JFET data sheets are similar to BJT data sheets-shearing with a device type number and a brief description of the device to indicate the most important applications.These datas are followed by maximum ratings,dc characteristics,ac characteristics,mechanical data etc. some of the important parameters specified are:
1.Saturation Current and pinch-off Voltage.The drain-source saturation current IDSS is sometimes termed the pinch-off current and is referred to as IDP at VGS=0.IDP may also be specified at values of VGS other than zero,an in this case VGS is also specified.
2. Transconductance. It is measured in siemens(S),formerly in mho (℧).Its typical value for a FET may lie between 1mS and 10 mS.
3.Drain-Source ON Resistance.The drain resistance rd is not to be confused with the drain-source ON resistance RDS,also designated RD(on).It is measured at a specified gate-source voltage VGS and drain current ID and is important when using the JFET as a switch.When the JFET is biased in its saturation or ohmic region of operation it exhibits a resistance between drain and source ranging from 10 to a few hundred ohms.
4.Gate cutoff Current and Input Resistance.The gate channel junction in a JFET is a P-N junction,since it is normally reverse biased,a minority charge carrier current flows.This is the gate-source cutoff current IGSS,also called the gate reverse current. IGSS is very small current of the order of few nanoamperes.The device input resistance (resistance of the reverse biased gate channel junctions) is inversely proportional to IGSS and its typical values for a JFET are 10 9 Ω at 25℃ and 10 7 Ω at 100℃.
5.Breakdown voltage:
There are several ways in which the FET breakdown voltage may be specified.BVDGO
is the drain-gate breakdown voltage with the source open circuited. BVGSS is the gate-source breakdown voltage with the drain shorted to the source.Both are a measure of the voltage at which the reverse-biased gate channel junctions break down.Typically values for each are of the order of 25 V.
6.Noise Figure:
FET usually has much lower thermal noise as compared to that in case of BJT.This is because, unlike the BJT ,there are very few charge carriers crossing a junction in the FET. As in the case of bipolar devices,the FET noise figure NF is specified as a spot noise figure at a particular frequency and bias conditions, and for a given value of bias resistance.
1.Saturation Current and pinch-off Voltage.The drain-source saturation current IDSS is sometimes termed the pinch-off current and is referred to as IDP at VGS=0.IDP may also be specified at values of VGS other than zero,an in this case VGS is also specified.
2. Transconductance. It is measured in siemens(S),formerly in mho (℧).Its typical value for a FET may lie between 1mS and 10 mS.
3.Drain-Source ON Resistance.The drain resistance rd is not to be confused with the drain-source ON resistance RDS,also designated RD(on).It is measured at a specified gate-source voltage VGS and drain current ID and is important when using the JFET as a switch.When the JFET is biased in its saturation or ohmic region of operation it exhibits a resistance between drain and source ranging from 10 to a few hundred ohms.
4.Gate cutoff Current and Input Resistance.The gate channel junction in a JFET is a P-N junction,since it is normally reverse biased,a minority charge carrier current flows.This is the gate-source cutoff current IGSS,also called the gate reverse current. IGSS is very small current of the order of few nanoamperes.The device input resistance (resistance of the reverse biased gate channel junctions) is inversely proportional to IGSS and its typical values for a JFET are 10 9 Ω at 25℃ and 10 7 Ω at 100℃.
5.Breakdown voltage:
There are several ways in which the FET breakdown voltage may be specified.BVDGO
is the drain-gate breakdown voltage with the source open circuited. BVGSS is the gate-source breakdown voltage with the drain shorted to the source.Both are a measure of the voltage at which the reverse-biased gate channel junctions break down.Typically values for each are of the order of 25 V.
6.Noise Figure:
FET usually has much lower thermal noise as compared to that in case of BJT.This is because, unlike the BJT ,there are very few charge carriers crossing a junction in the FET. As in the case of bipolar devices,the FET noise figure NF is specified as a spot noise figure at a particular frequency and bias conditions, and for a given value of bias resistance.
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