A External Trigger A M P Lifie R; Auto Trigger Circuit - Tektronix 2215 Instruction Manual

A External Trigger Amplifier
The A External Trigger A m p lifie r provides a means o f
triggering the instrum ent fro m an externally supplied signal
th a t can be applied to the E X T INPUT connector. Input
coupling to the A m p lifie r is selectable by the three-position
A EXT COUPLING switch, S401. In the AC position, the
dc com ponent o f the external trigger signal is blocked by
coupling the signal through C402. In the DC position, all
components o f the signal are coupled d ire ctly to the gate
o f Q411A through an in put divider composed o f R404 and
R408. Resistors R402 and R403 fo rm a voltage-divider
netw ork th a t attenuates the signal by a fa cto r o f 10 when­
ever S401 is set to DC-MO.
Field-effect transistors Q 411A and Q41 IB are a matched
pair. Source-follower Q 411A
impedance fo r the external trigger signal. Current-source
transistor Q411B causes Q 411A to operate at zero gate-to-
source bias, so the device functions w ith no dc offset
between the inp ut and o u tp u t signals. The o u tp u t signal
from the source o f Q411A drives the base o f e m itte r
fo llo w e r Q414. The em itte r-fo llo w e r stage lowers the
o u tp u t impedance o f the Trigger A m p lifie r and functions
as part o f the Trigger Source S witching c irc u itry .

Auto Trigger Circuit

When either A U T O or T V F IE L D triggering is selected,
the A u to Trigger c irc u it detects positive and negative peaks
o f the in p u t trigger signal and produces o u tp u t voltages
th a t set the A TR IG G ER L E V E L co n tro l range to w ith in
the peak-to-peak am plitude o f the triggering signal. The
peak detectors are disabled when S 611 is set to NORM, and
provides a high in p u t
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