Cascode amp question - RF Cafe Forums
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Post subject: Cascode amp question Posted: Mon Sep 25, 2006 3:40 pm
Joined: Mon Sep 25, 2006 1:51 pm
Location: Titusville, Fl.
This amplifier is intended to produce fairly high voltage pulses (60V) with fast transitions (<3nS). It comes close (4-5nS) but looks like there is a loss of bandwidth in the cascode stage. When looking at the collector/emitter junction of the two transistors with a properly grounded scope probe one can see very narrow voltage spikes at the transitions of the pulses. I would expect to see only a DC voltage here. To me it looks like the emitter of the top transistor is presenting an inductive load that is causing the voltage spikes when the current changes rapidly. Does that make sense? Connecting the scope probe (8pF//10M ohm) to this junction also increases the output level when the input is a 150MHz square wave. Or when I connect a small cap from this junction to ground. Would some type of matching network at this juction help increase the bandwidth? Thanks for any help.