mikeks said:
D.Self has abt. 11 pages on this in his book, most in favour of bipolars and quite convincing, so I would probably go bipolar with 60V rails.
OTOH I once had to design an amp with 400..500 V rails for scientific purposes and there simply was no contest. For high Vce, bipolar SOAR
melts at a whopping pace, even more for PNPs. By far the best p device I could get was the wimpy TO220 Moto/On MTP2P50E.
Still needed a lot of them in series and parallel.
BTW its spice model had the bias completely wrong IIRC.
Gerhard
Thats good Lumanauw, you have saved the post from Charles Hansen!
BTW, here's NP's mos.pdf paper, see page 9, figure 13!
Cheers Michael
EDIT: Oh, and while talking about P-ch FET flaws of IR, please add up your advices on vertical FETs with fairly good complementary pairs, or just P-ch others than IR in my thread Complementary V-FET's other than IRF's???, my hope is that we could make up a list under that thread.
BTW, here's NP's mos.pdf paper, see page 9, figure 13!
Cheers Michael
EDIT: Oh, and while talking about P-ch FET flaws of IR, please add up your advices on vertical FETs with fairly good complementary pairs, or just P-ch others than IR in my thread Complementary V-FET's other than IRF's???, my hope is that we could make up a list under that thread.
john curl said:
MikeB said:
Hi Guys,
If you guys are targetting complementary pairs in VFET's than P-channel Mosfet suffers most in RDS ..and Qg[Total Gate charge]
Though my application requires reliability much important, its evident for me to use N-channel devices only...so if you look at the SOA curve of these mosfets..they were simply amazing....
The APT20M22LVR
Just see the 10mS SOA at 100V =25Amperes
Bob,
We're just measuring speed in a different way. If you equate speed with ft then certainly VFETs are speedier than BJTs. And the source/emitter follower simulation you suggested is consistent with this.
I'm not just looking at speed in terms of current gain. The design approach I use, which I shall keep under my hat for now, causes me to look at it in a different way and this reduces the advantage of FET over BJT.
From what I have learned it does not surprise me that some have found more sonic success using bipolars than FETs. Many leading brands use bipolars. There are many brands that use FETs that sound mediocre. It is pointless to generalize about this.
Personally, I don't like either of them.
The tube folk are blessed.
MikeBettinger wrote:
Sorry.
Sound quality is hard to measure. That which is easy to measure tends to get the attention and tends to be "improved". Often improved much too far at the expense of that which is more important but hard to measure (or identify).
We're just measuring speed in a different way. If you equate speed with ft then certainly VFETs are speedier than BJTs. And the source/emitter follower simulation you suggested is consistent with this.
I'm not just looking at speed in terms of current gain. The design approach I use, which I shall keep under my hat for now, causes me to look at it in a different way and this reduces the advantage of FET over BJT.
From what I have learned it does not surprise me that some have found more sonic success using bipolars than FETs. Many leading brands use bipolars. There are many brands that use FETs that sound mediocre. It is pointless to generalize about this.
Personally, I don't like either of them.
The tube folk are blessed.MikeBettinger wrote:
Sorry.
Sound quality is hard to measure. That which is easy to measure tends to get the attention and tends to be "improved". Often improved much too far at the expense of that which is more important but hard to measure (or identify).Odd IRF p-channel behavior
Thanks, Nelson.
I have a question for you in regard to the discussion of the IRF p-channel devices. I am intrigued by the transconductance frequency response of the IRFP9240 that you show in your mos.pdf Figure 13 (very enjoyable reading, by the way). I must admit, I was unaware of this phenomenon.
I assume you were driving the gate with the 600 ohm source impedance, as you did for the n-channel device in Figure 8. I also take it that the low-frequency voltage gain of the test arrangement was around 20-25, so there would be significant Miller effect coming into play with Cgd. Was this transconductance frequency response a small-signal measurement or a large-signal measurement?
Has anyone ever speculated on the physical origin of this curious transconductance frequency response result, and how there could be a time constant at work in the 100 Hz to 1 kHz range? Do you believe that the observed effect is due to the nature of the Cgd changing with frequency, or do you believe that the underlying intrinsic transconductance is changing with frequency?
Did you try observing the effect with a smaller source impedance? One might expect that if the effect was due to strange behavior of Cgd, then the corner frequency of the effect would be then increased. On the other hand, if the effect was due to some frequency dependence of the intrisic transconductance (who knows, maybe some trapped gate charge effect?), then one might expect that the corner frequency of the effect would remain relatively unchanged with reduced source impedance. Since the corner frequency of the effect moves out with increased drain voltage in your measurement, I would tend to guess that the behavior is the result of strange Cgd behavior.
Thanks,
Bob
Nelson Pass said:
Thanks, Nelson.
I have a question for you in regard to the discussion of the IRF p-channel devices. I am intrigued by the transconductance frequency response of the IRFP9240 that you show in your mos.pdf Figure 13 (very enjoyable reading, by the way). I must admit, I was unaware of this phenomenon.
I assume you were driving the gate with the 600 ohm source impedance, as you did for the n-channel device in Figure 8. I also take it that the low-frequency voltage gain of the test arrangement was around 20-25, so there would be significant Miller effect coming into play with Cgd. Was this transconductance frequency response a small-signal measurement or a large-signal measurement?
Has anyone ever speculated on the physical origin of this curious transconductance frequency response result, and how there could be a time constant at work in the 100 Hz to 1 kHz range? Do you believe that the observed effect is due to the nature of the Cgd changing with frequency, or do you believe that the underlying intrinsic transconductance is changing with frequency?
Did you try observing the effect with a smaller source impedance? One might expect that if the effect was due to strange behavior of Cgd, then the corner frequency of the effect would be then increased. On the other hand, if the effect was due to some frequency dependence of the intrisic transconductance (who knows, maybe some trapped gate charge effect?), then one might expect that the corner frequency of the effect would remain relatively unchanged with reduced source impedance. Since the corner frequency of the effect moves out with increased drain voltage in your measurement, I would tend to guess that the behavior is the result of strange Cgd behavior.
Thanks,
Bob
Re: Odd IRF p-channel behavior
I have to admit I have not been aware of this too. It is precisely the very low frequency at which the dip occurs that is more than curious. It would be possible to eliminate Cgd as a factor reducing drain voltage swing to near zero (cascoding, small drain resistor, current probe...). Although, if it turns out it is Cgd, it must be a truly monumentally nonlinear capacitance, something akin to a ferroelectrical effect, but then this would actually account for the shelf in the gm curve, not the dip (lower output keeps Cgd swing less but DC component on Cgd more, possibly below the treshold of serious Cgd nonlinearity). I freely admit I am no expert at semiconductor processes and MOSFET physics, but I can't recall any fundamental mechanism that would change gm this way - not with such long time constants. Not even electrostriction effects would do this given the size of the die.
Whatever it is, it's certainly ne of those things you would have great difficulty achieving on purpose
Bob Cordell said:
I have to admit I have not been aware of this too. It is precisely the very low frequency at which the dip occurs that is more than curious. It would be possible to eliminate Cgd as a factor reducing drain voltage swing to near zero (cascoding, small drain resistor, current probe...). Although, if it turns out it is Cgd, it must be a truly monumentally nonlinear capacitance, something akin to a ferroelectrical effect, but then this would actually account for the shelf in the gm curve, not the dip (lower output keeps Cgd swing less but DC component on Cgd more, possibly below the treshold of serious Cgd nonlinearity). I freely admit I am no expert at semiconductor processes and MOSFET physics, but I can't recall any fundamental mechanism that would change gm this way - not with such long time constants. Not even electrostriction effects would do this given the size of the die.
Whatever it is, it's certainly ne of those things you would have great difficulty achieving on purpose
Bob Cordell said:
Looking at my original data, the circuit was set up as Common
Source with a variable -DC supply through a 4 or 8 ohm load
and then to the Drain. The DC bias was variable, as was the
AC Gate signal. After playing around to get the lay of the land,
I settled down to 4 curves.
In the first, the AC was 100 mV from 50 ohms, the Ids was 1 amp,
the load was 8 ohms and the Vds was varied from 4 to 10V DC.
I concluded that the effect was marginally dependent on the
Vds, with improvement at higher Vds.
In the second test, the same setup was used with Vds at 10V
but the Ids was varied from .5A to 1 amp. The effect was quite
a bit higher at higher Ids.
The third test was as the first, with 10V Vds and 1A Ids, but
with the source impedance at 25 versus 600 ohms. Not a lot
of difference.
The fourth test varied the Drain impedance from 8 to 4 ohms, and
there was less gain variation with 4 ohms.
I don't have any good insight as to why this is. Charles Hansen
said he saw a write-up on it once, but was unable to locate it for
us. An intriguing aspect of this is that only the IR P channel
parts seem to do this - I have not seen it elsewhere.
As a practical matter, I simply don't use IR P channel parts in
Common-Source gain stages if I can help it. I have not had
any dificulty with Common Drain applications, but then of course
it would be far less noticeable there.
I will follow this post with copies of the curves 1-4.
Nelson,
first thank's for your post and for putting up 4 hi-Q pics!
Now the question, you have before as in the quote above given similar comments regarding the IR P-ch anomaly and the difference between using it in CS or CD, could you clarfy that?
I mean that if using a global NFB topology in CD you have no voltage gain in that configuration but on the other hand the IR P-ch anomaly isn't that noticeably as you mention it.
But if runing it in CS it will be more noticeable, but we then also have much more voltage gain to fix the transconductance anomalies by means of NFB.
I'm not sure my conclusion is right but do I guess right that your comments regarding IR's P-ch transconductance anomalies and the differences depending on CD or CS is valid only in a NoFB amplifier, or should I say more noticeable?
Cheers Michael
first thank's for your post and for putting up 4 hi-Q pics!
Nelson Pass said:
Now the question, you have before as in the quote above given similar comments regarding the IR P-ch anomaly and the difference between using it in CS or CD, could you clarfy that?
I mean that if using a global NFB topology in CD you have no voltage gain in that configuration but on the other hand the IR P-ch anomaly isn't that noticeably as you mention it.
But if runing it in CS it will be more noticeable, but we then also have much more voltage gain to fix the transconductance anomalies by means of NFB.
I'm not sure my conclusion is right but do I guess right that your comments regarding IR's P-ch transconductance anomalies and the differences depending on CD or CS is valid only in a NoFB amplifier, or should I say more noticeable?
Cheers Michael
Nelson Pass said:
Hi Nelson, thanks very much for the detailed answer. I love a good mystery. I'll probably see if I can duplicate the behavior and poke around at it.
In reading your description above, it was a little unclear what the source impedance was for cases 2 and 4. Was it 600 ohms for those?
Thanks,
Bob