Hfe = forward Beta?

[Paul Burridge]

Hi gang,

I'm puzzled by the Fairchild data sheet for the 2N3904 transistor
(BJT). It states that Hfe is no better than 300 maximum under any
conditions, yet in the Spice model they supply on the same sheet is
shown the parameter for forward beta as being 416.4!
Two questions arise:

I thought FB and Hfe were essentially the same. Is that not the case?

and...

Why give such a *precise* figure for forward beta when it's such a
notoriously variable quantity not just dependent on circuit conditions
but varying considerably between devices within the same production
batch??

If anyone can shed any light on this seemingly impenetrable conundrum,
then I'll be mightily impressed.
p.

 

[Fred Bloggs]

Hi gang,

I'm puzzled by the Fairchild data sheet for the 2N3904 transistor
(BJT). It states that Hfe is no better than 300 maximum under any
conditions, yet in the Spice model they supply on the same sheet is
shown the parameter for forward beta as being 416.4!
Two questions arise:

I thought FB and Hfe were essentially the same. Is that not the case?

and...

Why give such a *precise* figure for forward beta when it's such a
notoriously variable quantity not just dependent on circuit conditions
but varying considerably between devices within the same production
batch??

If anyone can shed any light on this seemingly impenetrable conundrum,
then I'll be mightily impressed.
p.

The SPICE forward beta parameter pertains to the basic static Ebers-Moll
model of the intrinsic transistor. The actual device consists of this
intrinsic part and extrinsic parasitics resulting from the necessities
of the semiconductor engineering. The intrinsic model combined with
other SPICE parameters serves to model the end result with fairly good
accuracy.

 

[Jim Thompson]

Hi gang,

I'm puzzled by the Fairchild data sheet for the 2N3904 transistor
(BJT). It states that Hfe is no better than 300 maximum under any
conditions, yet in the Spice model they supply on the same sheet is
shown the parameter for forward beta as being 416.4!
Two questions arise:

I thought FB and Hfe were essentially the same. Is that not the case?

and...

Why give such a *precise* figure for forward beta when it's such a
notoriously variable quantity not just dependent on circuit conditions
but varying considerably between devices within the same production
batch??

If anyone can shed any light on this seemingly impenetrable conundrum,
then I'll be mightily impressed.
p.

Run a simulation sweeping IB for a fixed VCE, then display
IC(Q)/IB(Q).

Then comment out every parameter except IS and BF and re-run the
simulation.

Note the difference ;-)

[The main culprits are IKF (current crowding at the emitter edge) and
ISE (leaky base region).]

...Jim Thompson

 

[Scott Stephens]

I thought FB and Hfe were essentially the same. Is that not the case?

Nope, Hfe is a hybrid parameter of a 2-port network - H21=I2/I1
Beta is *delta*Ic/*delta*Ib

So you should see that it depends on the operating point (current bias)
of the transistor. And beta is for common emitter configurations, where
a hybrid parameter is for any ol network.

Why give such a *precise* figure for forward beta when it's such a
notoriously variable quantity not just dependent on circuit conditions
but varying considerably between devices within the same production
batch??

Convenience for techs not encumbered by the nuances of the Ebers-Moll
model? A crude figure of merit to contrast devices?


--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

 

[John Woodgate]

I read in sci.electronics.design that Scott Stephens

Nope, Hfe is a hybrid parameter of a 2-port network - H21=I2/I1
Beta is *delta*Ic/*delta*Ib

So you should see that it depends on the operating point (current bias)
of the transistor. And beta is for common emitter configurations, where
a hybrid parameter is for any ol network.

This is bit hair-splitting. The symbols are a bit off-beam; FB could be
mistaken for f-beta, which is something entirely different. There are
four h-parameter series - hfe is the *small signal* common-emitter
forward current gain, and hFE is the large signal value. These 'signals'
are a.c., and there is a corresponding pair of d.c. values, Hfe (at
small Ic) and HFE (at large Ic) (which are rarely quoted).

All of the h-parameters AND FB depend on the operating point.

h21 is the small signal forward current gain of ANY 2-port. hfe isn't.