Is high, or low, Hfe better for a CCS?

[Elvee]

Originally Posted by SY

Yes, but a high hfe and (relatively) high emitter resistance totally overwhelm even the highest collector resistances (1/hoe, basically).

No, and that is clearly shown in the graphs I posted: basically what counts is the ratio of the (external) emitter resistance to the dynamic emitter resistance (~=26/Ie).
The Hfe is only a higher order factor and plays a minor role

[counter culture]

Interesting how what at first looks obvious turns out not to be, as is so often the case in electronics.

[DF96]

Yes. I have certainly learnt something. Snag is, within a few months I will probably have forgotten it!

[SY]

Originally Posted by Elvee

No, and that is clearly shown in the graphs I posted:

I'll admit to being somewhat incompetent with sims, so it's not so clear to me; could you explain your graphs (e.g., what are they measuring and how, which curve corresponds to which hfe...)? I'm particularly confused by how much the resistances vary with very small increments of frequency.

[magnoman]

I understand that when cascoding the transistors then there is a beta multiplication of the output resistance.
With the simple current source as shown (with emitter resistors) I have trouble understanding why the output resistance would increase with decreasing beta. It would seem to me the opposite effect and mostly only with low beta.

From a transistor physics point of view (well outside my aptitude) I would think that minimizing the base width modutation from the CB would require a sacrifice in the the b-e doping concentrations and thus a lower beta may be associated with very high early voltage transistors (though this is seperate from just the circuit analysis of the simple current source and only varying the beta).


Thanks
-Antonio

[magnoman]

Elvee
Just zoomed in on your plots and now at least understand what your saying.
That is just increasing beta increases Ro slightly as expected, but that once the full model is reflective of the higher beta parts Ro decreases.
Do your models have more than just differences in VAF to account for this?

Thanks
-Antonio

[Elvee]

Originally Posted by SY

I'll admit to being somewhat incompetent with sims, so it's not so clear to me; could you explain your graphs (e.g., what are they measuring and how, which curve corresponds to which hfe...)? I'm particularly confused by how much the resistances vary with very small increments of frequency.

Simulators are good at plotting things, and even when all you really need is a single value, it is simpler to make a bogus frequency sweep, or something like that.

Here, I have chosen to analyze the behavior of the output impedance over the 10Hz to 20Hz interval: it is so small that no discernable change happens, this explains the flat lines (the solid ones, the dotted ones are the phase and are of no interest).

The first three (red, yellow and cyan traces) are "actual" transistors, BC 847A, B, and C. The suffix letter indicates the gain class: http://hep.fi.infn.it/PAMELA/pdf/bc847.pdf
A: typ. 180, 110 to 220
B: typ. 290, 200 to 450
C: typ. 520, 420 to 800
We see that despite the increase in Hfe, the output impedance decreases in large proportions: down to 1.9 Meg for the C.
This means that the effect of h22 increase with Hfe is overwhelming over the value of Hfe itself (the models come from NXP and are quite accurate).

I have then investigated the effect of a "pure" Hfe increase, ie I have tweaked the BF parameter in the BC847A model, without changing the other parameters that are normally associated with a higher Hfe.
There we see an increase in impedance (magenta and grey traces), but a very modest one.

That is just increasing beta increases Ro slightly as expected, but that once the full model is reflective of the higher beta parts Ro decreases.
Do your models have more than just differences in VAF to account for this?

Here are the models:

.model BC847A NPN(IS=9.677E-15 NF=0.9922 ISE=5.44E-15 NE=2 BF=182.1 IKF=0.14 VAF=143.8 NR=0.9935 ISC=5.236E-12 NC=1.53 BR=7.004 IKR=0.06 VAR=31.15 RB=10 IRB=5.00E-06 RBM=4 RE=0.78 RC=0.656 XTB=0 EG=1.11 XTI=3 CJE=1.443E-11 VJE=0.733 MJE=0.3514 TF=6.04E-10 XTF=8.94 VTF=3.78 ITF=0.2711 PTF=0 CJC=3.287E-12 VJC=0.5444 MJC=0.3954 XCJC=0.6193 TR=0.00000011 CJS=0 VJS=0.75 MJS=0.333 FC=0.789 Vceo=45 Icrating=100m mfg=Philips)
.model BC847B NPN(IS=1.822E-14 NF=0.9932 ISE=2.894E-16 NE=1.4 BF=324.4 IKF=0.109 VAF=82 NR=0.9931 ISC=9.982E-12 NC=1.763 BR=8.29 IKR=0.09 VAR=17.9 RB=10 IRB=5.00E-06 RBM=5 RE=0.649 RC=0.7014 XTB=0 EG=1.11 XTI=3 CJE=1.244E-11 VJE=0.7579 MJE=0.3656 TF=4.908E-10 XTF=9.51 VTF=2.927 ITF=0.3131 PTF=0 CJC=3.347E-12 VJC=0.5463 MJC=0.391 XCJC=0.6193 TR=9.00E-08 CJS=0 VJS=0.75 MJS=0.333 FC=0.979 Vceo=45 Icrating=100m mfg=Philips)
.model BC847C NPN(IS=2.375E-14 NF=0.9925 ISE=5.16E-16 NE=1.3 BF=524.9 IKF=0.09 VAF=49.77 NR=0.9931 ISC=7.064E-12 NC=1.78 BR=10.04 IKR=0.132 VAR=16 RB=10 IRB=5.00E-06 RBM=5 RE=0.653 RC=0.78 XTB=0 EG=1.11 XTI=3 CJE=1.132E-11 VJE=0.7685 MJE=0.3733 TF=4.258E-10 XTF=6.319 VTF=6.4 ITF=0.1845 PTF=0 CJC=3.379E-12 VJC=0.5444 MJC=0.3968 XCJC=0.6193 TR=0.000000095 CJS=0 VJS=0.75 MJS=0.333 FC=0.999 Vceo=45 Icrating=100m mfg=Philips)

You can see that practically all parameters are affected by the change in doping, not only VAF.

[SY]

Great, many thanks for clarifying!

[stinius]

Originally Posted by Elvee

Simulators are good at plotting things

And a disaster if they are used wrong.

Could you please post a readable plot with white background?

S

[magnoman]

So for a Wilson (or cascode) the final current stage transistor should be a "C" version, not sure about the other 2 transistors.

Thanks
-Tony