External Trigger Amplifier

The External Trigger Amplifier provides a means of triggering the instrument from an externally supplied signal that can be applied to the EXT INPUT connector. Input coupling to the Amplifier is selectable by the three-position EXT COUPLING switch, S401. In the AC position, the dc component of the external trigger signal is blocked by coupling the signal through C402. In the DC position, all components of the signal are coupled directly to the gate of Q411A through an input divider composed of R404 and R408. Resistors R402 and R403 form a voltage-divider network that attenuates the signal by a factor of 10 whenever S401 is set to DC-MO.

Field-effect transistors Q411A and 0411B are a matched pair. Source-follower 0411A provides a high input impedance for the external trigger signal. Current-source transistor 0411B causes Q411A to operate at zero gate-to-source bias, so the device functions with no dc offset between the input and output signals. The output signal from the source of 0411A drives the base of emitter follower Q414. The emitter-follower stage lowers the output impedance of the Trigger Amplifier and functions as part of the Trigger Source Switching circuitry.

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