The Blomley Class B amplifier


2010-11-18 9:12 am
Here's not the case as TR2 act on the 2n3904/06
bases before TR3-Tr6 conveyor act on the emitters so the speed of switching is determined by the Tr3-Tr6 delay mainly and they are clearly slower than 2n3904/06. That sequence is deliberate to forcefully discharge the base of the trz that needs to shut off, the circuit doesn't rely only on the underbias for that matter .As with the capacitances involved Low cob trz are usually low on beta too because they are mainly destined for common base common base connection miller's capacitance is just a fraction of cob as the voltage gain is below 1X so cob or rather ccb in this case is irrelevant .
I remember that the
UHF-VHF amplifiers I was building in childhood to catch the russian's channels barely had 10db gain per 1 trz stage made with some BFR90...91 trz used in common emitter connection so its CCE would be important while multiplied with the stage voltage gain which was like 1/5 of the dc current gain thus 10 db gain was kinda big deal at 800Mhz .
BF422/423 would be a more natural choice than BFR trz which I'm not sure how stable can be in audio while bf422/3 definitely work in audio, but I'm pretty sure VHF trz can easily be toast due to very low CE max voltages common to these types of transistors.Unless you're going to use high power RF trz you won't get big VCE ranges...and big power trz are usually used in common base connection or cascoded "two on a die" to reduce the influence of their inherently higher cob ...
I modelled this amplifier using 2N3904 and 2N3906 transistors. In the original Blomley design the pairing was 2N3904 and 2N3905. There are some subtle differences in the specifications for the latter two devices. 2N3905 has lower minimum hFE than 2N3906 and lower fT. T on is the same for both but T off for 2N3905 is less than for 2N3906. If it takes marginally longer for this to turn of it will take marginally longer for it to turn on again. One might create a .model for 2N3905 and substitute the difference using 2N3906 as a template for the common performance parameters and run a new simulation, however Mouser have listed a 2N3905 made by Central Semiconductor that could be worth trying.

Since the choice has to align with 2N3904 which has a low hFE the datasheet for this would need to be checked to see which is the best match out of 2N3905 and 2N3906 - see attached.


  • 2N3905.PDF
    384.2 KB · Views: 17