Transistors 151
the drain current is progressively reduced as the gate– source voltage is made more negative. At a certain value of VGS the drain current falls to zero and the device is said to be cut-off. 20 18 16
12 10 8
Drain current, ID (mA)
14
6 4 2 ⫺5.0 ⫺4.5 ⫺4.0 ⫺3.5 ⫺3.0 ⫺2.5 ⫺2.0 ⫺1.5 ⫺1.0 ⫺0.5
0
As in the case of the bipolar junction transistor, the output characteristic curves for an n-channel FET have a ‘knee’ that occurs at low values of VDS . Also, note how the curves become flattened above this value with the drain current ID not changing very significantly for a comparatively large change in drain–source voltage VDS . These characteristics are, in fact, even flatter than those for a bipolar transistor. Because of their flatness, they are often said to represent a constant current characteristic. The gain offered by a field effect transistor is normally expressed in terms of its forward transconductance (gfs or Yfs ) in common-source mode. In this mode, the input voltage is applied to the gate and the output current appears in the drain (the source is effectively common to both the input and output circuits). In common-source mode, the static (or d.c.) forward transfer conductance is given by: gfs =
0
Gate−source voltage, VGS (V)
whilst the dynamic (or a.c.) forward transfer conductance is given by:
Figure 12.17
gfs = Figure 12.18 shows a typical family of output characteristics (ID plotted against VDS ) for a small-signal general-purpose n-channel FET operating in commonsource mode. This characteristic comprises a family of curves, each relating to a different value of gate–source voltage VGS . You might also like to compare this characteristic with the output characteristic for a transistor operating in common-emitter mode that you met earlier in Figure 12.10. 20
VGS ⫽ 0 V
18 Drain current, ID (mA)
16 VGS ⫽ ⫺1 V
14 12
VGS ⫽ ⫺2 V
10
ID (from corresponding points on the graph) VGS
ID (from the slope of the graph) VGS
(Note that ID means ‘change of ID ’ and VGS means ‘change of VGS ’.) The method for determining these parameters from the relevant characteristic is illustrated in worked Problem 8 below. Forward transfer conductance (gfs) varies with drain current. For most small-signal devices, gfs , is quoted for values of drain current between 1 mA and 10 mA. Most FET parameters (particularly forward transfer conductance) are liable to wide variation from one device to the next. It is, therefore, important to design circuits on the basis of the minimum value for gfs , in order to ensure successful operation with a variety of different devices. The experimental circuit for obtaining the common-source characteristics of an n-channel JFET transistor is shown in Figure 12.19.
8 VGS ⫽ ⫺3 V
6 4
VGS ⫽ ⫺4 V
2 0
0
VGS ⫽ ⫺5 V 2 4 6 8 10 12 14 16 18 20 Drain−source voltage, VDS (V)
Figure 12.18
B1
⫺ ⫹
ID
mA
TR1 g
d
VR1 V VGS
Figure 12.19
s
⫹ B2
V VDS
VR2
⫺
Part 1
151
Transistors
12 10 8
Drain current, ID (mA)
14
6 4 2 ⫺5.0 ⫺4.5 ⫺4.0 ⫺3.5 ⫺3.0 ⫺2.5 ⫺2.0 ⫺1.5 ⫺1.0 ⫺0.5
0
As in the case of the bipolar junction transistor, the output characteristic curves for an n-channel FET have a ‘knee’ that occurs at low values of VDS . Also, note how the curves become flattened above this value with the drain current ID not changing very significantly for a comparatively large change in drain–source voltage VDS . These characteristics are, in fact, even flatter than those for a bipolar transistor. Because of their flatness, they are often said to represent a constant current characteristic. The gain offered by a field effect transistor is normally expressed in terms of its forward transconductance (gfs or Yfs ) in common-source mode. In this mode, the input voltage is applied to the gate and the output current appears in the drain (the source is effectively common to both the input and output circuits). In common-source mode, the static (or d.c.) forward transfer conductance is given by: gfs =
0
Gate−source voltage, VGS (V)
whilst the dynamic (or a.c.) forward transfer conductance is given by:
Figure 12.17
gfs = Figure 12.18 shows a typical family of output characteristics (ID plotted against VDS ) for a small-signal general-purpose n-channel FET operating in commonsource mode. This characteristic comprises a family of curves, each relating to a different value of gate–source voltage VGS . You might also like to compare this characteristic with the output characteristic for a transistor operating in common-emitter mode that you met earlier in Figure 12.10. 20
VGS ⫽ 0 V
18 Drain current, ID (mA)
16 VGS ⫽ ⫺1 V
14 12
VGS ⫽ ⫺2 V
10
ID (from corresponding points on the graph) VGS
ID (from the slope of the graph) VGS
(Note that ID means ‘change of ID ’ and VGS means ‘change of VGS ’.) The method for determining these parameters from the relevant characteristic is illustrated in worked Problem 8 below. Forward transfer conductance (gfs) varies with drain current. For most small-signal devices, gfs , is quoted for values of drain current between 1 mA and 10 mA. Most FET parameters (particularly forward transfer conductance) are liable to wide variation from one device to the next. It is, therefore, important to design circuits on the basis of the minimum value for gfs , in order to ensure successful operation with a variety of different devices. The experimental circuit for obtaining the common-source characteristics of an n-channel JFET transistor is shown in Figure 12.19.
8 VGS ⫽ ⫺3 V
6 4
VGS ⫽ ⫺4 V
2 0
0
VGS ⫽ ⫺5 V 2 4 6 8 10 12 14 16 18 20 Drain−source voltage, VDS (V)
Figure 12.18
B1
⫺ ⫹
ID
mA
TR1 g
d
VR1 V VGS
Figure 12.19
s
⫹ B2
V VDS
VR2
⫺
Part 1
151
Transistors
