One relatively easy way is to connect the drain to the supply, the gate to ground, and the source to a current source that has the other end grounded. Then just measure the voltage across the current source to find Vgs.
On the one hand you want to come reasonably close to the actual operating conditions of the transistor, which will typically be 50 Vds at 100 mA or so. However, this would dissipate 5 watts in the device which would destroy it unless heatsinked. So instead pick something smaller. I would suggest 10 Vds and 25 mA. This gives a total device dissipation of 250 mW, which will be fine.
You can make a current source from a 3-pin regulator if you read the app notes.
Don't try to match the P-channel devices to the N-channel devices. Just match each polarity to itself.
On the one hand you want to come reasonably close to the actual operating conditions of the transistor, which will typically be 50 Vds at 100 mA or so. However, this would dissipate 5 watts in the device which would destroy it unless heatsinked. So instead pick something smaller. I would suggest 10 Vds and 25 mA. This gives a total device dissipation of 250 mW, which will be fine.
You can make a current source from a 3-pin regulator if you read the app notes.
Don't try to match the P-channel devices to the N-channel devices. Just match each polarity to itself.
The method lucpes posted is really only suitable for vertical devices. With this approach you will only apply Vgs (threshold) across the device. This is OK for vertical devices as you will have 3 or 4 volts Vds. But for lateral devices you will only have a few hundred millivolts, which is kind of sketchy.
However if you don't want to build the current source as I proposed in my previous post, you could just use a resistor as lucpes proposed. Please note that it should replace the current source in my posting and be connected between source and ground.
Also, please be aware that the pinouts in lucpes' diagram are for vertical MOSFETs. The lateral MOSFETs have the source and drain pins interchanged.
However if you don't want to build the current source as I proposed in my previous post, you could just use a resistor as lucpes proposed. Please note that it should replace the current source in my posting and be connected between source and ground.
Also, please be aware that the pinouts in lucpes' diagram are for vertical MOSFETs. The lateral MOSFETs have the source and drain pins interchanged.
Charles Hansen said:
I'm pretty sure that IRF uses a 20uS pulse -- I haven't gotten around to making a programmable pulse generator to automate the process, i.e. to ramp the 20uS pulses in voltage to measure transconductance etc.
I think (correct me if I'm wrong) they (IRF, not Hitachi) also characterize the threshold at 100mA.
the thought of a programmable pulse generator is percolating somewhere between the front and back burners right now...
Sorry, my previous instructions were not correct. They would work for a depletion mode JFET, but not for an enhancement mode MOSFET.
You will need to add a second supply as shown in the attached schematic to test lateral MOSFETs. You can either use a true current source, or just substitute a resistor. I would recommend +/- 10 volts for the supplies. Then for the 2SK1058/2SJ162 parts I would recommend either a 25 mA current source or a 350 ohm resistor. These values are not critical and can easily be varied +/- 50% or more.
You will need to add a second supply as shown in the attached schematic to test lateral MOSFETs. You can either use a true current source, or just substitute a resistor. I would recommend +/- 10 volts for the supplies. Then for the 2SK1058/2SJ162 parts I would recommend either a 25 mA current source or a 350 ohm resistor. These values are not critical and can easily be varied +/- 50% or more.
Attachments
Charles Hansen said:
I have always got the impression that it is recommended to
match the N and P devices and now you recommend against
this. Could you please elaborate a bit on why you make this
recommendation? This has immediate practical consequences
which I have failed to get satisfactory answers to on previous
occasions.
There are a couple of reasons for this. The first is a practical one, in that it is almost impossible to match Vgs across both polarities. I suppose that you could do it by throwing away 60% to 80% of the parts you buy.
The next reason is that the N-channel parts have a lower output conductance than the P-channel parts. This is especially true for the Toshiba brand MOSFETs (as opposed to the Magnatec clones which are *much* better in this regard). This means that whatever reading you get will depend slightly on Vds for the N-channel parts, but significantly for the P-channel parts. So you could only match them for one specific Vds condition.
But guess what? When you are running them as followers in an audio amp, Vds is constantly changing!
Besides, about the only thing you are losing is a slight DC offset by not matching them. This is easily compensated for elsewhere in the circuit. The bottom line is that there is no point whatsoever to try and match across polarities in the output stage.
The next reason is that the N-channel parts have a lower output conductance than the P-channel parts. This is especially true for the Toshiba brand MOSFETs (as opposed to the Magnatec clones which are *much* better in this regard). This means that whatever reading you get will depend slightly on Vds for the N-channel parts, but significantly for the P-channel parts. So you could only match them for one specific Vds condition.
But guess what? When you are running them as followers in an audio amp, Vds is constantly changing!
Besides, about the only thing you are losing is a slight DC offset by not matching them. This is easily compensated for elsewhere in the circuit. The bottom line is that there is no point whatsoever to try and match across polarities in the output stage.
Charles,
Thankyou for the explanation. Yes the practical problem of
matching devices is what has worried me, especially if one
ends up with a bunch N devices from the same batch and a
number of P devices from the same batch. I have tried to get
opinions on this problem several times without getting much
wiser. It would simplify
things a lot if one doesn't have to worry about matching.
Your point that matching won't result in equivalent behaviour
is a good point and I guess it applies also to BJTs for instance.
A counterargument might be that one shouldn't match for a
certain parameter to be equal, but aim for a deliberate mismatch
that gives the best overall compromise, but it is probably difficult
to find the optimal compromise.
I don't really know about the MOSFETs, but I did some simulations
about the effect of mismatching BJTs in class A OPS. I found that
mismatching NPN and PNP devices resulted in increased
even order distorsion, but almost unaltered odd-order distorsion.
Of ocurse, those are only simulation results and have to be
taken as such.
BTW, I had planned on using the Exicon clones of the Hitachi
K162 and J1058 devices. They seem to be the cheapest
alternative and have slightly better spec's than the Hitachis.
Do you have any experience of them? The Magnatecs are also
much cheaper than the Hitachis, of course and seem rather
easy to get.
Thankyou for the explanation. Yes the practical problem of
matching devices is what has worried me, especially if one
ends up with a bunch N devices from the same batch and a
number of P devices from the same batch. I have tried to get
opinions on this problem several times without getting much
wiser. It would simplify
things a lot if one doesn't have to worry about matching.
Your point that matching won't result in equivalent behaviour
is a good point and I guess it applies also to BJTs for instance.
A counterargument might be that one shouldn't match for a
certain parameter to be equal, but aim for a deliberate mismatch
that gives the best overall compromise, but it is probably difficult
to find the optimal compromise.
I don't really know about the MOSFETs, but I did some simulations
about the effect of mismatching BJTs in class A OPS. I found that
mismatching NPN and PNP devices resulted in increased
even order distorsion, but almost unaltered odd-order distorsion.
Of ocurse, those are only simulation results and have to be
taken as such.
BTW, I had planned on using the Exicon clones of the Hitachi
K162 and J1058 devices. They seem to be the cheapest
alternative and have slightly better spec's than the Hitachis.
Do you have any experience of them? The Magnatecs are also
much cheaper than the Hitachis, of course and seem rather
easy to get.
The Magnatecs are quite a bit better than the Hitachi parts. The Hitachi N-channel parts are fine, but the P-channel parts are very "triode-like" in their characteristic curves (high output conductance). This lowers their PSRR and makes them a poorer match for the N-channel parts. Also the zero tempco point differs by about 25% between polarities on the Hitachi parts but is virtually identical for the Magnatecs.
As far as the mis-matching of opposite polarity's impact on distortion, this really only applies to mismatches of transconductance. But MOSFETs are pretty uniform in this regard, so this is not really a problem. Where they differ is in Vgs (threshold), which as I said only has an effect on DC offset. That is easily corrected elsewhere in the circuit.
The main reason to match output devices is just to make several separate (same polarity) transistors act as one big composite device. You need to match them so that they are sharing the work more-or-less equally.
The main reason to match output devices is just to make several separate (same polarity) transistors act as one big composite device. You need to match them so that they are sharing the work more-or-less equally.
Charles Hansen said:
OK, well the Hitachis are not my first choice anyway. Although I
can buy them off-the-shelf locally, they are about three times
as expensive as ordering Exicons or Magnatecs from the UK.
I take it you haven't tried the Exicons then? I have seen some
people report they sound slightly better than the Hitachis.
Charles Hansen said:
No, not at all, unless they sell the same devices undet two
different brand names and with different device numbers.
As I understand it Magnatec is owned by Semelab and sell
Magnatec lateral MOSFETs (as well as other things). They also
sell other brands, but not Exicon. It is not clear if the devices
are manufactured by Semelab of Magnatec themselves.
Exicon is a brand name owned by Profusion, who in addition
also sell a lot of other brands including semelab products, but
not the Magnatex LMOSFETs. It is unclear also here who
manufactures the Exicon devices.
http://www.magnatec-uk.co.uk/
http://www.semelab.co.uk/
http://www.exicon.com/
http://www.profusionplc.com/
It could, perhaps, be that both Magnatec and Profusion are
just distributors and for some reason buy identical devices
from Semelab, but branded by their own name.
OK Charles,
You seem so certain so I guess I'll have to believe you. There is
nothing in any those links I posted that reveals any such
connections, but there is nothing that contradicts it either.
So what we have then are two different brand names using
two different sets of device numbers for exactly the same
physical devices (either die or whole package)?
You seem so certain so I guess I'll have to believe you. There is
nothing in any those links I posted that reveals any such
connections, but there is nothing that contradicts it either.
So what we have then are two different brand names using
two different sets of device numbers for exactly the same
physical devices (either die or whole package)?
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