It's not all there! What? Your parts manufacturer provides an incomplete I_DSS range for your FET? Then he's telling you to go fish!

What's a guy to do? You need some fishng gear. Homebrew an I_DSS meter. Measure your FETs, instead of depending on incomplete datasheets. Sound good? Below are schematics for meters that reel in the data.

♦ CAUTION. No enhancement devices. These circuits measure parameters for JFETS or depletion MOSFETs. But the circuits won't measure enhancement MOSFETs. If you plug in a MOSFET, it won't turn on. You might mistakenly assume that it's dead. (Examples: No 2N7000s! No BS170s!)

Build in minutes. Test in seconds! The simple I_DSS meter (Figure 1) allows you to read I_DSS. Depending on the FET you're measuring, I_DSS could run anywhere from a few milliamps to 100 milliamps. (Very few FETs have an I_DSS below 1 mA. Example: PN4117.) In this circuit, you may use a handheld multimeter to measure I_DSS current. To find V_{GS (off)}, Malvino provides this handy formula (1.)...

Fig. 1. Simple I_DSS meter

1. In the 8-11-2020 tests, the author used a digital meter.

2. In the 8-20-2020 retests, the author used an analog meter.

3. The 8-20, 9V tests agree with the 8-11-2020 test results.

4. The 8-20, 15V tests disprove the hypothesis that a lower V_DD is responsible for I_DSS readings below the maximum spec on the datasheets. (2N3819: 20 mA max. 2N5457: 5 mA max.)

Technician's Cadillac. The swanky I_DSS meter appears at Figure 2. You can use the power voltage (B2) of your choice. The 20-volt supply is a typical example. (DC power packs are fine, too. A particular FET might also require a larger battery B1. For example, Hayes and Horowitz use -15V. (2.))

Fig. 2. Swanky I_DSS meter (4.)

♦ CAUTION: FIGURE 2. Never connect the gate battery B1 backwards! This battery must provide a zero to negative voltage to the gate. The voltage must never rise above zero volts. Otherwise, the gate junction could melt, destroying the JFET.

2. Set your DVM to read current.

3. Adjust Pot VR1 so that it shorts the FET gate to the source. Current through the test FET will reach I_DSS (maximum safe value).

4. Press TEST BUTTON S1. Current will flow through your DVM (M1).

5. While pressing the button, read I_DSS on your DVM. For most small-signal FETs, the reading will be in milliamps. (Some FETS operate in the sub-milliamp region.)

Repeating Fig. 2. Swanky I_DSS meter (4.)

What is V_P? V_P, or more precisely, V_P0, is the pinch-off voltage at I_DSS. At V_P0, the drain saturates with current. (5.) V_P0 is equivalent to -1[V_{GS (off)}]. (6.) See the drawing (right).

What is I_DSS? Looking at Figure 3, let's start at the origin, bottom-left. Moving right along the X-axis, we spot V_P, the pinch-off voltage. Here, we arrive at the vertical line. Now, we follow the line upward to the drain curve I_DSS. Here, we see that V_P points to the knee in the curve. At this point, the current through the FET flattens out. We've reached current saturation, or I_DSS!

Fig. 3. I_DSS occurs at pinch-off voltage V_P. (7.)

Flat section. On the device datasheet, there are more drain (I_D) curves beneath the I_DSS curve. Each drain curve has a saturation mode and a pinch-off voltage. We need the FET to operate on the flat, or “saturation mode” section of the curve.

Select a drain curve. In design work, we can operate our FET on any drain curve. On Figure 4, right, the top drain curve is I_DSS. Parallel to, and below I_DSS are four horizontal curves. Each curve produces a different drain current I_D. We identify these curves by the negative gate bias voltage V_GS that produces them...

Deriving R_S from G_M

Source resistor R_S bias allows us to select a drain curve. R_S bias reduces I_D. A good, starting value for R_S is (1 / G_M). This statement assumes that we use the G_M (transconductance) value at I_DSS. (Datasheets sometimes refer to G_M as G_FS or Y_FS.) The datasheet provides G_M as a range of values at I_DSS. Yet we must choose only one G_M value! We can cope by selecting an average or “typical” G_M value. (Sometimes the manufacturer provides the typical value.) (10.)

What is G_M? There are two types of G_M. Dynamic G_M is the change in gate voltage that causes a given change in drain current. Static G_M expresses the drain current per volt of gate voltage. The measurement is in amperes (or more likely milliamps). Yet we divide the ratio (mA / 1V) or (µA / 1V), expressing the result as a decimal number. The unit for this new term for inverse resistance is the Siemens, S (or U). Example: 3,000 µS or 3 mS. Both of these mean “3 milliamps per volt.”

Fig. 4. Drain family curves (9.)

Fig. 5. G_M curve: R_S selects drain curve. (11.)

Fig. 6. R_S & static G_M

Source resistor connection. The reciprocal of any static G_M value is a source resistor value. Malvino walks us through the easy R_S formula. (12.) Be sure to ignore the V_GS sign...

R_S= (-V_GS / I_D)

R_S examples. Figure 5 includes three examples of source resistors. The table below gives the calculations to find these three resistor values...

Tour of the G_M Curve. At Figure 5, notice that transconductance (G_M) is a parabola. As the curve ascends, the transconductance increases. As the curve descends, the transconductance decreases. Here's what this means for source resistor values: A large source resistor locks in a high reverse bias on the gate. With this gate voltage, the transconductance is low, but the current drain is economical. With this large resistor, be careful not to approach V_{GS (off)} too closely, or the device might clip. A small source resistor allows far more drain current to flow. The transconductance is high, but the battery drain and heat might be excessive. A heat sink might be necessary. If the design approaches I_DSS too closely, clipping could again become a problem.

Consider the drain current of your circuit. Drain current affects the operating transconductance of the FET. Manufacturer specs usually give the transconductance figure at I_DSS. Are you operating at a lower I_D than I_DSS? Then use a transconductance (G_M) value that's less than the device maximum.

Example: The manufacturer powers his model MPF102 at I_DSS, 20 mA. But your project FET draws 2 mA, average. The datasheet G_M range is 2 to 7.5 mS. You choose 2 mS, giving an R_S of 500 ohms: (1 / 0.002)= 500Ω. You can then use a 470-ohm or 560-ohm standard resistor.

1. When V_GS = 0, you achieve I_DSS: Maximum drain current for normal operation. (13.)

2. See Figure 7. Your FET amplifier must operate inside a zone with these boundaries...
   

   •Max IDSS	• Min ID
   •Vp	•Breakdown voltage

   (Minimum I_D occurs at V_{GS (off)}.)

3. To use a lower I_D (drain) curve, increase the source bias voltage, V_GS. (With N-channel devices, make V_GS more negative. For P-channel devices, more positive.) The R_S will increase.

4. To use a higher curve, reduce the source bias voltage, V_GS. (With N-channel devices, make V_GS more positive. For P-channel devices, more negative.) The R_S will decrease.

5. For an N-channel device, V_GS is negative. For a P-channel device, V_GS is positive.

6. The reverse-biased gate prevents gate current. No gate current flows. (15.)

7. V_P is a drain-source voltage.

8. V_{GS (off)} is a gate-source voltage. It has the same magnitude as Vp, but the opposite sign.