VMOS Amplifiers The results of Example 8-8 show that the voltage gain of a MOSFET amplifier, like that of a JFET amplifier, is generally rather small. Voltage gains of FET amplifiers are small because transconductance values are small, typically 3 to 4 mS. On the other hand, VMOS FETs (Figure 7-48) have comparatively large
transconductance values, on the order of 100 mS. Thus, amplifiers constructed with these devices have voltage gains 20 to 30 times greater than those based on other types of FETs.
L\s discussed in Section 7-8, short-channel power FETs have a linear transfer aracteristic. Figure 8-36 shows an example, the transfer characteristic of the “l\115N35 discrete power FK1 at 25°C. Note that values of drain current are fjcantly higher than those shown on transfer characteristics for the small-signal J\.lv ~FETs discussed earlier. The maximum drain-to-source voltage for this device is specified to be 350 V. We should note that, technically speaking, the MTM15N35 is not a MOSFET, since it has a silicon gate rather than a metal gate. Silicon gates are commonly found in FETs designed for switching-type applications because of certain advantages they enjoy where Vr is the threshold voltage and gm is the transconductance. In practice, the threshold voltage is generally assumed to equal the value where an extension of the linear characteristic intersects the VGs-axis, as shown by the dashed line in Figure 8-36. 1. Find the equation of the transfer characteristic of the MTM15N35 power FET. 2. Use the equation to determine the drain current when the gate-to-source voltage is 5 V.
1. Examination of the transfer characteristic in Figure 8-36 reveals that the dashed
line intersects the VGs-axis at Vr:=3.75 V. The slope of the linear characteristic
2. Substituting Vr.s = 5 V into the equation for the transfer characteristic, we find