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Hi All,

I have a question about reading the datasheet of the 2N3903 BJT. It tells me several hFE values:

hFE VCE=1.0V, IC=0.1mA 20 - 40 - hFE VCE=1.0V, IC=1.0mA 35 - 70 - hFE VCE=1.0V, IC=10mA 50 150 100 300 hFE VCE=1.0V, IC=50mA 30 - 60 - hFE VCE=1.0V, IC=100mA 15 - 30 -

Does this mean that for a collector current of .1ma AND a VCE of 1V the hFE is going to be between 20 and 40? and does it have to meet both parameters (that is VCE=1 and IC=.1ma)?

Question #2

What is the maximum current that an opamp can source and sink?

Thanks,

Q1.

Transistor parameter measurement is a very tricky business.

Ideally hFE would be constant but in practice it varies from device to device and for a given device it varies with Vce. By making base and collector current measurements with a fixed Vce the variability due to Vce is removed. A Vce = 1V is often chosen because it is higher than the maximum Vbe so that the bc junction is never forward biased and so no bc current flow gets involved and obscures the desired base and collector currents.

Most device parameters are also highly temperature sensitive. The use of a low Vce reduces the effect of self heating due to the device power dissipation. Some parameters are pulse tested using pulses rather than DC for the same reason; to reduce average power dissipation and hence self heating.

"Does this mean that for a collector current of .1ma AND a VCE of 1V the hFE is going to be between 20 and 40?"

Yes but only if you measure it the same way the manufacturer did to get the value they quote in their datasheet.

" ... and does it have to meet both parameters (that is VCE=1 and IC=.1ma)?"

Not quite sure what you mean but see above!

Have a read of:

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/intro/electron.htm

In particular:

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/page1.html

and:

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/bpcur.html

Q2.

There is no single answer to the question:

"What is the maximum current that an opamp can source and sink?"

Simple answer: the short circuit current as given in the device datasheet.

Longer answer:

It depends on the device. It also depends on what voltage you want out of the device.

You need to look at the graphs for Vout vs Iout for output sourcing current (i.e. an output driving current into a load. e.g. an opamp with +/-15V supplies and positive output voltage connected to a grounded load) and for output sinking current (i.e. an output pulling current from a load. e.g. an opamp with +/-15V supplies and negative output voltage connected to a grounded load).

Beware CL opamp models: they do not model Vout vs Iout accurately:

This may help with some opamp background:

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/first11/part10/page1.html

and:

http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part8/Page1.html

Clarification:

I've checked data sheets from Fairchild, Philips, and ST, all consistently rate a beta range of 100 to 300 at 10mA, and only list minimum hfe at other currents. But the min hfe at 0.1 mA is 40 (Fairchild) or 60 (ST, Philips). Don't know who would sell standard product that performed so poorly as beta=20 at low currents (reflects a very poor bipolar process!)