Hawes Amplifier Archive by James T. Hawes, AA9DT
Design a Depletion-Mode FET Preamp, Part 2

Step-by-Step Process

  1. Pick the gate resistor value (RG). Let RG equal the desired input impedance (Z). Don't know what resistance to use? Start with one megohm.

  2. Pick the power voltage (VDD) from the VDS values on the graph (right).

  3. Pick the maximum drain current ID. This value must fall between zero milliamps and the device's IDSS value. (IDSS is the maximum current through the device while shorting the gate and source. Manufacturers specify IDSS as a range of values. For example, 1 to 3 milliamps.) You'll find the range of IDSS values on the datasheet for your device. Or: Look here: IDSS Finder.

    Don't know your device's exact IDSS value? Set ID to the minimum IDSS value on the datasheet: For our example, 1 mA. This value is likely one of the ID values for either IDSS value. Also, 1 mA uses battery power economically.
    Table: Example Idss value from LND150 datasheet

  4. Draw the load line for your drain resistor. Run the line between the maximum power voltage (VDS) and the maximum current (ID).

 Graph: Start & end of load line on drain curves; mouse over to see load line.

How to draw a load line. Mouse over the curves to see a load line.
  1. Find the drain resistor value (RD). Ohm's Law determines the drain resistor value: RD = (VDD / IS).

    2. For example: (300 / 0.005) = 60KΩ → Use the closest, standard value. For example, 62K.
  2. Use the load line to select your bias curve. Halfway down the line is the quiescent voltage. This is the drain voltage with no signal on the gate. (Another name for this location is the Q-point.) On the X-axis, locate a point midway between zero and the power voltage point. Draw a vertical from this point to intercept your load line. With luck, this point will also intercept a bias curve. (Your curve might not appear on the graph. Rest assured that your curve “is there.” The engineer just didn’t draw it in. From two explicit, neighboring curves, you may estimate your curve. You may then sketch in your curve.)

  1. Find the source resistor value (RS). The horizontal curve provides your bias voltage value. Divide this voltage value by average current on the Y-axis. The result is your source bias resistor value (approximately).

  2. Adjust the drain resistor value (RD). Does the source resistor value exceed a tenth the value of the drain resistor value? No: Use a nearby standard (RD) value within 20 percent. Yes: Subtract RS from RD. The result is your new (RD) value. For both RD and RS, use nearby standard values within 20 percent.

  3. Tweak the source resistor value. Usually the graph is too small to yield an exact resistor value. Besides, actual device performance will vary from the graph. Some manufacturers provide maximum and minimum graphs. Then you can come up with an average resistor value. Test this value and tweak it for your best value.

How to Tweak Your Source Resistor

The Simulator Can't Find It for You!

♦Caution. Your Spice simulator can’t tell you the exact, best source resistor value for your device. For years, this fact has been the crux of my frustration with simulators.

 Schematic with resistor labels; on mouseover: schematic with voltage labels

Amplifier schematic with standard resistor labels. Mouse over image for standard voltage labels.

On a real circuit, you absolutely must tweak the source resistor value. If no source resistor within 5X works, then the drain resistor value or B-plus voltage is wrong. Or you have a lemon device. When I say “5X,” I mean a factor or multiple of 5.

The best RS value produces a quiescent (no signal) drain voltage of approximately half the power supply voltage. Yet: If (RS > 1/10 RD), the ideal VD exceeds (1/2 VDD).

Load Line Design Example

The load line actually represents the sum of the drain and source resistors (RS + 1/10 RD). Let’s skip the source resistor for now. In this circuit, the effect of source resistor RS is negligible. (RS only makes a difference when it’s more than 10 percent the size of the drain resistor.)

  1. Pick the gate resistor value (RG). See above.

  2. Pick the power voltage (VDD). Suppose that you have a power voltage of 234 volts.

  3. Pick the maximum drain current ID. For this example, the maximum current is 0.002 amp and change.

  4. Draw the load line between 234 volts on the bottom and 0.002A on the left. Now you have a line whose slope stands for your resistor RD. The halfway point, 117 volts, is your Q-point. (That's Q for “quiescent voltage point”: In the real world, it’s your “target” quiescent voltage.)

  5. Find the drain resistor value (RD). By Ohm’s Law, your drain resistor is close to the standard value of 100K.

    6. RD= (234 / 0.002) = 117KΩ → Use a nearby standard value within 20%. For example, 100K.
  6. Use the load line to select your bias curve. Your Q-point also intersects with your bias voltage curve. For a depletion FET, the bias voltage at saturation is a horizontal line. If the line isn’t on the graph, you may draw in the line.

  7. Find the source resistor value (RS). The intersecting bias line (some -0.3 volts) is the source voltage drop. Remember that you drew the load line for maximum current (ID). Dividing the bias line value by half this current gives you a ballpark source resistor value.

    8. ID / 2= 0.001
    VS= 0.3V
    (0.3 / 0.001) = 300Ω → Use a nearby standard value within 20%. For example, 330Ω.
  8. Adjust the drain resistor value. Our source resistor is much smaller than 10 percent of the drain resistor. For this example, RD adjustments are unnecessary.

 Load line; on mouseover: Finding source resistor value

Draw the load line. To find the Q-point, roll over the drawing.



Load line; on mouseover: Finding source resistor value

To find the source resistor value, roll over the drawing.
  1. Tweak the source resistor value. These small curve charts can't accurately pinpoint the resistor value. Also, device variation will compel you to try neighboring resistor values. But the chart will give you a starting point. (See above.)

Perfect Your Design

Got it working? Now get it perfect! Click... Tweak Your Amp Design.

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♦ WARNING. This is your project. Your achievement is entirely yours. I assume no responsibility for your success in using methods on these pages. If you fail, the same is true. I neither make nor imply any warranty. I don't guarantee the accuracy or effectiveness of these methods. Parts, skill and assembly methods vary. So will your results. Proceed at your own risk.

♦ WARNING. Electronic projects can pose hazards. Soldering irons can burn you. Chassis paint and solder are poisons. Even with battery projects, wiring mistakes can start fires. If the schematic and description on this page baffle you, this project is too advanced. Try something else. Again, damages, injuries and errors are your responsibility. — The Webmaster



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