Scaling RG germanium test.

[mac]

In my previous post,
http://www.diystompboxes.com/smfforum/index.php?topic=45433.0
I mention that some 2N388 I have increase their gain by a factor of 1.7 - 1.9 if the base current ib is ten times the standard 4uA RG germanium test uses Note that 4uA is the base current of a magic fuzz face, so RG test gives you the values of your germaniums oparating in a fuzz face. Nice.
But some transistors have more, or less, juice at higher base currents, that is, hfe is a function of the collector current ic, hfe = hfe(ic).
Ok. Most of us use the test almost exclusively for our fuzz faces. But there are other projects, like a class A, A/B amps with higher base, and collector currents, and you may want to know the gain under this conditions. RG test may be scaled.

     |                   |   Vcc
     |                   |
     /                   /
     /   Rb            /  Rc
     /                   /
     |                   |
     |                  /
     --------------- /       Ge
                      \
                        \
                         |
                       –––
                         –  gnd
.
I used to sleep deeply in the classroom, so hopefully, if I understood RG, eqs are:

vc = vcc - ic*.Rc
ic* = ic + iL, iL the leakage current
ib = (vcc- vbe)/Rb

vc = vcc - hfe.ib.Rc - iL.Rc

solving for hfe,
hfe = (vcc-vc).Rb/[Rc.(vcc - vbe)] - iL.Rb/(vcc - vbe)


the value of iL is
iL = (vcc - vc*)/Rc
vc* the value at the collector when Rb is disconnected.

so hfe is
hfe = (vcc-vc).Rb/[Rc.(vcc - vbe)] - (vcc - vc*).Rb/[Rc.(vcc - vbe)]

hfe = (vc* - vc).Rb/[Rc.(vcc - vbe)]    vc, vc*: respect to gnd

Assigning Rb sets the base current ib.Then you can choose a value of Rc so as to make the factor Rb/[Rc.(vcc - vbe)] close to 100, or the number you want. Reading the voltage drop across Rc with Rb not connected, vcc - vc*, and when connected, vcc - vc, substracting and multiplying by 100 or whatever factor you choose gives you the gain under this conditions.

Corrections welcome.


mac

[zbt]

May I, Sir?
sorry for the long post

Assume the measurement is repeated, and this is what i did

1. Measure the battery voltage is 9V
    I once used a drop battery (ouch!), so I used a stable supply voltage of 9V, no need worry again

2. What is the leakage value
    Need calculator, vc/2K472, so I used 1K 1% resistor series with 2K trimmer set to 1K472
    V1K * leakage (ex 100uA) would read 0.1V read as 0.1mA, a lot easier

3. What is the actual value of hfe
    perhaps 10uA for ib?

[mac]

2. What is the leakage value
    Need calculator, vc/2K472, so I used 1K 1% resistor series with 2K trimmer set to 1K472
    V1K * leakage (ex 100uA) would read 0.1V read as 0.1mA, a lot easier

Cool!

3. What is the actual value of hfe
    perhaps 10uA for ib?

1k and 890k.
No quantum precision needed so 890k can be 820k + 68k + 2k2.
Leakage is as your 2) and hfe the collector differences by 100 factor.

mac

[zbt]

If 10uA then thing would a lot easier



I should change to 10K for leakage then 1K for hfe

[zbt]

added a feature, now i got a stethoscope 

[mac]

Add 1M anti-pop resitsors @ input & output.

mac

[antonis]

Anti-pop for 5 & 10 nF caps..??

[mac]

Anti-pop for 5 & 10 nF caps..??

Yes.
When I built my RM I did not consider an anti-pop resistor at the output because we are used to have a volume pot to gnd there. But the RM is different, it took me ages to find the problem, I soldered a 100k to gnd, pop gone.

mac