BJT or Darlington Tranisistors for Cap Multiplier?

[chozo4]

I have been working things out over time and then feeling stupid one way or another and even posting my lack of knowledge only to look back and realize 'wow, I sound like an idiot'. However, I have been looking over literally every transistor mouser has offered for my needs as they sparingly list any useful gain specs outside the datasheet so had to check through several hundred datasheets last night. Looking for high gain as the C|B biasing will be done via a 150-250ohm resistor with a 12v vCE and want to avoid too much voltage drop with higher current requirements up to 1A as worst cases scenario.

There is one question that kind of bugs me overall...
How exactly would one read the DC gain section of a transistor datasheet? After scouring google and trying to take in as much as I can I'm still quite confused as there aren't any simple or straight forward answers outside being used for audio amplification purposes only.

For example: Under the BSP52T1G datasheet it states the following:
Vce=10v, Ic = 150ma (1000gain min)
Vce=10v, Ic = 500ma (2000gain min)

While it states input voltage and input current - how does this in any sense pertain to affecting the gain when the input voltage can be much higher? Are these simply stating a minimum voltage and minimum current needed for said gain values or a maximum before performance is affected?

Thus far I've been paying attention only to the Ic taking it as a 'maximum current' for that gain and ignoring all but the minimum guaranteed gain value. Am I right in doing so in this case for general needs?


So far after all the searching the BSP52T1G is no longer a viable candidate (despite using it as part of the example) however due to the gain amount and found many close alternatives to my needs in a BJT package rather than Darlington with far far lower saturation values. The smallest I can use is an SOT-89 package for the space needed as well, an SOT-223 is just about pushing the limits to almost too large but can still use for other purposes in an eventually planned power filter for multiple devices. I still selected at least one Darlington for toying with however: The options are as follows:

- ROHM - 2SD2537T100V - BJT NPN 25V 1.2A @ 860~1800gain
- ROHM - 2SD2153T100U - BJT NPN 25V 2A @ 860~1800gain
- DiodesInc - FZT600BTA - Darlington NPN 140V 3A @ 5000~10000gain

Are the options provided through ROHM semiconductor viable or are there better alternatives out there that I'd have missed? Thus far results have been very satisfactory with a TIP120 Darlington in a TO-220 package but it's far too large and the voltage drop of 1.5v is undesirable. The voltage only fluctuates at most 0.3v however under load due to the bias and gain amount.

[redwire]

Re: BJT or Darlington Tranisistors for Cap Multiplier?

A capacitance multiplier (for a ripple-filter?) application runs the transistor in a linear mode, not saturated. It will have a large (several volts) voltage drop and dissipate heat, like a linear voltage regulator.
Using a single high beta transistor vs a Darlington lowers your (best) losses by ~1V but they still need a few volts minimum. Your 1.5V loss is as good as it gets and I think your enemy will be heat for the small SMD part.

Some design guidelines here: http://sound.westhost.com/project15.htm

I design using the datasheet lowest beta value, which covers temperature and lot variations.
In a capacitance multiplier, note this transistor fails when the power supply has a step load applied and the cap dumps too much current into the E-B junction. So you need that extra resistor to limit base current between the cap and the transistor.

[budm]

Re: BJT or Darlington Tranisistors for Cap Multiplier?

https://www.badcaps.net/forum/showpo...2&postcount=33

[PeteS in CA]

Re: BJT or Darlington Tranisistors for Cap Multiplier?

Well, if a low V(CE Sat) is undesirable, don't use a Darlington. If your circuit current is going to be ~1A, use a BJT rated for a much higher current (e.g. an MJE13009). As for device beta, also pay attention to the graph in the datasheet that shows beta vs. collector current. That device with a nice beta of over 100 may have a beta of 5 or 10 when operated near its max rated current.