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MOSFET transistor analysis

dolgaleb

Member level 1
I am trying to find the drain current for a circuit employing the MRF1517 transistor. Below is what I have done so far.

Since I have not being able to either include the circuit in an attachment or paste it in here, I will provide a description of the circuit since it is very simple.

First, the gate has a 2000 ohms connected to it; the other side of this resistor goes to a 2 V DC source.
Second, the transistor's drain is connected to a 6 V DC power supply.
Third, the transistor's source is grounded.

Vth = 1 V

VS = 0V VD= 6V VG = 2V

VGS = 2 V VGD = -4V VDS = 6 V

For transistor to be in saturation mode the following must be true:

• VGS > Vth 2 >1 ok
• VGD < Vth -4 < 1 ok
• VDS ≥ VGS – Vth 6 ≥ 2 – 1 = 1 V ok
So the transistor is in amplification mode.

How do I find the drain current? In Amplifier mode the drain current is given by:

ID= 0.5 [Kn’ W/L(VGS – Vth)2]

I have VGS and Vth but I don’t know the Kn’ W/L factor. Any ideas as to how to find the current?

So you want the SPICE model. Assigning geometry is designer's choice.

Not really, although it wouldn't hurt to have it.

What I need are the equations or methodology to find the drain current of the circuit.

Since Vgs(th) always has a wide-tolerance and a tempco for RdsOn , there is a wide tolerance on Id.

Gate Threshold Voltage
(Vds= 7.5 Vdc, Id = 120 μAdc)
Vgs(th) = 1.0min 1.7typ 2.1max Vdc

In the saturation region, Ids = ß * (Vgs - Vt)^2 / 2
Beta or k is somewhat inversely proportional to Rds

To determine the current, you have to measure with something like a 50 mV shunt resistor for max current swing on Vdd or a current sense IC.
Then you regulate Vgs to achieve your target current.

To get Rds = 50 Ohms = 6V/120 mA you need 120 mA

To get Id = 1.5A you need a good heatsink and Rds= 6/1.5 = 4 ohms

Then you may need to regulate Vgs from 2 to >3V to match your target current and output power.
Often twice the threshold voltage is needed as a rule of thumb.