# How to get hFE value?

Discussion in 'Electronic Basics' started by NoClue, Dec 6, 2006.

1. ### NoClueGuest

Hi,

I made a sample circuit and I want to calculate the value of hFE. How
can I get the value.
I know hFE=Ic/Ib and Ib is (Vcc-Vbe)/Rb which is about 100uA. In order
to get Ic, I have to know hFE and a sample circuit is not the same
condition from datasheet. If I draw this sample circuit to PSPICE, it
give me the Ic so that I can calculte the value of hFE. But how, what,
and who know to get hFE without using PSPICE.

+------+-----+
| | |
| | |
Rb Rc |
| | |
| /C +
+----|B V
\E -
| |
| |
+-----+

Rb is 343k, Rc is 810, Vcc is 35V, and transistor is P2N2222A.

clueless Ken.

2. ### Charles SchulerGuest

Use 200 for that transistor.

3. ### John FieldsGuest

---

Measure the voltage on the collector, subtract that voltage from the
supply voltage and divide that by the collector resistor's
resistance. That will be the collector current. Call it Ic.

Measure the voltage on the base, subtract that voltage from the
supply voltage and divide that by the base resistor's resistance.
That will be the base current. Call it Ib.

Divide Ic by Ib. The quotient will be beta.

4. ### DaveGuest

Extrapolate using the data sheet. Which says, btw, that at I(c)=150mA,
V(ce)=10V, min hfe=100, max hfe=300. Hence the previous poster's suggestion
to use 200.

hfe varies with current, voltage, temperature, etc. Generally highest
around the mid-point of voltage and current ranges, lower towards the ends.

5. ### Phil AllisonGuest

"NoClue"

** Honest at least ....

** You cannot.

Hfe is a variable parameter of *actual devices* - it varies widely from
sample to sample and is also significantly affected by the voltage current
and temperature levels experienced by that device.

What you have drawn is suitable for TESTING the Hfe figure of individual
PN2222 devices - since the base current is fixed and the collector current
can be noted from the voltage across Rc.

........ Phil

6. ### chrisGuest

The simplest and most accurate way to determine the hFE for the
transistor you are using is to simply measure the current flowing
through Rc and Rb. Then divide Rc/Rb.
You could as suggested else-where measure the voltage across the
resistors, however you then open yourself to errors when you remember
the tolerance values applied to the resistors.

Chris

7. ### Phil AllisonGuest

** Top Posting Groper Alert !!

** Better try Ic / Ib.

** With standard 1% resistors and a decent DMM, the result will be within
1% which is likely to be as good or better than using the same DMM as a
current meter.

When matching the Hfe of two or more devices, accurate test figures are
irrelevant.

....... Phil