J201 Fet

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Unfortunately, Tillman's amplifier requires a J201 FET. The J201 is a very special device and an excellent choice. Yet it's uncommon to hobbyists. But my site solves the mystery. I've designed a similar preamplifier with an everyday JFET. ABOUT MPF102 SUBSTITUTES. N CHANNEL JFET, -40V, TO-92; Breakdown Voltage Vbr:-40V; Gate-Source Cutoff Voltage Vgs (off) Max:-1.5V; Power Dissipation Pd:350mW; Operating Temperature Range:-55°C. FET Preamp Parts List Q1 J201 N-channel JFET R1 3.0M ohm 1/4-watt 5% resistor R2 2.2K ohm 1/4-watt 5% resistor R3 6.8K ohm 1/4-watt 5% resistor. J201 Datasheet pdf - N-Channel JFET General Purpose Amplifier - Calogic Quick jump to: 1N 2N 2SA 2SC 74 AD BA BC BD BF BU CXA HCF IRF KA KIA LA LM MC NE ST STK TDA TL UA Up1 LM317 LM339 MAX232 NE555 LM324 7805 2N3055 LM358 2N2222 74LS138 TDA7294 TL431 IRF540 1N4148 5Q0765RT.

  1. J201 Fet
  2. J201 Jfet Pdf
  3. J201 Fet Datasheet

A Guitar Preamp
with the MPF102 JFET

Now, with Easy to Find Parts! Some time ago, J. Donald Tillman designed an elegant JFET guitar preamplifier. He posted the circuit on the Web. Overnight, this circuit became very popular. And it's still a winner today. The reasons why are easy to see. This amp is easy to build. The circuit works like a champ. And the JFET's tube-like warmth sounds great. Plus, the high-impedance input won't load your guitar. And the medium output impedance combats high-frequency losses due to loading and long cables. And by the way, this FET preamp is great for microphones, too!

Bad News..and Good. Unfortunately, Tillman's amplifier requires a J201 FET. The J201 is a very special device and an excellent choice. Yet it's uncommon to hobbyists. But my site solves the mystery. I've designed a similar preamplifier with an everyday JFET.


Many Advantages of MPF102. I've read the forums. Hobbyists want to use the famous MPF102 in Tillman's original design. The MPF102 is an excellent choice. It handles more current than the J201, making the MPF102 more rugged. The MPF102 can provide a stronger signal into a lower impedance load. And like many other JFETs, the MPF102 is a low-noise device. The MPF102 has a lower gain than the J201: An advantage, because it won't overload as fast as the J201 does. Still, Tillman optimized his design for the J201. The specs for an MPF102 are very different. Ignore the differences, and your amplifier might not work. For example..

  • The J201 requires only 0.2mA to 1 mA of drain current.
  • The MPF102 operates on 2 to 20 mA. Run it on less, and you may get an attenuator instead of an amplifier! I can prove that statement. See.. Proof

Summary of the Differences

On the table, notice that the MPF102 requires more current than does the J201. The current requirement is a major reason why you can't just plug an MPF102 into Tillman's circuit. This is a very poor substitution. It will disappoint you. (You might as well strum piccolo music on a bass guitar. You'll have more luck.)

Device IDSS
(Drain current, mA) 		VGS GFS (Gain) RDS Typical
Vdd
J201 0.2 - 1 -40 0.5mS + 750Ω 10
MPF102 2 - 20 -25 2 - 6.5mS 150Ω 15

Achieve results that you can be proud of! When you build a circuit, you go to a lot of work. Make the effort pay off! My page offers a new preamp design, starting with the MPF102's specs. Now you have the right design for an MPF102 JFET preamplifier. Build carefully. You'll wind up with a satisfying project that you'll appreciate for years.


Oncoming Scarcity of MPF102 & J201

Best download app. ♦ CAUTION. Major manufacturers are exiting the JFET market. Others are ceasing production of leaded JFETs or MOSFETs. For instance, Siliconix has already left the JFET market.

In 2011, Fairchild obsoleted the MPF102 and J201. Other JFETs that are compatible with the MPF102 (2N3819, 2N5458, etc.) are also obsolete! When current stocks run out, Fairchild will not provide more. For the time being, you can still buy these devices from vendors or factory representatives (see below.)

In February 2013, Radio Shack discontinued the MPF102 (part 276-2062). Soon thereafter, stores ran out their stocks forever. Other vendors sell various substitutes, many by NTE. You'll have to order online. I recommend the NTE457. For more suggestions, click MPF102 Substitutes.


Substitute Parts

Fairchild continues to manufacture the J113, a possible substitute for the MPF102. Vendors such as Mouser and Digi-Key sell the J113.

Other sources continue to make the MPF102 and J201..

  • InterFET, the former producer of the Siliconix JFET line, continues to manufacture the J201. InterFET also provides devices that seem compatible with the MPF102: Use the Interfet 2N3822 or J113. Note that device pinouts might not match the MPF102 pinout. For an InterFET sales representative in your area, contact InterFET.
  • Central Semiconductor continues to manufacture the 2N3819. The 2N3819 is a reliable substitute for the MPF102. (The pinout differs.) Central also offers the J201, which you'll need for the Tillman circuit. Prices for these “end-of-life” devices tend to be high.
  • Ebay Electronics Parts can find you deals on JFETs from China. Some of these deals are impressive. If you can arrange for air shipment, your order can arrive in a few days or weeks. I don't know if the parts come from Chinese foundries or elsewhere. Chinese MPF102s plug right into my circuit. I've bought 2N5457, J113, and 2SK370 JFETs from China. So far, the I've received excellent parts.

Catalog and Online Vendors..

Datasheet
•Allied •Avnet •Digi-Key
•Mouser •Newark •Radio Shack

Find a Vendor in Your Area..

•Contact Central, Fairchild, or InterFET for a local representative!

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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

Copyright © 2007 by James T. Hawes. All rights reserved.

•URL: http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm
•Webmaster: James T. Hawes
•Revision—November, 2020 •Page design tools: HTML, Notepad & Explorer

Experiment: MPF102 vs. J201 in Tillman Circuit

Objective

To find out if the MPF102 has any positive gain in the Tillman JFET preamplifier circuit.

Hypothesis

In the Tillman circuit, the MPF102 will attenuate (lose) signal strength, rather than amplifying.

Discussion

The Tillman JFET preamplifier uses a J201 JFET. Many builders substitute the more available MPF102 part. These builders usually leave the rest of the circuit as is. For more satisfactory operation, some people replace the source resistor with a potentiometer. This pot adjusts the JFET bias. By adjusting the pot, the user can set the amplifier's operating point for the lowest distortion output. Adding the pot is a means of coping with differences between the J201 and the MPF102.

Schematic, JFET test circuit

Bias isn't the only difference, though. Current is also very important. For best operation, the MPF102 requires at least four times the current that the J201 requires. For this reason, in the Tillman circuit, the MPF102 will probably attenuate. The Tillman circuit causes the MPF102 to operate outside its specifications. Operating the MPF102 this way will likely produce non-optimal performance. According to the datasheet, the MPF102 needs a minimum of 2 mA of current. The maximum current figure is 20 mA. The recommended voltage is 15 volts. The J201 is another matter. The 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.

Procedure

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.

Results

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 source voltage.

MPF102 Performance in Tillman Circuit

J201 MPF102 MPF102
Device # VD VS VD VS RS VD VS
1 7.68 0.41 2.90 1.95 8.2K 7.07 2.43
2 7.53 0.46 2.70 2.03 8.2K 6.89 2.60
3 7.46 0.48 2.69 2.03 8.2K 6.85 2.61

Table Key

J201 Fet

  • VD = Drain voltage, between drain and ground.
  • VS = Source voltage, between source and ground.
  • Rs = Source resistor value, in kilohms.
  • Power voltage: 9 VDC, from regulated supply.
  • Circuit: JFET with 3 carbon resistors (RG, RD and RS), on plug board.
  • Devices: 3 of each type. Fairchild devices from same, 2007 Mouser Electronics order.
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The output (drain) voltages on all three J201 FETs are above 7 volts. I' d="" expect="" all="" three="" to="" clip="" when="" the="" gate="" goes="" negative.="" apparently="" all="" three="" devices="" have="" more="" gain="" than="" tillman's="" j201="" had.="" for="" better="" performance,="" a="" builder="" should="" decrease="" the="" source="" resistor.="" the="" source="" voltage="" will="" then="" drop,="" causing="" the="" drain="" voltage="" to="" drop,="">

In every test of the MPF102, the Tillman 2.2K source resistor was too small. This too-small value didn't produce enough voltage to properly bias the device. The output voltage of all three MPF102 devices dropped to around 2 volts. This voltage will guarantee clipping when the gate goes positive.

To correct the clipping problem, I provided a second set of MPF102 tests (right columns). In these tests, I increased the source resistor. I tried several resistors, with 6.8K producing about the best bias. (In one MPF102 test, VD = 6.6 volts and VS = 2.36 volts.)

MPF102
Device # RS VD VS
1 6.8K 6.60 2.36

To approach Tillman's results, though, I substituted an 8.2K source resistor. With this resistor, my drain voltages rose close to 7 volts. To match results from the J201, I'd have had to increase the source resistor even further. I could plainly see the trend. In any case, the high, 8.2K source resistor value causes signal attenuation. A larger value would attenuate even more. My preferred value of 6.8K produces an amplifier with a gain of just less than 1 (unity). This 'best case' is a no-gain amplifier!

The voltage gain formula for these experiments is..

[ ( RD / ( RS + ( 1,000 / GFS ) ) * -1 ]

RD & RS in ohms GFS in millisiemens

Conclusions

As the table shows, using an MPF102 in the Tillman circuit causes severe clipping. You can remedy the clipping by increasing the source resistor. Unfortunately, the new source resistor reduces gain to 1 or even attenuates the signal. The upshot: As an amplifier, the circuit fails. There are two ways to correct this problem..

  • Decrease the source and drain resistors, allowing more current to flow through the JFET. Then adjust the source resistor for optimal bias and gain. For gain, the source resistor must be smaller than the drain resistor.
  • Bypass the source resistor with a large-value, electrolytic capacitor. 100 to 1,000 μF should be sufficient. Unfortunately, the gain now varies across the bandwidth. An amplifier with a bypassed source resistor favors higher frequencies. In this circuit, all capacitors are high-pass filters. For better low-frequency response, use a large capacitor value. That's why I suggest a value as large as 1,000 μF.

J201 Jfet Pdf

Notes

Effect of Load. The voltage gain formula describes gain into a high-impedance (virtually infinite) load. In the real world, a load of less than some 100K affects the gain. As the load drops below 10 times the drain resistor, the gain decreases noticeably. Because the load parallels the drain resistor, the load must affect the gain.

Transconductance. Transconductance varies with the amount of current through the device. For the MPF102, the Fairchild datasheet gives the transconductance (GFS)as 2.0 to 6.5. But Fairchild measures these figures with 15-volt, 2 mA power to the device. In the Tillman circuit, the device average power is 9 volts at 0.5 mA. With such power, the MPF102 transconductance must be less than the datasheet spec. This lower transconductance further decreases preamplifier gain.

J201 Fet Datasheet


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