How to Tweak the Source Resistor
for Best Performance

This Bias Procedure adjusts circuit performance for the MPF102 device that you've bought. With one resistor change, you can make the circuit work well despite broad device variations. Buy DVM. For this procedure, you need a DVM. Buy one at Radio Shack. Then turn on your circuit. Measure the no-signal, DC voltage between the Q1 drain and ground. Ideal Vd. If drain voltage (Vd) is between 4 and 7 volts, the circuit will work. The ideal Vd point is halfway between the drain voltage and source voltage (Vs).

To Optimize Bias, Tweak Source Resistor Value

Headroom. What I mean by "ideal" is maximum headroom. This is the voltage that allows the most gain before clipping. For most devices in this circuit, the ideal drain voltage is 5 to 5.25 volts. (For some devices, this voltage might be lower or higher. See the table above.)

Best performance. In the discussion below, I'll assume that 5 volts is the ideal Vd voltage. If you want the best performance out of your amplifier, though, determine the ideal voltage for your own JFET. The formula is...

Ideal Vd = [ ( ( 9 - Vs ) / 2 ) + Vs ]

If you ever change JFETs in this circuit, repeat procedure below for the new device.

If Drain Voltage is Too High...

1. Suppose that you see more than seven volts at the drain. You need to reduce the drain voltage (Vd). You must decrease the source resistor value. Buy two insulated clip leads and these resistors...

• 470Ω	• 390Ω	• 330 Ω	• 270Ω	• 220Ω
• 180Ω	• 150Ω	• 120Ω	• 100Ω	• 82Ω

2. Desolder and remove the source resistor from the circuit.

3. In succession, clip in each resistor in the sequence. Don't let the source lead short to any other pin. After you make a connection, power up the circuit and retest the voltage.

4. When the voltage is close to 5 volts, test the next resistor in the sequence.

• The new drain voltage isn't as good: Go back one value and solder that part into the source circuit. You're done with the optimization process.

• The new drain voltage is better: Repeat this step.

If Drain Voltage is Too Low...

1. Suppose that you see less than 5 volts at the drain. You need to increase the drain voltage (Vd). You must increase the source resistor value. Buy two insulated clip leads and these resistors...

• 680Ω	• 750Ω	• 820Ω	• 1KΩ	• 1.2KΩ
• 1.5KΩ	• 1.8KΩ	• 2.2KΩ	• 3.3KΩ	• 3.9KΩ

2. Desolder and remove the source resistor from the circuit.

3. In succession, clip in each resistor in the sequence. Don't let the source lead short to any other pin. After you make a connection, power up the circuit and retest the voltage.

4. When the voltage is close to 5 volts, test the next resistor in the sequence.

• The new drain voltage isn't as good: Go back one value and solder that part into the source circuit. You're done with the optimization process.

• The new drain voltage is better: Repeat this step.

• Save the test resistors. If you ever replace the JFET, you'll need to tweak the bias on the replacement part.

Resistor-Swap vs. Potentiometer Method. In case you wonder, the "resistor swap" is a better method than a potentiometer in the FET's source leg. With the resistor-swap method, you wind up with the exact, best standard resistor. With the potentiometer, you find a match, but then you must convert the match to a standard part. Also, you run the risk of measuring the pot the wrong way. Then you have a poorly biased FET. Add to these problems the fact that the potentiometer is a noise source. Plus, a knock or a bump can throw off your careful adjustment. A fixed part is much more dependable.

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