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Old 21st Apr 2021, 9:16 am   #27
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,965
Default Re: Buffer Amplifier for Digital Frequency Counter

My guess would be Miller capacitance from the first 2N2222 as the output 2N2222 has limited gain due to the -ve feedback. You can prove this by lifting the emitter decoupling capacitor, C4. This will reduce the gain to negligible but would allow you to confirm if this is the cause of the roll-off.
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