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
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?
Quote2. 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!
Quote3. 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
If 10uA then thing would a lot easier
(https://i.postimg.cc/bDFm3MjH/hfe-Tester.png) (https://postimg.cc/bDFm3MjH)
I should change to 10K for leakage then 1K for hfe
added a feature, now i got a stethoscope
(https://i.postimg.cc/GBRFQ4Zp/rmtester.png) (https://postimg.cc/GBRFQ4Zp)
Add 1M anti-pop resitsors @ input & output.
mac
Anti-pop for 5 & 10 nF caps..??
QuoteAnti-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