Current Regulator w/ FET, NPN transistors

crutschow said:

To elaborate on my previous comments about bipolar transistors: The base-emitter voltage is always around 0.6-0.7V (typical of a foward biased diode) when the transistor is on. It's the base current that determines the collector current. The base voltage will vary slight with base current of course (again similar to a diode) but it's the current that is the determining factor.

If you look at the gain curves for a transistor it will typically peak over some collector current range and droop at the high and low current levels. Typically you want to bias the transistor so its collector current is somewhere in the peak range of gain, in this case 1mA was selected.

In your calculation for R1 you used 10mA, it should be 1mA. Also the 2.4V gate voltage is actually gate-to-source voltage. Thus you need to add the 0.7V source voltage (across R2) to the 2.4V gate-source voltage to give 3.1V required at the gate (to common). Thus R1 = (6V-3.1V) / 1mA = 2.9k ohm.

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Here is a spec sheet im looking at for the NPN:
https://www.fairchildsemi.com/ds/2N/2N4123.pdf

There is a graph called base emmiter on vs collector current. Is taht the one i look at? In order to get 1ma @ 25C i would need a ~.68V at the base correct? If i wanted 10ma, i would need ~.72V . So if i set my R2 resistor @ 1A to drop .6V, i should be getting 0.1mA would i not?

Yes you are correct about the 3.1V needed because of the 0.6V drop.

Also, is the method i chose before correct. By looking at the FET specs and looking what type of output current is given for a given gate voltage? On some of the FETS i was considering (power mosfets), the graphs they give do not typically show the region of 1A. If they do show it, it is at the leftmost part of the graph and its hard to read what type of Voltage i need. Do you have any suggestions on how to get an accurate voltage i need to apply to the gate for it to output 1A? Are there emperical equations that you can use to calculate it rather than trying to read it from a small graph?

stryker1080 said:

There is a graph called base emmiter on vs collector current. Is taht the one i look at? In order to get 1ma @ 25C i would need a ~.68V at the base correct? If i wanted 10ma, i would need ~.72V . So if i set my R2 resistor @ 1A to drop .6V, i should be getting 0.1mA would i not?

Yes. And then you would need to change R1 to give 3.1V across it at 0.1mA.

Also, is the method i chose before correct. By looking at the FET specs and looking what type of output current is given for a given gate voltage? On some of the FETS i was considering (power mosfets), the graphs they give do not typically show the region of 1A. If they do show it, it is at the leftmost part of the graph and its hard to read what type of Voltage i need. Do you have any suggestions on how to get an accurate voltage i need to apply to the gate for it to output 1A? Are there emperical equations that you can use to calculate it rather than trying to read it from a small graph?

Click to expand...

The value is not critical in your application because of the negative feedback. The FET generally also has significant differences in gate voltage from unit to unit and this will also be compensated by the negative feedback. That is, it only takes a small change in voltage across R2 to correct for a much larger change in the FET gate voltage due to the gain of the bipolar transistor.

Ok, this makes sense now and i finally know how to calculate the values i need now. Thank you everyone for all your help, i will post a topic once i order the parts i need and build a circuit!!!!