View Single Post
Old 8th Mar 2018, 7:41 pm   #8
MrBungle
Dekatron
 
MrBungle's Avatar
 
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
Default Re: Testing unknown transistors for max frequency response

Another dragon:

Watch out for scope probe capacitance. Your average not rubbish probe has a 12pF load capacitance. At more than a few MHz it's going to look like a pretty annoying resistor in the circuit across collector to ground. This will, with the collector load resistor affect the frequency response in a low-pass fashion. It's worth adding a follower on the output with a known good high fT transistor (2n5179 is a good one if you can get it to stop oscillating) to isolate the circuit from the probe. Also keep the probe ground very short - use the little spring clip if you have one.
MrBungle is offline