# Maximum Current Gain Possible for a 100 mA PNP transistor

I have a pulse charging circuit where the maximum current is limited by the hFE of the PNP transistor (DTA144EE). The typical hFE at room temperature is ~220 as shown in the plot below. There is of course no spec on the max Hfe at room or any other temperature.

My calculations show that the circuit will correctly limit the current to 100 mA due to the current gain as long as the gain is not more than 500.

Is > 500 a physically realizable gain value at 100 mA for a low power discrete PNP transistor, or am I safe to assume that the gain will never be more than 500? Note that I am operating in fast pulse mode, so 100 mA is in the safe operating area for the ~100 ms charge time for the circuit.

Note that PNPs with 500 gain exist, but I could not find one near that at 100 mA of collector current. The high gain was in the 1 mA range.

Also, doesn't gain typically go down at higher currents?

• PNP's suffer a carrier mobility problem. I know that some folks, two decades ago or so, were working on ideas that would enhance hFE while at the same time improve their high frequency performance. These were based on some ideas then called SALTran. But I think these were low current devices, too. I don't know what your circuit issues are and this may be a very dumb question because I'm pretty sure you already have considered it and set it aside. But what does a Darlington or Sziklai configuration fail to do for you? (Just want to dot that i, cross that t, so to speak.)
– jonk
May 28, 2021 at 5:24
• where the maximum current is limited by the hFE of the PNP Show us that circuit. In my experience, designing circuits that rely on the $h_{FE}$ of a BJT is simply asking for trouble unless you know and realize that $h_{FE}$ varies "all over the place" and that that's OK. Also, doesn't gain typically go down at higher currents? Yes it does. And it varies over temperature. I would not be surprised if the phase of the moon also has some influence ;-) Now you know why I never rely on the $h_{FE}$ of a BJT. May 28, 2021 at 6:59
• I suspect that a selected BC327-400 may come close. I'd need to check. Usually having a PNP with beta under 500 at 100 mA would be expected. May 28, 2021 at 9:23
• BC327-40 beta 250-600 at 100 mA. digchip.com/datasheets/parts/datasheet/456/BC327-40.php May 28, 2021 at 9:28