![]() ![]() The charge carriers enter the channel at source and exit via the drain. The MOSFET works by varying the width of a channel along which charge carriers flow (electrons or holes). When the FET is ON, the input signal will appear at the output and when it is OFF the output is zero. It acts as a closed switch if the control voltage is zero and open switch if control voltage is negative. In this circuit FET acts as a series switch. When we apply the negative voltage which is equal to the pinch off voltage at the gate, FET operates in the cutoff region and acts as a high resistance device and the output voltage is equal to the input voltage.Since the resistance R D is very large, the output voltage is approximately considered as zero. The output voltage across the across the FET is V OUT = V in *. When the VGS applied is zero the FET is turned ON by operating in saturation region and its resistance is very small nearly 100 ohms.Similarly, when the VGS applied is more negative than the pinch off voltage, FET operates in cutoff region and doesn’t allow any current flow through the device. Hence it is like a fully switched ON condition. When the V GS is zero the FET operates in saturation region and maximum current flows through it. Μ = ∂ V DS / ∂ V GS = μ = ∂ V DS / ∂ I D x ∂ I D / ∂ V GSįET can be used as a switch by operating it in two regions, they are cutoff and saturation region. Μ = ∂ V DS / ∂ V GS at constant I D There is a relation between transconductance (g m ) and dynamic output resistance (r d ) and that can be established in the following way. The amplification factor is defined as the ratio of change of drain voltage ( δ V DS ) to change of gate voltage ( δ V GS ) at a constant drain current (I D = Constant). This is the ratio of change of drain to source voltage ( δ V DS ) to the change of drain current ( δ I D ) at a constant gate to source voltage (V GS = Constant). The expression of drain current (I D ) is The transconductance at any other value of gate to source voltage (g m ) can be determined as follows. This maximum value (g mo ) is specified in a JFET data sheet. Gm = ∂ ID/ ∂ V GS at constant V DS This value is maximum at V GS = 0. Transconductance is the ratio of change in drain current ( δ I D ) to change in the gate to source voltage ( δ V GS ) at a constant drain to source voltage (V DS = Constant). This current is fixed for a JFET and this is called shorted gated drain current and generally denoted by I DSS. This is the maximum drain current that can flow through the channel when the gate terminal is in ground potential. But after pinch-off voltage (V p ), the drain current would not be increased further and gets a constant value. When the gate terminal is grounded (V GS = 0) and positive drain to source voltage (V DS ) is being increased slowly in case of n channel JFET, the drain current gets increased linearly. This voltage is known as Cut Off Gate Voltage (V GS(off) ). The relation between gate to source voltage and drain current is given below.Īfter a certain gate to source voltage (V GS ), the drain current I D becomes zero. If the gate to source voltage decreases from zero in n channel JFET, the drain current also gets decreased accordingly. At a fixed drain voltage, the drain current (I D ) of a JFET depends on the gate to source voltage (V GS ). ![]()
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