BC109 HFE

[mcentee2]

Hi - long time lurker, first post, so be gentle

I've read a lot abot how the hfe of a transistor may be important to know (depends on usage), so bought a multimeter (Uni-Trend UT132A) to do just that - tell me the hfe of a set of BC109C's I have.

Note: These are Silicon, not germanium hence the reliance on my MM and not any constructed test circuit re leakage etc.

I was hoping they would be >500 or so to fit into a new build that are recommended for it - previous hobby builds have never really specified anything so have just built them, but this one does.

On the MM these only measured 260ish - hmm I thought, something's fishy here.

Inspecting the MM manual told me the hfe reading was based on "10ua and Vce 2.5v" (didn't say if DC or AC, but am assuming it was DC).

Inspecting generic BC10x factsheets shows there are 2 or 3 different sets of hfe figures for B109C:

1) DC at Ic=2ma/Vce=5v, hfe=520),
2) DC at Ic=10uA / Vce=5v, hfe=270
3) Small Signal Current version(AC?) at Ic=2mA / Vce= 5v, hfe=550

So it looks like my multimeter is measuring the 2) figure above.


My questions therefore are:

1) Am I correct in that a lot of the "talk" on the internet forums with regard to hfe where it mentions "500 upwards" is related to the Small Signal AC current hfe and not the other two ? Are a lot of people discussing or mentioning the "wrong" figures ?

2) if that is the case - is there a way to either extrapolate the figure I do get from my multimeter to guage what the Small Signal Current hfe is ?

3) If 2) is a no-go - is there another "quick" way to assess a transistor's hfe without a constructed test circuit ?

4) is my MM any use now for measuring the correct hfe required ? and do lots of other DIY pedal makers have the same problems ?

Why isn't there many references to the above on the internet forums ? ...is it just me ?


Many thanks

[DrNomis]

Different makes and models of Digital Multimeters use different currents, and methods, to measure Transistor Hfes,it really depends on the battery used to power the Multimeter (most tend to use a 9V battery), some Semiconductor Analysers use a 12V battery, you could try consulting your Multimeter's user manual...


Most of the time the Hfe of a Transistor refers to it's DC current gain, the AC current gain is what you're probably more interested in if the Transistor is going to be used in some kind of Audio Amplifier circuit, the figures quoted for a Transistor's AC current gain is what the Transistor is theoretically capable of, what current gain you actually get out of a given transistor that's used in a circuit is determined by the circuit itself, and the range of frequencies being amplified, a given Transistor also has a specification listed as Ft, this is the frequency where the AC current gain of a Transistor falls to 1 due to an effect called "Parasitic Capacitance"...

Parasitic Capacitance is an unfortunate fact of life when Transistors are manufactured, it can be reduced but not avoided...

Incidentally, most of the BC109C transistors I've ever tested in my life have usually had an Hfe that measured around 480, to be honest, you should just try using the Transistors in your build, providing the Transistors are working, and see what results you get....


Hope that helps.....

[mcentee2]

Thanks - more surfing last night showed that most of the test circuits are DC based, with a 9v battery small resistors (xk and xM ranges) leading to micro-amp currents.

So this matches the DMM setup at "10ua and Vce 2.5v" - spot on values of 250-260 vs the pdf.

So I can't believe that there are B109C's out there showing DMM readings of >550, as that sounds ridiculously off the spec limit!

If the test rig seems to match what the DMM is doing - how on earth do people measure these things to get >550 ?

So am still wondering if i am looking at "apples" and "Pears" in different places.

Eventually (time permitting) I'll get a test rig together and see what comes out!

[DrNomis]

Thanks - more surfing last night showed that most of the test circuits are DC based, with a 9v battery small resistors (xk and xM ranges) leading to micro-amp currents.

So this matches the DMM setup at "10ua and Vce 2.5v" - spot on values of 250-260 vs the pdf.

So I can't believe that there are B109C's out there showing DMM readings of >550, as that sounds ridiculously off the spec limit!

If the test rig seems to match what the DMM is doing - how on earth do people measure these things to get >550 ?

So am still wondering if i am looking at "apples" and "Pears" in different places.

Eventually (time permitting) I'll get a test rig together and see what comes out!

I think the reason why a figure of >550 for Hfe was quoted was just that the Transistor they were testing just happened to have an Hfe of >550(it was probably a BC109C, which is speced as a High-Gain Low-Noise NPN Small Signal Si Transistor) , the Hfe of a given batch of BC109 Transistors will vary a bit from device to device, if you look up BC109 in a Semiconductor specs book you'll see that it will quote something like Hfe = 300 to 550, or it may list the minimum and maximum Hfe values for each Transistor type, so what that means is that for a given Transistor it's Hfe will measure anywhere between those quoted minimum and maximum values, I've come across some BC549C transistors that measured as high as Hfe 820, the BC549C is essentially the same as a BC109C except it's encapsulated in Black plastic.....

[mcentee2]

Hi DrNomis -indeed, the bits I quote re the hfe (HFE) values are from the pdf factsheets for BC109C's hence my confusion about what is being measured.

In the RG Keen article, and my DMM, and they do both seem to be measuring DC at uA ranges, which equate to 260-270 in the pdf.

So where are the >500 numbers coming from in forums etc - who is testing them under what conditions - as I can't see that the RG Keen layout can do AC Small signal measuring.

Are people measuring based on DC 2mA (not uA) - and getting numbers >500 /If so, that isn't the RG Keen method.....

I just want to be able to measure something - and know what it is I have measured

[tonyharker]

Testing a number of my old BC109Cs on my DCA 55 show gains in the 500s with a test collector current of 2.5mA.

[mcentee2]

Testing a number of my old BC109Cs on my DCA 55 show gains in the 500s with a test collector current of 2.5mA.

Ah - ok, think I am getting somewhere - I need to be able to test at "mA" levels, and not "uA" levels to get those >500 figures, and my DMM tests at 10uA!

Not sure - but for pedal circuit designs are we then looking at the base bias currents being in the mA regions rather than uA ? (which the AC signal then fluctuates about).

Anyone know how to build a mA tester ? (Is the RG Keen circuit still good to use but change the resistor values down to 3600K and 360R to get in the 2.5 mA base current range ?

[DrNomis]

Testing a number of my old BC109Cs on my DCA 55 show gains in the 500s with a test collector current of 2.5mA.

Ah - ok, think I am getting somewhere - I need to be able to test at "mA" levels, and not "uA" levels to get those >500 figures, and my DMM tests at 10uA!

Not sure - but for pedal circuit designs are we then looking at the base bias currents being in the mA regions rather than uA ? (which the AC signal then fluctuates about).

Anyone know how to build a mA tester ? (Is the RG Keen circuit still good to use but change the resistor values down to 3600K and 360R to get in the 2.5 mA base current range ?

Generally, the current flowing into the Base of a Transistor (assuming it is biased correctly for minimum distortion when the Transistor is being used in an amplifier circuit) is in the microamps, on the other hand the current flowing through the collector and emitter is in the milliamps (for a simple one Transistor amplifier a Collector/Emitter current flow of about 10 milliamps max is typical when the Transistor is fully-on or saturated).


Mathematically, the current-gain of a Transistor is defined by the following formula:

Ic=Beta X Ib


Where:

Ic equals the Collector current.

Beta equals the Current gain.

Ib equals the Base current.


So, say hypothetically we know that 1 microamp is flowing through the Base of the Transistor, and we know that 1 milliamp is flowing through the Collector and the Emitter of the Transistor, we can therefore determine that the current gain of the Transistor is 1mA divided by 1uA, or 1000, in other words, the Collector current is 1000 times larger than the Base current.


When a Digital Multimeter is measuring the Hfe of a transistor it is actually measuring the Collector/Emitter current and comparing it to the Base current (which is usually a fixed amount).....

I think R.G. Keen uses a figure of 1uA Base current (one microamp) for his Transistor current-gain measuring circuit article....

[grrrunge]

Testing with base currents in the mA range makes no sense for pedal building IMO.
A BC109C will have a ß ranging from 450 to 900 according to SE's datasheet. In the extreme case of ß=900, a base current of 2.5mA would result in a collector current of 2.25A
That's a bit much current to draw from a standard 9V battery if you ask me

[mcentee2]

I think R.G. Keen uses a figure of 1uA Base current (one microamp) for his Transistor current-gain measuring circuit article....

Ok, sort of following so far....so if I test my BC109C with RG Keen's setup - based on 1uA Base current (2.2MO and and 2.47KO Res), then what is the corresponding hFE figure in the factsheet that I compare this with to see if I have high/low numbers ?

At some point at the end of this, I need to understand if the R G Keen figure results are "in spec" (whether high or low or whatever) and also give me a "gain figure" I can use to compare against the >500 requirement....

[mcentee2]

OK, couple of clarifications.

Yes, the fact sheet PDFs show it is Ic in mA not the base current, my bad.

Also, the RG Keen article works at around 4uA base current, not 1uA from what is written there I think.

Done lots of testing this afternoon and managed to mangle 2 of my Bc109's already somehow with no clear set of results yet.

[grrrunge]

Could we see what it is You're building, that needs a specific ß to work? It's a lot easier for us to help you understand what it is you need to understand that way

For small-signal transistors it should be safe to assume that the ß you measure in a DC situation is steady throughout the audible spectrum, as their parasitic capacitances are usually very small.

You should also consider that it's generally considered bad practice to design circuits, that are largely dependent on specific variables, that vary a lot during component production - transistor ß being one of them.
For that reason they spend quite a lot of time in the analog electronics classes here at DTU, teaching methods to design circuits that are as independent as possible of such factors as possible.

[mcentee2]

Hi grrrunge, its is the guitarpcb DSOTM fuzz based on the Lunar Module.

I noted in one of the threads when theLM was traced, regarding the HFE of the BC109C required.


In the past hour I just set up an RG Keen test circuit ( not exact same R's but close to work through the maths Rb 2.047MO Rc2.49kO, 9v batt).

Turns out I have my remaining BC109C's turn in figures of:

603
534
531
522

So pretty close to 550 to give me hope!

Or should I buy and test some more?

[grrrunge]

Any of those will work fine

[DrNomis]

Hi grrrunge, its is the guitarpcb DSOTM fuzz based on the Lunar Module.

I noted in one of the threads when theLM was traced, regarding the HFE of the BC109C required.


In the past hour I just set up an RG Keen test circuit ( not exact same R's but close to work through the maths Rb 2.047MO Rc2.49kO, 9v batt).

Turns out I have my remaining BC109C's turn in figures of:

603
534
531
522

So pretty close to 550 to give me hope!

Or should I buy and test some more?

Those Transistors should all work fine, in practice the Hfe of a Transistor isn't as critical to circuit operation as you would think.....


Put it this way, the Lunar Module isn't going to suddenly refuse to work properly just because there's only a difference of about 30 or so between the Hfes you've listed and the specified Hfe value of 550, provided that you've got the Transistors connected up correctly and you haven't made any mistakes, it should work fine.....

[mcentee2]

Many many thanks guys.

One last thought - is the highest hFE tranny best placed in one of the the positions?

My guess would be lower ones in Q1 and Q2 with the larger at Q3.

I will socket and see anyway.

[grrrunge]

Those Transistors should all work fine, in practice the Hfe of a Transistor isn't as critical to circuit operation as you would think.....

Exactly. With those numbers in this circuit i can't see it should make any other difference than a slight deviation in DC operating points and bias currents. Nothing crucial for small signal operation.
To be honest I'm not sure i would even bother testing my transistors for ß before sticking them in this one.
At the moment I'm doing a lot of calculations on how to optimize and linearize class AB BJT power stages. Suddenly it matters a lot if ß is 20 or 30 Power BJT's are a completely different bitch to handle. Totally similar in function, but it just takes much more consideration... That's another story though.

[DrNomis]

Many many thanks guys.

One last thought - is the highest hFE tranny best placed in one of the the positions?

My guess would be lower ones in Q1 and Q2 with the larger at Q3.

I will socket and see anyway.

You could try that and see what results you get......

[grrrunge]

One last thought - is the highest hFE tranny best placed in one of the the positions?

If you had to go fence-pissing about high input impedance and stuff like that, the input resistance of the first stage is slightly dependent on ß.
Higher ß will yield a marginally higher input resistance but it's dominated heavily by the bias network, so i wouldn't bother
Socketing and testing sounds like a great solution, but i doubt you will hear miraculous differences with such similar transistors. Probably more so with different transistors

[DrNomis]

Bottom line is just try it and see....