While building the N-Channel amplifier (Anthony Holton's), I used the method described by him to match the output devices, but the voltage drop across the load resistors in the actual circuit vary much. The devices were matched for the same readings on the voltmeter as described by Anthony, ie., Gate and Drain tied together and connected to a 15volt supply through a 150 ohm resistor with the Source tied to ground. Anthony's specified voltage readings were not obtained but I read between 10.36 and 10.42 volts.
Is there another (read better) method that folks here might be using say in the case of Aleph devices etc?
Thanks for any inputs.
Is there another (read better) method that folks here might be using say in the case of Aleph devices etc?
Thanks for any inputs.
cp642,
I did not wait for a long time, since I was making sets out of a lot of 30 IRFP250s. I agree to thermal drift. However, I am not satisfied with the method suggested by Anthony Holton which is as follows:
With a 15 volt DC source connected through a 150 ohm 1 watt resistor to the MOSFET's Drain and Gate tied together and the Source connected to Ground, the voltage drop across the resistor is to be measured. Any other methods? What are the methods used for the Aleph projects?
Thanks,
I did not wait for a long time, since I was making sets out of a lot of 30 IRFP250s. I agree to thermal drift. However, I am not satisfied with the method suggested by Anthony Holton which is as follows:
With a 15 volt DC source connected through a 150 ohm 1 watt resistor to the MOSFET's Drain and Gate tied together and the Source connected to Ground, the voltage drop across the resistor is to be measured. Any other methods? What are the methods used for the Aleph projects?
Thanks,
Sam,
Perhaps you might want to check out Passlabs homepage on matching MOSFeTS in the second part of the A75 article and the DIY OP AMP article.
Also do check out Mark's homepage on his Aleph4 project. there's also an article on MOSFETS matching
http://members.ozemail.com.au/~dkfinnis/passlabs/aleph4_construction.htm
Perhaps you might want to check out Passlabs homepage on matching MOSFeTS in the second part of the A75 article and the DIY OP AMP article.
Also do check out Mark's homepage on his Aleph4 project. there's also an article on MOSFETS matching
http://members.ozemail.com.au/~dkfinnis/passlabs/aleph4_construction.htm
Mosfet Matching and Other Device Matching
Lets face it device matching is a pain so over the years I have come up with what I feel is the best way to handle this. It involves using a little bit of automation.
The method I like to use involves measuring the current through the MOSFET and using a servo system to drive the gate to obtain a preset selected current. Any current can be set from fractions of milliamps to amps.
All one has to do is to measure the gate drive voltage required to obtain the present current which was set by a comparitors reference voltage. Either drain or source current sensing can be used to match the intended purpose in the final application.
Such a system can be automated with some gating and sample and hold circuitry for both a low and high current limit. For power MOSFETS the low could be 50 MA and the high 300 MA. Thus each device has two matching points. The sample and hold circuit for the low and high set points feeding a seperate voltmeter.
Thus the drive voltage required to reach the low and high current setting can be read nearly simulatiously. A simple linear ramp voltage source can be used as a drive source. The sample and hold circuits hold the reading when the low and high currents are reached. The whole proces can take less than a second and does not heat up to MOSFET to any degree.
There is no nead to use methods that short leads together to match MOSFETS. This is not normally the way they will be used in the circuit so why attempt to match using such stone age methods?
With a small microprocessor that has a built in A to D converter the whole measuring circuit can be simplfied lot. A simple program could preset currents, read the drive voltages required to obtain the preset currents and remember and also display the results. Now we have a fully automatic matching system that could actually compare readings to a present standard and give the differences in percent, voltage or current. External power supplies are required to actually supply power to the device being tested.
This would be a real good project for someone to design and share with everyone who needs to do device matching.
I will start a new thread on the subject under Device matching automation.
John Fassotte
Alaskan Audio
[Edited by alaskanaudio on 11-25-2001 at 07:12 PM]
Lets face it device matching is a pain so over the years I have come up with what I feel is the best way to handle this. It involves using a little bit of automation.
The method I like to use involves measuring the current through the MOSFET and using a servo system to drive the gate to obtain a preset selected current. Any current can be set from fractions of milliamps to amps.
All one has to do is to measure the gate drive voltage required to obtain the present current which was set by a comparitors reference voltage. Either drain or source current sensing can be used to match the intended purpose in the final application.
Such a system can be automated with some gating and sample and hold circuitry for both a low and high current limit. For power MOSFETS the low could be 50 MA and the high 300 MA. Thus each device has two matching points. The sample and hold circuit for the low and high set points feeding a seperate voltmeter.
Thus the drive voltage required to reach the low and high current setting can be read nearly simulatiously. A simple linear ramp voltage source can be used as a drive source. The sample and hold circuits hold the reading when the low and high currents are reached. The whole proces can take less than a second and does not heat up to MOSFET to any degree.
There is no nead to use methods that short leads together to match MOSFETS. This is not normally the way they will be used in the circuit so why attempt to match using such stone age methods?
With a small microprocessor that has a built in A to D converter the whole measuring circuit can be simplfied lot. A simple program could preset currents, read the drive voltages required to obtain the preset currents and remember and also display the results. Now we have a fully automatic matching system that could actually compare readings to a present standard and give the differences in percent, voltage or current. External power supplies are required to actually supply power to the device being tested.
This would be a real good project for someone to design and share with everyone who needs to do device matching.
I will start a new thread on the subject under Device matching automation.
John Fassotte
Alaskan Audio
[Edited by alaskanaudio on 11-25-2001 at 07:12 PM]
matching mosfet
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If the devices are used in an amplifier where the devices pass a fixed value of bias current, then one of the test currents must be that bias current. Vgs of parallel devices must be selected to be equal at that bias/test current.
If you go for extra test currents then these can be higher or lower than the bias current. These will in effect match the slope of the Vgs vs Id curve of the devices as well as Vgs @ bias current. I think that matched pairs/triples/quads that have same Vgs and same average conductance will perform better in the paralleled situation.
If you go for extra test currents then these can be higher or lower than the bias current. These will in effect match the slope of the Vgs vs Id curve of the devices as well as Vgs @ bias current. I think that matched pairs/triples/quads that have same Vgs and same average conductance will perform better in the paralleled situation.
I use a similar method for matching J-fets. This is a great use for one of the old 741 op-amps you have lying around.....it is a known fact we all have these lying around in a 'junk box'.
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