The characteristic IV curve of an example FET transistor. Note that there is a finite output conductance, which is proved by the slope of the CCS region.
This finite output conductance is the result of channel length modulation (the performance is similar to the advance effect of bipolar transistors), which will cause the device current to increase with the increase of VDS.
The gain surface of the FET-based amplifier comes from the product of the transconductance of the device and the load resistance of the transistor. Include a limited output resistance that distorts the CCS region of the surface.
The change of the operating surface of the amplifier in. The CCS operation portion of the surface is no longer planar, indicating an increase in the sensitivity of the power supply voltage of the amplifier.
The power supply voltage sensitivity of the amplifier within the range of possible device operating conditions (VGS,VDS). Compared with figure 5-5, the sensitivity of the power supply voltage is increased due to the limited output resistance:
(a) Linear graph, you can see a slight tilt of "floor" at high VGS and high VDS.
(B) A diagram recording the same data to emphasize the increase of PSS in the CCS operating area.
This more realistic transistor reduces the sensitivity of the power supply voltage.
In (a), the "floor" rises above zero at the higher values of VGS and VDS. There seems to be little difference on this scale, so ET still works well. Better understand the sensitivity of the actual power supply voltage as shown in figure 5-8.
(B) as shown, where the logarithm of the power supply voltage sensitivity is drawn on the vertical axis. Even at high VGS, the sensitivity (pss,power supply sensitivity) of the supply voltage is still less than 0.1. The existence of other operating modes with higher supply voltage sensitivity is still very obvious.
It is very important to be very clear at this point. If the value of pss becomes non-zero, it means that the voltage value of the power supply does have some effect on the actual output signal envelope. Whether this extra modulation is important depends entirely on the application using the ET system. It is difficult to find a RF power transistor that is close enough to an ideal current source and can be thought of as having virtually zero pss. This means that the output signal of the amplifier usually depends on the change of the supply voltage. This is something we have to provide customer service.