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6487 and 2400 share negative/ground output

2502, 6485, 6487 - Support
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txz033000
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Joined: October 20th, 2014, 12:58 pm
Country: United States

6487 and 2400 share negative/ground output

Post by txz033000 » October 20th, 2014, 1:11 pm

Hi, All,
I have a device like a transistor , need to bias the gate-source to +50V, and at the same time to bias the drain-source to +300V. the source is set as zero voltage. I plan to use Keithley 6487 to apply the drain-source voltage and use Keithley 2400 to apply the gate-source voltage. Then I had to connect the source of my "transistor" to the negative output of the two instruments mentioned above. is this going to be a problem? I am worried there may be some interference between the 2 meters because I also need to measure the current between gate-source, and the current between drain-source.
can someone shine some light?

regards,
tao

Andrea C
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Re: 6487 and 2400 share negative/ground output

Post by Andrea C » October 21st, 2014, 12:21 pm

Hello tao,

When using the 500 volt source range of the 6487 to apply Vds of 300 volts, the voltage supply will have no more than 2.5mA of current available. Is your expected Ids current less than this?

Connect the LO of the Amp Meter to the LO of the voltage supply. Connect the HI of voltage supply to the drain and the Amp Meter HI to the Source.

You would connect the LO of the 2400 to the LO of the 6487. Your actual applied Vgs at the device would differ from the 2400 source voltage by the voltage burden of the amp meter, but the voltage burden should not exceed 200uV, so this is probably not significant.

You also need to consider device breakdown scenarios:
The 200 volt max input of the 2400 could be damaged if the 300 volts at the drain was applied to this terminal due to device breakdown.

Andrea

txz033000
Posts: 13
Joined: October 20th, 2014, 12:58 pm
Country: United States

Re: 6487 and 2400 share negative/ground output

Post by txz033000 » October 21st, 2014, 1:55 pm

Andrea,
thanks for the reply.
when we use 6487 to output 300V, we should not expect a current over 1mA. if it is over 1mA, we would be really surprised and happy. the 200uV voltage burden of the ampere meter is not a problem, but good to know.
about the "breakdown" scenario, I think it is probably not going to happen because we have a "transistor-like" device. in reality, the resistance between the gate and drain is that of cm-long vacuum, and should not change unless we have a vacuum-breakdown. but thanks for the reminder.

regards,
tao

txz033000
Posts: 13
Joined: October 20th, 2014, 12:58 pm
Country: United States

a new development

Post by txz033000 » December 2nd, 2014, 10:41 am

Andrea,
we still plan to use Keithley 6487 and 2400 to test our device, which is filed electron-emitter. so a metal piece is used as electron emitter. a nearby metal ring as a gate are in positive potential relative to emitter to extract electron out of the emitter. A metal plate which is a few mm above the gate are acting as collector to collect the extract electrons, which is in the high positive potential relative to the gate. basically I need to apply two voltage ; V gate-emittor is +30V; V collector-gate is +400V. I am thinking to use 6487 for V collector-gate(+400V) and use 2400 for V gate-emitter(+30v). but I wonder how I should connect the two meters. I have two options: 1st one as I mentioned before, connect the negatives of the 2 meters together, but the disadvantage is if the gate potential is changed , I need to change the collector potential to maintain the 400V difference, which is hard to do because I am the sweeping the V gate-emitter during test. 2nd connection scheme, is to connect the negative output of 6487 to the positive terminal of 2400, then no matter how gate potential varies, the V collector-gate is fixed. but I am not sure if the 2nd method is all right or allowed. could you shine some light?

regards,
tao

Andrea C
Keithley Applications
Keithley Applications
Posts: 1490
Joined: October 15th, 2010, 10:35 am
Country: United States
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Re: 6487 and 2400 share negative/ground output

Post by Andrea C » December 8th, 2014, 5:58 am

Hi tao,

If you float the 6487 on top of the 2400, you will have a series path for the currents, so you will not be able to know the gate current separate from the "drain" or "collector" current. Otherwise, I think it is fine.

Andrea

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