Position Indicator; Trigger Amplifier - Tektronix 2A61 Instruction Manual

Circuit Description—Type 2A61
appear across R424 and R524, so the mean plate-to-cathode
voltage is constant.
The gain of the bootstrap amplifer Q504 and Q514 is
determined in part by the voltage divider action of the
impedance of D516 and the load resistor R431. The loss of
gain due to the divider action may be offset by increasing
the gain of the basic amplifier. This gain adjustment, R502,
sets the overall gain of the bootstrap amplifier at 1.
For high frequencies, the Zener impedance and compensat-
ing gain adjustment are bypassed by C516, ensuring unity
gain.
The voltage changes at the terminals of C437 are identical
so the capacitor C437 and resistor R440 float with the volt-
age changes.
In differential operation C437 acts as a short circuit to the
signal allowing signal amplification to be determined by the
value of the degeneration resistor, R440.
FILTERS
Frequency Response Filter
Two sections of filtering are provided to attenuate
unwanted signals.
C443, C543, A through E, together with R450 and R550
comprise the FROM section of the Frequency Response Filter
and set the low frequency response.
C445, A through D, together with R435 and R535 comprise
the TO section of the Frequency Response Filter and set the
high frequency response.
Power Line Frequency Filter
The power line frequency filter, consisting of R452, C452,
R453, C453, R552, C552, R553 and C553 may be switched
into the circuit with the FILTER switch on the front panel.
Signals arrive at each output terminal via two paths; the
direct path, R452-C452 (R552-C552}, and the crossover path,
R553-C553 (R453-C453) which is driven from a source 180°
out of phase with the direct path source. These signals tend
to cancel and at one frequency the cancellation can be made
almost complete. Maximum rejection is at 60?Hz with
attenuation greater than 50:1.
OUTPUT AMPLIFIER
Trace Restoration
The high system sensitivity may cause large transients
resulting from swich operation or input signals to drive the
trace off the CRT. The large time constant in the low fre-
quency filter will then cause a long time delay before the
trace returns. Depressing the push button switch SW450, con-
centric with the POSITION control, will bleed off the capaci-
tor charge and quickly return the trace to the CRT.
First Section
The first section of the Output Amplifier is a two stage
feedback amplifier consisting of V464, Q474 and Q574. Its
2Also available at 50 Hz and 400 Hz.
3-2
gain is set by R476, R576 and switched resistor R470 which
controls the gain of the Type 2A61 from .01 millivolts/div
through .2 millivolts/div. The .01 millivolt GAIN control
R579, in parallel with R470, compensates for "'less than
infinite" loop gain in the amplifier and acts as a gain track-
ing adjustment for R470.
The triode section of V484 and its shunt resistor R459
form a simple series voltage regulator which supplies the
first section of the output amplifier.
The output signal of the first section may be attenuated by
the two section VARIABLE control R482A and B which is used
in a balanced voltage divider connection.
The DC BAL adjustment, R465, compensates for differences
in the grid bias of V464A and B, and ensures that no trace
shift occurs when R470 is switched or when the VARIABLE
control is operated.
Second Section
Output from the Variable Gain control is fed to the grids
of V484A and V584A, a cathode coupled plate loaded con-
figuration.
The cathodes of V484A and V584A are coupled together
through R585 and Gain Adjust control R485, a cathode
degeneration control.
R485 sets the overall gain of the Type 2A61, and is adjusted
to match the Type 2A61 output to the deflection factor of the
CRT in the 560-series Indicator Oscilloscopes.
The cathode current of V484A and V584A is through R489
and the POSITION control, R488. R488 can vary the quies-
cent current of V484A and V584A to position the trace ver-
tically on the CRT.
V484A and V584A are coupled through pins 17 and 21 of
the interconnecting socket to the CRT vertical deflection
plates. L484 and L584 are shunt peaking inductances in the
plate circuits of V484A and V584A. Ferrite cores L483 and
L583 in the plate leads of the output tubes are parasitic
suppressors.
Trace Position Indicator
The DC balance of the output stage is sensed by Q494.
The cathode with the higher voltage drives Q494 through
either D491 or D591. The emitter of Q494 is returned to a
divider which stays at the average voltage of both cathodes.
When the trace is beyond the position control range, the
DC unbalance is sufficient to drive Q494 and light the neon
lamp, B494.
Trigger Amplifier