# Eqn. for current

#### pratapkollu

Joined Dec 13, 2006
42
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

#### Ron H

Joined Apr 14, 2005
7,014
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.

#### pratapkollu

Joined Dec 13, 2006
42
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.
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

#### Ron H

Joined Apr 14, 2005
7,014
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
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.

#### pratapkollu

Joined Dec 13, 2006
42
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.
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

#### beenthere

Joined Apr 20, 2004
15,819
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.

#### pratapkollu

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

#### Ron H

Joined Apr 14, 2005
7,014
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
I didn't even look at your equation. I just suggested that
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.
You should be able to do that from the output current equation that I put on the schematic. Just substitute R1 for 499, etc.

#### hgmjr

Joined Jan 28, 2005
9,029
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

#### Ron H

Joined Apr 14, 2005
7,014
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
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".

#### hgmjr

Joined Jan 28, 2005
9,029
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".
Oooooppppssss!!!!!! You're correct. I did have the control voltage in mind when I typed power supply voltage.....

hgmjr