# Eqn. for current

Discussion in 'The Projects Forum' started by pratapkollu, Jan 9, 2007.

1. ### pratapkollu Thread Starter Active Member

Dec 13, 2006
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Some please tell me the equation for CURRENT OUT in the circuit that is given in attachment. And please tell me how to calculate the transfer function( equation for current)

Thanks and regards
Pratap

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2. ### Ron H AAC Fanatic!

Apr 14, 2005
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If you understand the following three things, you can understand the analysis in the attached schematic.
1. Ohm's law.
2. Operational amplifiers (and feedback theory).
3. MOSFETs.
If there is something you don't understand, post it here.

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3. ### pratapkollu Thread Starter Active Member

Dec 13, 2006
42
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HI, Thanx for your explanation, can u please give the eqn for current as function of resistors and input voltage? for ex: I = [r2/ (r1.r3)]. Vi

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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Why don't you provide a schematic with reference designators (r1, r2, etc.), figure out what you think the transfer function is, and we will tell you if it's correct or not.

5. ### pratapkollu Thread Starter Active Member

Dec 13, 2006
42
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I dont know the exact eqn for current , I just gave u that eqn to let u know that I need the eqn for current as function of resisor newtwork and input voltage

Thanks and regards
Pratap

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6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
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Hi,

What resistor network? The output depends on the gate voltage of the second FET. That comes from the output of the second op amp, which is being driven by the first op amp. The limit is 100 milliamps into a 50 ohm load.

If you can get a transfer curve for the FET, find the voltage on the gate that corresponds to the D-S resistance necessary to allow 100 milliamps into your chosen load. Hint: the circuit is dynamic, so no calculation will be valid except as it applies current into some load. As there is no load in the schematic, output current is zero. Put some resistance between the terminals and see what happens. One suspects further that the 5 volt control level may correspond to 100 ma into 50 ohms.

7. ### pratapkollu Thread Starter Active Member

Dec 13, 2006
42
0
Thank you very much for the replies.

8. ### Ron H AAC Fanatic!

Apr 14, 2005
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I didn't even look at your equation. I just suggested that
You should be able to do that from the output current equation that I put on the schematic. Just substitute R1 for 499, etc.

9. ### hgmjr Moderator

Jan 28, 2005
9,030
214
I think what Ronh is asking of you is that you examine the annotations from his attachment and use your schematic containing the reference designators (R1, R2, R3, etc.) to produce the circuit's transfer function.

Ronh's has done a stellar job of annotating the schematic. He has even left the values of all of the resistors and the power supply voltage in the final expression.

hgmjr

10. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Yes, that's what I am asking.
Thanks for the compliment. I should point out that the power supply voltage does not appear in the final expression. Perhaps you meant "control voltage".

11. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Oooooppppssss!!!!!! You're correct. I did have the control voltage in mind when I typed power supply voltage.....

hgmjr