I want a current of 100mA to flow through a transistor. For that, I polarize it with a voltage divider.
The transistor I use is the BS270. According to its data sheet, for a current of 100 mA to flow, it is necessary to apply a gate-source voltage of about 3.2 V. Since I want it to operate in the saturation region, VDS>=VGS-Vtn. The worst case occurs when the threshold voltage (Vtn) is minimum, which is 1V (datasheet). So: VDS>=3.2-1=2.2V. Therefore, we choose a value for VDS of, for example, 2.5V. As we use a power supply (measured) of about 5.1V (Arduino), the remaining voltage 5.1-2.5=2.6V is distributed equally in the resistors RD and RS. With which, both must be worth RS=RD=(2.6V/2)/100mA=13 ohms. The necessary gate voltage can be obtained as: VG=VGS-VS=3.2-1.3=1.9V. We get this voltage using as RG1=1M and RG2=470k+100k+10k+2*4.7k.
However, when assembling the circuit, the voltage values VGS, VDS and the voltages on the resistors RD and RS are not what they should be. For example, the supply voltage drops all between drain and source and none across RD and RS. I don't know if it's the design that's wrong. Could someone tell me why?
To power the circuit I use four 5V outputs from the Arduino (one specific for more powerful projects) in parallel, so that it can supply the 100mA current.
The image of the designed circuit is as follows: