CIRCUIT DESCRIPTION
SECTION 4
Type 1A1
Change information, if any, affecting this section can be found at the
rear of the manual.
AMPLIFIERS
Introduction
The Type 1A1 Dual-Trace Plug-In Unit consists of two
switched-amplifier channels and an output amplifier. Chan
nel 1 is identical to Channel 2 except for additional stages
which provide signal and trigger outputs for Channel 1.
Therefore, only Channel 1 is described in the following de
scription.
NOTE
Voltages and currents given in the circuit descrip
tion are approxim ate. Throughout the circuit de
scription discussion, refer to the block and circuit
diagrams located in Section 10.
Input Coupling
The signal to be displayed is applied to Input Source
Follower Q122 via INPUT SELECTOR switch SW101 and
VOLTS/CM switch SW105. In the DC position of the IN
PUT SELECTOR switch, input coupling capacitor C l02 is by
passed so the input is DC coupled. In the AC position,
the signal must pass through Cl 02, which blocks the DC
component. Capacitor C l02 limits the low-frequency re
sponse to less than 2 Hz at —3 dB. In the GND position,
the signal path is open and the input circuit of the channel
is grounded.
Input Attenuation
VOLTS/CM switch SW105 and SW129 is a 12-position
two-section rotary switch. The first section (SW105), con
taining attenuator networks, is electrically connected in the
gate circuit of Input Source Follower Q122; the second sec
tion (SW129), containing emitter resistors, controls the gain
of Input Amplifier Q124/Q144. A special mechanical cou
pling between the two sections holds the first section
(SW105) stationary while the second section (SW129) ro
tates through the first four positions (.005, .01, .02 and .05).
Then, the mechanical coupling transfers the switch drive
from the second section of the switch to the first section.
As a result, the second section will remain stationary at the
.05 position while the first section rotates through its posi
tions.
In the first four positions of the VOLTS/CM switch, the
signal is coupled straight through the first section of the
switch without attenuation to the Input Source Follower
Q122. When the signal arrives at Input Amplifier Q124/
Q144, emitter resistors inserted by the second section of the
switch set the gain of the stage and hence the amount that
the signal is amplified.
In the remaining positions of the VOLTS/CM switch (.1
through 20) individual attenuator networks are switched into
the gate circuit of Input Source Follower Q122 so the sig
nal applied to the gate is always 0.05 volt for each centi
meter of CRT deflection, providing the VARIABLE VOLTS/
CM control is set to the CALIB position and the gain of the
Type 1A1 and associated oscilloscope is set properly.
The attenuator networks are frequency compensated RC
voltage dividers. Their attenuation factor can generally be
expressed as follows:
Attenuation
_ total divider resistance (including R116)
Factor
Ground-leg resistances (including R116)
Using the X 2 attenuator as a specific example (see Fig. 4-1),
the formula is:
Attenuation Factor =
(R105C) (R116) + (R105C) (R105E) + (R105E) (R116) _ n
(R105E) (116)
At low frequencies the dividers are resistive because the
impedance of the capacitors is high and their effect in the
circuit is negligible. As the frequency of the input signal
increases, however, the impedance of the capacitors de
creases and their effect in the circuit becomes more pro
nounced.
For high-frequency signals, the impedance of the capaci
tors is low in comparison to the resistance of the circuit and
the attenuators become capacitive voltage dividers. For
these frequencies, the attenuation factor is similar to the
resistance case, except that the capacitive reactances are
the dominant factors involved. A variable capacitor in each
attenuator, such as C105C in the X 2 attenuator (see Fig.
4-1), provides a method for adjusting the capacitive reac
tance ratios equal to the resistance ratios.
The variable capacitor at the input to each attenuator
(see Fig. 4-1), provides a means for adjusting the input RC
of the attenuator to an arbitrary standard value of
15 pF x 1 MO when using a 15 pF input time constant normal-
izer as a reference. Similarly, C l04 provides a method
for normalizing the input time constant when the VOLTS/
CM switch is set to any of the input straight-thru positions.
In addition to providing the same input capacitance, the
resistance values of the attenuators are chosen to provide
an input resistance of 1 MQ for each setting of the VOLTS/
CM switch. Thus, an attenuator probe, when connected
to the input connector of the Type 1A1, will work into the
same time constant to eliminate the need for readjusting
the probe capacitance compensation for different VOLTS/
CM switch settings.
© 2
4-1