Transistor testing using GEOFEX method - NPN?

[phector2004]

Hey everyone,

I recently ordered some NOS Germanium transistors from a Russian guy on ebay, and while they came with detailed Russian instructions that I manage to decipher, I decided to test them out for leakage (tried building a Rangemaster with trimpots... no success!)

I remembered seeing a transistor tester on RG's 'Technology of the Fuzz Face' and built one to the best of my abilities, but I think I'm missing something...

a) FF transistor tester is designed for PNP, mine are NPN. Do I need to make any modifications to the tester? Tried swapping E with C and the voltages started going all over the place...
b) Don't have too many resistors on hand, so I found a 2.2k and a 220Ω and put them in series, giving me... 2.37KΩ     Are there any correction factors I can use? Any relevant formulas?
c) I got 8.18V testing the transistors. Russian datasheet states hFe is in the 60-150 range, so I'm assuming its either extremely leaky or I'm doing something wrong.
d) when I close my makeshift stripped wire SPST to test total gain, resistor voltage drops to ~8.15-8.17V
e) other two tested showed similar results

I'm scratching my head right now, wondering if its just me or the few transistor's I've tested happened to be complete duds...

I'll try again with others tomorrow, but until then, can anybody shed light on this? I have some others I'd like to test, hopefully I can sort this out.

Thanks

[R.G.]

a) FF transistor tester is designed for PNP, mine are NPN. Do I need to make any modifications to the tester? Tried swapping E with C and the voltages started going all over the place...

To test NPNs, you leave everything else the same but reverse the battery. Note that the emitter of the NPN now attaches to the negative pole of the battery, which is where the positive pole was for the PNP version. The collector still connects to the smaller-value collector resistor.

b) Don't have too many resistors on hand, so I found a 2.2k and a 220Ω and put them in series, giving me... 2.37KΩ     Are there any correction factors I can use? Any relevant formulas?

The funny value of the resistor was just to make the numbers come out pre-calculated. Any similar resistor value works, including just a 2.2K. The only difference is that you have to note the voltage across the resistor and calculate the currents by Ohm's law, I = V/R.

The base resistor causes a base current of (Vbattery-Vbe)/Rbase to flow. If you use a 9.0V battery and a 2.2M resistor, and Vbe is 0.2V, you get
Ib = (9.0-0.2)/2.2M = 4.0uA. If your battery voltage is different, or your base resistor is not exactly 2.2M, the actual base current will be different. You can ignore that and get usable results, or you can measure all of the battery voltage, base resistor and Vbe under test and calculate the base current more accurately. Generally, using 9.0V and 2.2M nominally will be close enough for pedal work.

The collector resistor converts collector current to voltage by its resistance value, whatever that is. So a 2.2k resistor makes 2.2 volts per milliampere, or
2.2mV/uA.

You first measure the voltage on the collector resistor and convert that to a current: Ic = (meter voltage) / Rc. That's your leakage current. Write that down.
As an example, if you used just the 2.2K resistor and measured 114mV with the base open, that would be a leakage current of Ileak = 114mV/2.2K = 51.8uA
of leakage. That's a good, low leakage device for germanium.

You then push the base current button, and that sends 4uA into the base. The collector current increases by the HFE times the base current, so you measure the collector voltage again, and it will be bigger than it was when measuring the leakage, because it contains the leakage plus the amplified base current. Again, you measure the voltage, divide by the resistor value to get the new collector current.

To get gain, you subtract the first-measurement leakage current from the second-measurement gain-plus-leakage current, and divide that by the base current.

If you happened to read, for example, 0.862V with the base current connected, that would be 862mV/2200 = 392uA of gain current plus leakage.

Subtracting out the leakage of 114uA gives a gain-current of 340uA. Dividing this by the 4uA of base current gives the real DC gain: 340/4 = 85.

Your actual battery voltage, base resistor, collector resistor and everything else will be slightly different, but you can plug in your measured numbers and get what matches your setup.

[phector2004]

Thanks, R.G!

Got it all worked out now 

[bt2513]

Since it looks like his questions were answered, I guess this isn't hijacking....

BUT

I am trying to fashion a calculator of sorts out of a spreadsheet that will asist me in calculating DC Gain.  For this, I am trying to figure out what the constants are.  Ideally, I would like to be able to measure the resistors' ACTUAL resistance, the battery's ACTUAL voltage output, then input the voltage readings on the 2.2k resistor with and without base current applied to have the spreadsheet calculate an accurate hfe reading.

Now, I have applied the forumulas in R.G.'s article and have the calculator working correctly.  However, is Vbe a constant value or does this need to be measured prior to the test?  According to my math, if I'm using a slightly week battery, my measurements will be much different.  I just want to fully understand what is happening...

[Hides-His-Eyes]

Just measure the battery with a multimeter or use a power supply!

[bt2513]

Just measure the battery with a multimeter or use a power supply!

Yes, I know, but.....

The base resistor causes a base current of (Vbattery-Vbe)/Rbase to flow. If you use a 9.0V battery and a 2.2M resistor, and Vbe is 0.2V, you get
Ib = (9.0-0.2)/2.2M = 4.0uA. If your battery voltage is different, or your base resistor is not exactly 2.2M, the actual base current will be different. You can ignore that and get usable results, or you can measure all of the battery voltage, base resistor and Vbe under test and calculate the base current more accurately. Generally, using 9.0V and 2.2M nominally will be close enough for pedal work.

In this context, where do I measure Vbe?

[R.G.]

I am trying to fashion a calculator of sorts out of a spreadsheet that will asist me in calculating DC Gain.  For this, I am trying to figure out what the constants are.  Ideally, I would like to be able to measure the resistors' ACTUAL resistance, the battery's ACTUAL voltage output, then input the voltage readings on the 2.2k resistor with and without base current applied to have the spreadsheet calculate an accurate hfe reading.

I'l be very honest with you - that tester is not an "instrument" by any means. It's design goals are very limited indeed. It is solely intended to

(a) identify transistors which are "too leaky" by some arbitrary definition, or at least to provide a warning
and
(b) to provide an indication of the transistor gain that is close enough to get good sound in typical effects circuits which use germaniums.

It's important to keep that in mind, because it was never intended to be a precision measuring device. I did this deliberately because (1) sorting wheat from chaff is important when you are buying decades-old electronics that may or may not have ever been good and (2) more precision may be counterproductive as well as misleading. This last is because gain and leakage both change depending on the operating conditions and temperature. To do an accurate characterization of a transistor, you need to measure many more things. Gain and leakage are not single numbers; they are better described as 3-D surfaces in a mathematical "phase plane". But a one-value number is OK as an indicator. That's what the circuit does.

However, is Vbe a constant value or does this need to be measured prior to the test? 

Vbe is really only useful because I use the difference between Vbe and the power supply through a large resistor as a pseudo-constant current source. In a more accurate instrument, that would be a real current source with some precision and not affected by Vbe. As it is, Vbe can vary from about 0.15 to 0.3V as it will for germanium, without huge changes in the results. So - is it constant? No. Does that matter? Some, but not a whole lot.

According to my math, if I'm using a slightly week battery, my measurements will be much different. 

That is quite true. Another of those simplifying assumptions I made was that a 9V battery will be between 9V and 9.4V, those being what a reasonably fresh battery does. If I was doing this as an accurate measurement instrument, the battery voltage would not matter because the base current would be held constant by an active circuit. The current through a transistor collector is not terribly dependent on the voltage across the collector-emitter. Some, but after the first volt or so, not too much. The variation of collector current with changing collector-emitter voltage at constant base current is wrapped up in the "Early voltage", and described in other ways as other parameters.

Mostly, yeah - use a fresh battery.

Using a spreadsheet to get very accurate measurements from this is kind of like using a hand grenade to swat flies. It may work, but it's vastly overqualified for the task and may have unintended side effects.  

[slacker]

Vbe, if you still want to measure it after what R.G. has said,  is the voltage across the base and emitter.

[bt2513]

Thank you both!!

I guess the term spreadsheet conjurs images of hundreds of cells and graphs...

Its actually just a small 2x8 grid:

Constants
Battery Voltage = X.XX
Vbe = X.XX
R1 (~2.2M) = XXXX
R2 (~2.47k) = XXXX

Measurements
R2 no current = X.XXXXX
R2 with current = X.XXXX

Results
Leakage Current = XXX
DC Gain = XXX

I just used the formulas to solve for the Leakage Current and DC Gain.... this way, there is less room for me to make a mathematical error when testing some transistors.  All I have to do is check the battery voltage, Vbe, and actual resistor values before I start and with my laptop in front of me, just type in the measurments and, bam, there's the gain.

[Brymus]

Thats pretty neat.
And a good idea.
Any chance of sharing the program?
That would be easier for sorting through transistors rather than writing with pen and paper.

[phector2004]

Made one of those in Excel a week ago, but I didn't care to save it...

Don't forget your measurements are only valid for the temperature you record at. Handling the transistors made leakage and "gain" shoot way up. Results were pretty consistent after letting them sit a minute or so, but they're definitely gonna be different now that I turned the heating on.

Thank you, Canada.

[bt2513]

Thats pretty neat.
And a good idea.
Any chance of sharing the program?
That would be easier for sorting through transistors rather than writing with pen and paper.

Don't mind sharing it at all.. however, can't upload an excel file here and don't have web space to host it on.  If anyone wants it, PM me with your email address.

[Hides-His-Eyes]

Google Docs will let you host spreadsheets for free.

[mac]

Let me a few days to get back in my country, I could write a HTML/javascript page or Mac OSX/Windows application to do the math, instead of a spreadsheet. I could also add NPN/PNP wiring graphs.

It could be hosted here, in my gallery.

mac

[mac]

I uploaded a HTML page in my gallery. There is a graph in the images folder. Download both and create a new folder "images" at the same level as the HTML page and put the wiring.gif  pict into the folder.

I included the link to RG's article.

Let me know what do you think, errors, etc.

mac