode Follower stage by means of
a Mode switch. These voltages add differentially to the
signal applied to the other Input Cathode Follower when
differential-comparator mode of operation is used.
The low-capacitance, high-impedance input of the Cath­
ode Follower stage isolates the input circuitry from the
amplifier stages. The Input Cathode Followers are required
to handle input signal voltages as great as ± 1 0 0 volts,
without distorting. Special Constant-Current circuits prevent
the cathode followers from cutting off or drawing grid
current with large signals. Screen Voltage Regulator circuits
maintain a constant voltage between the cathode and
screen grid of the cathode followers. The Constant-Current
and Screen-Regulator circuits permit the cathode followers
to handle large signals without operating nonlinearly.
The output of the Input Cathode Followers is applied to
the control grids of the Differential Amplifier stage.
stage also employs a Constant-Current circuit and a screen
Voltage Regulator to permit the stage to handle large signals
without distortion.
The output of the Differential Amplifier stage is applied
to the bases of the transistors in the Output Amplifier stage.
Signals from the Output Amplifier stage are applied
through voltage dividers to the Output Cathode Follower
stage. The voltage dividers provide the proper dc operating
voltages at the grids of the Output Cathode Follower stage.
Output signals from this stage are then applied to the input
of the oscilloscope main vertical amplifier through pins
1 and 3 of the interconnecting plug. O verall gain of the
preamplifier is 2, push-pull.
You may wish to refer to the schematic diagram located
near the rear of the manual and the block diagram of
Fig. 3-1 during the following discussion.
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D E S C R I P T I O N
DETAILED CIRCUIT DESCRIPTION
Comparison Voltage Regulator
Regulation of both the
The
is necessary to maintain specified voltage accuracy.
regulation system makes the comparison voltage independent
of differences in regulated power supply voltages from one
oscilloscope to another.
The comparison voltage is developed across V7689, a
highly stable gas tube, type O G 3. Because the drop across
the tube is less than 100 volts, three zener diodes are placed
in series to increase the reference voltage to slightly more
than 100 volts. In addition to increasing the total voltage
drop in the circuit, the zener diodes provide temperature
compensation for V7689.
A constant-current source of 6 ma is required to maintain
a constant voltage across the gas tube, V7689.
— 100-volt comparison voltage reference the
vided by a constant-current transistor, Q8674, which operates
as a common-base amplifier.
lished with respect to the — 150-volt supply by zener diode
D8679. Since the base-to-emitter voltage of Q8674 is es­
sentially constant, the current through the transistor is de­
termined and maintained constant by the voltage across
resistors R8673 and R8674. By adjusting R8674, the current
through V7689 is set to the specified 6 ma.
Q8672, a diode-operated 2N1102, provides temperature
compensation for Q8674.
This
For the + 100-volt reference, a constant current of 6 ma
is furnished by Q7674 in conjunction with zener diode D7675
and resistors R7670 and R7671. This circuit operates similar
to the — 100-volt reference previously explained.
As the COMPARISON VO LTAG E Polarity switch is moved
from -f position to — position, it is important that the load
on transistors Q8674 and Q7674 remain the same. In the
+ and 0 positions, the current from the negative current
regulator, Q8674, passes through R8670 instead of the gas
tube and associated zener diodes.
current from the positive current regulator, Q7674, passes
through R8670. In calibration, the voltage at Test Point A
is set equal to the voltage at Test Point B in all positions
of the polarity switch, so that a constant load is always pre­
sented to Q7674 and Q8674. The voltage is then applied
across the COMPARISON VO LTAG E potentiometer, R7686.
The voltage applied across potentiometer R7686 must
be exactly 100 volts.
C I R C U I T
and — comparison voltages
6
ma is pro­
The base voltage is estab­
Transistor
In the — position, the
Since the voltage obtained by the
This
For the
3-1