Wideband Level-Independent Trigger Preamplifier

0 5 WIDEBAND LEVEL-INDEPENDENT TRIGGER PREAMPLIFIER This circuit eliminates the difficulty in re-adjusting the trigger level of an os- cilloscope or frequency meter any time the amplitude of the input signal changes. The block diagram shows that the trigger pulses are supplied by a fast comparator that compares the instan- taneous input signal amplitude with a reference voltage deduced from the dif- ference between the peak amplitude of the positive and negative half cycles of the rectified input signal. The circuit is fast enough to handle input signals with a frequency of up to 100 MHz, and has a sensitivity of 100 mVpp. With reference to the circuit diagram, the input signal is raised in a wideband preamplifier based around a UHF dual- gate MOSFET, Ti, fed by constant cur- rent source Tz. Presets Pi and Pz define the potential at the source of Ti, and hence form the fine and coarse offset compensation adjustments for the direct-coupled chain of opamps IC 1- IC2-IC3. The signal ...