I've been looking some pre-biased dual NPNs to use as buffers. My criteria is to have a very small package (i would prefer their width be less than 3mm. 2mm is very good) and that they ought to have a max. collector current rating of 200mA.(adsbygoogle = window.adsbygoogle || []).push({});

Imayhave found such an NPN in the http://www.nxp.com/documents/data_sheet/PIMN31.pdf" [Broken] but I fear I may not be overdriving it enough. I'll first describe how I'm calculating if the BJT reaches saturation. The max. collector-emitter voltage at saturation for the PIMN31 is 0.3V. Hence, load current is:

My collector current is:

[itex]\frac{Vcc - Vce}{47}=\frac{3.3 - 0.3}{47}=63mA[/itex]

The min. Hfe for the PIMN31 is 70. Therefore, required Ib:

[itex]\frac{Ic}{β}=\frac{63}{70}=0.9mA[/itex]

The PIMN31 has two biasing resistors. R1 is the input resistor connected to the base and R2 is connected between the base and emitter. I convert this to a thevien equivalent circuit:

[itex]Vth = \frac{R2}{R1 + R2} * Vin =\frac{11K}{11K+1.3K}*3=2.68V[/itex]

[itex]Rth = R1||R2=\frac{11K*1.3K}{11K + 1.3K}=1.16K[/itex]

Therefore, actual base current:

[itex]\frac{Vth-Vbe}{Rin}=\frac{2.68-0.6}{1.16K}=1.78mA[/itex]

Now, if my calculations are correct (are they?), the transistor is saturating. But...

Overdrive Factor: 1.78/0.9 = 1.97 ~ 2.

Is this enough? I have read online online that a overdrive factor of 10 is recommended. The issue is, I can't find a transistor with a small enough input resistance and a high enough current rating. If I do, they aren't in a small enough package.

My other option is a http://www.fairchildsemi.com/ds/FD%2FFDG6317NZ.pdf" [Broken]. I have found one in a very small package, but the trouble is I will need external resistors for the gate and a pull down resistance between gate and source.

It's not that I'm hesitant to include these resistances, it's just I need 300 of these buffers on the board. If 2 per transistor are required, that's 600 on the board. Of course, this begs the question: are these resistances really needed?

Some of you will inevitably be wondering what the application is. Please see https://www.physicsforums.com/showthread.php?t=548271" where I provide quite a bit of information.

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# What is a good Overdrive Factor for a BJT?

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