1. FETs are also made of N type and P-type semiconductors.
3. FETs usually have a much higher input impedance than bipolar transistors.
4. Usually, FET input voltages do not control the output current as exactly as bipolar input currents control the output current.
2. FETs work on the idea of an electric field. As the voltage on the gate (P-type) changes, then the reverse bias of the PN Junctions change. This reverse bias makes an electric field called the depletion region. This depletion region will open the N-type channel or close it for current.
· The gate to source voltage (Vgs) will control the drain to source (Id) current. Vgs should be 0V or negative.
· The equation for the Drain current of the FET is:
Id = Idss ( 1 – Vgs / Vp) 2 where Idss = the maximum (or saturation drain current) and Vp = maximum gate voltage
· For circuits: use Ig = 0 and Id = Is (Idrain = Isource)
The blue section in Figure4 is called the ohmic region. Here is the N-channel has a changing resistance.
In Figure4, the section to the right of the blue, shows the saturation region, where Id = Idss and is maximum.
Figure3 shows the parabolic graph for how Vgs makes Id.
2. The P-channel FET has a positive Vgs. The Drain is connected to the negative side of a power supply. So the drain current goes in the source and out of the drain.
Id = Idss ( 1 – Vgs / Vp)2 = 10mA (1 - -2.5V/ -5V) 2 =2.5mA
Is = Id = 2.5mA
Vds = Vb – R2 x Id = 10V – 2K x 2.5mA = 5V
Starting with
Ig = 0
2. MOSFETs also work on the idea of an electric field. As the gate voltage becomes more negative, it will push electrons in the channel to go into the P-type and be combined with holes. So the Drain current will be less. The Gate voltage will open the N-type channel or close it for current Id.
N-Type
channel
· The gate to source voltage (Vgs) will control the drain to source (Id) current. For depletion mode, Vgs should be 0V or negative. For enhancement mode, Vgs is >0.
· The equation for the Drain current of the MOSFET is:
Id = Idss ( 1 – Vgs / Vp) 2 where
Idss = the saturation drain current.
Vp = maximum gate voltage
Drain
Figure6
Source
Gate
For a MOSFET Ig=0A, so Vr1 =0V and Vgs = -Vr3 = -IdxR3.
Next make a graph for the MOSFET: using Vgs=Vp/2 = -2V then Id=Idss/4=1mA, when Vgs=Vp then Id=0, when Vgs=0 then Id=Idss
Now for the circuit: When Id=0 then Vgs=0,
When Vgs= -3V, then Id = -Vgs/ R3 = - -3V/ 1K = 3mA. So draw a line.
Where the lines meet: Id = 1.5mA
The source of the N –channel MOSFET is connected to Ground (0V).
When Vin=0V, then for the N-channel, Vgs=0V so it is OFF. But for the P-channel Vgs= -5V so it is ON. So Vout = Vcc.
When Vin=5V, then for the N-channel, Vgs=5V so it is ON. But for the P-channel Vgs= 0V so it is OFF. So Vout = 0V.
Pchannel
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