Electronics > Beginners
Jellybean hobbyist general purpose transistors..
orolo:
--- Quote from: T3sl4co1l on October 13, 2017, 10:15:23 pm ---
--- Quote from: orolo on October 13, 2017, 08:23:32 pm ---But there is a catch, that I first heard of from Bob Pease: the higher the beta, the lower the output impedance. It can be seen in this datasheet: the 547A has 55.5k typical output impedance, the 547B 33.3k, and the 547C 16.6k, and it could go as low as 9k!. This can be very important when building current sources/sinks, active loads, etc.
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Only relevant to pure current mirrors without degeneration.
Tim
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Sorry for going a bit off topic, but honestly, I'm confused beacuse this beta thing has been bothering me for a time. Do you mean that there is a workaround for low rout, or that it's not relevant for current sinks and active loads? I've gone back to basics to see if I got this wrong, redoing example 2 here: http://leachlegacy.ece.gatech.edu/ece3050/notes/ISources/isources.pdf [There is an errata in the exercise: they changed m for n]. I attach the python code I used:
--- Code: ---beta = 100.0 # Current gain.
Va = 75.0 # Early voltage.
Vbe = 0.65 # Base-emitter voltage.
Vce = 8.00 # Collector-emitter voltage.
rx = 40.0 # Base spreading resistance.
Ic = 1.5e-3 # Collector current.
Vcc = 15.0 # High rail.
Vdd = -15.0 # Low rail.
n = 2.0 # Emitter resistor Vbe multiplier.
m = 10.0 # Stiff current multiplier at the base.
Vt = 300.0*1.3806e-23/1.6022e-19 # Thermal voltage.
alpha = beta/(1+beta)
Re = (beta*n*Vbe)/((1+beta)*Ic)
R1 = beta*(Vcc-Vdd-(n+1)*Vbe)/((m+1)*Ic)
R2 = (beta*(n+1)*Vbe)/(m*Ic)
r0 = (Va + Vce)/Ic
rep = (R1*R2/(R1+R2) + rx)/(1+beta) + alpha*Vt/Ic
rout = (r0 + rep*Re/(rep+Re))/(1.0 - (alpha*Re)/((rep+Re)))
print "beta ", beta
print "va ", Va
print "Vt ", Vt
print "Re ", Re
print "R1 ", R1
print "R2 ", R2
print "r0 ", r0
print "rep ", rep
print "Output resistance: ", rout
--- End code ---
For the same circuit, if beta=100, Va=75, the resistance of the sink is 379k, in agreement with the exercise. Now take beta=200, Va=75/2=37.5. The resistance drops to 215k. Even if beta=400, Va=37.5, the resistance is still 218k. Beta does not compensate for Va drop. If we go to the extreme, beta=600, Va=12.5, resistance drops to 100k. At least we can say that high beta parts underperform in this role. Isn't it just the same with a common emitter with active load: will a piece with 15k-9k output impedance give the same gain than a piece with 50k?
Found Pease's article warning about high beta: http://www.electronicdesign.com/power/whats-all-vsubbesub-stuff-anyhow-part-2 .
Edit: Aha, I found it! What was spoiling the resistance of the current source was not the Va, but the choice of resistors R1 and R2. For high beta you need a much stiffer current at the base divider, m=100 or so. I think I'll need to think this over, much deeper. :-+ Thanks!
Vtile:
How disturbing it is that VHF means very high frequenzy in a 30 MHz to 300 MHz range while every dog collar today transmit in Gigahertz range Wifi. Anyone else annoyed. >:D ^-^
Vtile:
--- Quote from: edavid on October 13, 2017, 10:54:48 pm ---
--- Quote from: Vtile on October 13, 2017, 09:18:14 am ---The 4117 were my list just because it is what it is and it still is available in the discrete form. Unfortunately the A-subtype is not hobbyist friendly priced in any form, while MMBF4117 is. (unfortunately in SMD, but luckily SOT23 is still somewhat usable, without PCB and professional equipment.) :)
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There is a US seller who has PN4117As at a decent price:
http://www.ebay.com/itm/QTY-100-PN4117A-N-Ch-JFET-General-Purpose-Amplifier-/371603063222
Maybe he would ship you some, if you ask nicely.
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If the seller do refuge to sell outside the US. why you would suggest he would change his mind if someone in Europe would asks. Will no happen, especially since the products are in "high-tech" category, which have had traditionally paranoid export limits / licencing.
In general ordering something from US ebay to Europe is far from hobby priced if the miracle happens and you find a seller who will ship to Europe.
PS. I'm not angry to you, your intention is/was good and someone in US propably will benefit on that information.
bd139:
Indeed. China to Europe is cheaper than US to Europe!
David Hess:
--- Quote from: bd139 on October 14, 2017, 12:07:52 pm ---Indeed. China to Europe is cheaper than US to Europe!
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China to the US is cheaper than the US to the US.
But buying semiconductors and other electronic parts of questionable heritage does not strike me as being particularly economical.
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