UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

ppppenguin · 6th Sep 2010, 1:08 pm

I think there may be a confusion between hFE(large signal) and hfe(small signal). hFE is relevant to setting the bias point. hfe will determine the input impedance for signals. hfe is strongly frequency dependent when the signals are a significant fraction of the cutoff frequency of the device. The hfe figure is quoted for low frequencies. At fT, hfe is unity by definition. For devices like the OC71 I think that hfe will be starting to fall noticeably in the audio band.

6th Sep 2010, 1:08 pm	#12
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Transistor Choice in radio project I think there may be a confusion between hFE(large signal) and hfe(small signal). hFE is relevant to setting the bias point. hfe will determine the input impedance for signals. hfe is strongly frequency dependent when the signals are a significant fraction of the cutoff frequency of the device. The hfe figure is quoted for low frequencies. At fT, hfe is unity by definition. For devices like the OC71 I think that hfe will be starting to fall noticeably in the audio band.