FET Preamplifier

J201 JFET works best at 10 volts. The minimum current is 0. 5 mA. These are all ideal specifications. The specification spread of FETs is normally three to five times. I`ve personally seen a five-X variation in brand-name devices. The devices that I`ve recently tested are from the same batch. These devices perform more like one another. I intend to operate the test circuits from a 9-volt, regulated power supply.

My goal for proper bias of the MPF102 is 4. 5 to 7 volts at the FET drain. If the MPF102 doesn`t bias properly, I`ll perform a second set of tests. In these tests, I`ll take the MPF102 circuit and swap in source resistors. When I find a source resistor that produces drain voltage in the range, I`ll rerun the source and drain voltage tests. Then I`ll check to see if the MPF102 produces voltage gain or attenuates the voltage signal. If the drain resistor is substantially larger than the source resistor, the circuit has voltage gain. This statement assumes that when using these resistor values, the circuit biases the device correctly. I built the circuit on a plug board. Then I plugged in one test device. I powered the device and measured the drain and source voltages. I then replaced the test device with the next device of the same type, and repeated my tests. As I expected, I had to adjust the bias on the MPF102 transistors. The table below summarizes my results. I tested three of each type of JFET. For each transistor, the table has two columns. In the left column, I give the drain voltage. In the right column, I give the source voltage. Also note that I have a second "MPF102" column on the right. Under this heading are three columns. These three columns detail my adjustment to the MPF102 bias. The leftmost column is the new source bias resistor value. The middle column is the new drain voltage. The right column is the new...

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