Sziklai pair FT ratio?

[dreamth]

I need to replicate a Sziklai pair in a power regulator .The original pair is comprized of a 100mHz driver for a 15 mHz power trz.I only have 5 Mhz, 8 Mhz and 180Mhz power transistors at hand to replace the original.Which one would you choose?

 

[dreamth]

i never knew what makes a sziklai pair to be prone to oscillations so i thought the FT missmatch might be a reason.

 

[U230421]

The main bjt must be the best (high Ft), the 'glued' one is to enhance the transconductance of this bjt, most often of simpler grade (lower Ft). Oscillations can originate from many causes, to simple to point to a Sziklai (or other compound whatsoever). By increasing this transconductance, the risk of oscillations is supressed because of the lower impedance at the emittter of the main bjt. One have to keep a keen eye at the collector - in general a mundain load resistor (say up to 10k) will do no harm.

 

[dreamth]

You mean the driver ,the lower power one as the main transistor?

 

[bogdan_borko]

Driver higher Ft, power transistor lower Ft, so 5 or 8MHz one you have should be fine.

 

[dreamth]

Thank you!