I'm posting this here because I do not know where else to put this.
After a recent discussion with JM Fahey about JFET variations, I started thinking about a test rig idea I came up with a while back. Here is a schematic showing my idea next to a much simpler test rig similar to the one shown on the Fetzer valve page at ROG and, I believe, similar to one JM posted:
With the simple rig on the left with the switch positioned as shown, the absolute value of the JFET's threshold voltage will be shown on the multimeter. In the other position, the 1 Mohm R1 is shunted, and you divide the voltage shown on the Multimeter by the 100 ohm value of R2 to get the IDss.
My rig is shown on the right. U3 (multisim wants to label these as Ux since these parts are not in there master database) enables or disables the test rig according to its gate voltage. U5 is used for shunting R3 to switch between test modes. I added a couple of LEDs to indicate the test mode. The two MOSFETs in each LED setup work as an AND gate. The gate of the top MOSFET in each setup recieves the same enable signal as U3. The bottom MOSFET of the first LED setup recieves the same ON signal as U5. The bottom MOSFET of the second setup recieves an ON signal when the first setup is off.
At first glance, it may seem like I am working on a Rube Goldberg type JFET test rig. After all, this isn't any easier to use than the simpler version, it requires more parts, and the LED indicators are kinda just eye candy. But imagine if you replace the mechanical switches labeled J2 and J3 with the digital outputs of an Arduino board or some other similar board. Then, instead of a multimeter, use an analog input of the Arduino for measurement. Maybe add a button for one of the Arduino digital inputs to tell it when to take the measurements. Then, you simply pop in a jfet, hit a button, and the arduino takes both measurements in an instant. You might not even need the button to tell the Arduino when to take the measurement. You could have an optical switch that is interupted when a JFET is placed in the socket to trigger the measurements. You could write an application to take the Arduino output and automatically populate a spreadsheet or database with your measurements. Then, measuring a bag of JFETs can be done much more quickly.
Take this a step further. Set up a reel feeder to run a reel of JFETs through the tester. Advance the reel one JFET at a time using an optical switch to determine when a JFET is in position. Have a three prong probe or socket move up to engage the pins of the JFET. Then take the measurements. Then advance to the next JFET. Write an application that generates a unique identifier for the reel and print a label with that identifier to put on the reel when you are done measuring. When each measurement is taken, have your application on your PC record the unique identifier, the offset of the JFET from the beginning of the reel, and the JFET's measurements stored in a database. You can then very quickly catalog a whole reel of JFETs.
Tonight, I breadboarded the simple rig and measured several JFETs and recorded them to use for comparison. I then built my rig and demonstrated that it worked. Tomorrow, I'll take measurements of the same JFETs. Then, if I have time, I'll try to get it working with my Arduino. I'll post more stuff later.
I decided to refine my CMOS Power Amplifier design (See CMOS Power Amp for the original version). I played with different ideas, and dropped the idea for a while. Then, tonight, I came up with this (See above schematic). While the schematic for the old version has a MOSFET buffer, I left it out of this design. I'm not that concerned with that part of the design. Several buffers can be used, and might not be needed depending on whatever preamp one uses.
This design has several similarities with the original. The CMOS Inverter is midpoint biased and has variable feedback for gain control. It uses the same MOSFETS.
However, there are some major differences. This looks less like a digital CMOS inverter than the original. While the original has a Class A Bias, this has a Class AB bias using pots R2 and R3 to adjust the bias. The gain pot is between the input and output of the inverter stage, rather than in front. I also decided to see how this works with a 24V supply. Because the biasing pots have some effect on the feedback ratio of this amp, I added R1 to make adjustments to where one has unity gain with R6 set to 50%. The two 1uF capacitors, C1 and C3 help the signal to bypass the biasing pots to reach the gates of U1 and U2. I'll explain pots R5 and R8 in a bit.
Here is a screenshot of this design in a running simulation (paused actually) in MultiSim:
Even with double the voltage, this amp is MUCH more efficient than the original. (That being said, I did not mind that the original was inefficient. I liked having a quiet overdriven amp, and it was still kinda too loud.) This behaves like one would expect a unity gain amplifier to behave. The simulated function generator fed a 24 Vpp (12 Vp see Function Generator window) sine wave at 1 kHz. The output voltage was about 22.7Vpp and was clipping hard. With a slightly smaller input voltage, or with more feedback, the inverter put out a nice sine wave that matched the voltage of the input. The frequency response was fairly flat.
At one point, before adding R5 and R8, I noticed that I was not getting the nice sine wave I had expected, but rather a heavily rounded, clipped output like you might see from a midpoint biased CMOS inverter. This didn't make sense until I noticed that I forgot to connect C3 and C1 to the junction between R1 and R6. After connecting them, I got a slightly but abruptly clipped sine wave like in the above screen shot. It occurred to me that I could use switches to disable C3 and C1 to allow for control of the clipping characteristics of the inverter. Disabling only one could allow for some asymmetrical clipping. Then, I thought, why not use pots to fine tune the clipping characteristics.
The following screen shot shows the results:
Notice how the oscilloscope window shows an output wave form like one might expect from a CMOS inverter. Adjusting R5 and R8 allows one to get something in between this and a hard-clipped sine wave.
R7 simulates an 8 ohm speaker load (not very well of course.) It looks like this can produce a bit more than 8 watts of clean output with an 8 ohm load. If I tweak the bias and try different speaker impedances, I might get more.
This screen shot shows a quiescent current of about 250 mA. After taking this screen shot, I got it to a bit less than 70 mA, which isn't horrible.
Of course, this is all simulation. I haven't bread-boarded any of this yet. When I try to, I might find out that this design doesn't work the way I expect, or maybe doesn't even work at all. I can't wait to find out.
Recently, I built a simple two-stage preamp circuit. Each stage is basically built using the basic criteria for the Fetzer Valve (see runoffgroove.com). If I were to use a more general biasing scheme (in other words, not using the Fetzer Value calculations for Rd and Rs), I imagine I could get all the gain I need with one stage. I chose the Fetzer scheme based on its supposed tube like qualities. There have been recent discussions on this site about possible better ways to emulate a valve/tube with a JFET, but I wanted to keep the circuit simple. I also have not yet compared this design to one using a general JFET biasing scheme. Because it is very simple and makes a good clean preamp or booster, I figured some on this site might find this design useful. The design is certainly not god's gift to guitar-related electronics and isn't all that original, but might be simple enough for a beginner to build while still providing good results.
If anyone uses this design and sticks with an 18 V supply, you may want to scale it down to 9 V if anything you put after this circuit cannot handle an output voltage approching 18 Vpp (The output of this will probably be a volt or two less).
Here's an idea for a mosfet booster. According to multisim, this should have a lot of gain with a very flat frequency response. Of course, real builds may differ substantially. I haven't built this yet. When I do, I'll let you know how it turns out.
In the course of experimenting with MOSFET's, I've come up with a MOSFET Mu Follower booster circuit. You can read more about it here: http://www.riemer.us/index.php?option=com_content&task=view&id=30&Itemid=42. There might be no advantage to using this circuit over a simple Op Amp based booster, but I kinda like it. When critiquing my design, please keep in mind that I do not have a lot of experience with designing electronics, so please keep the criticism constructive. Using two of these stages together makes some crazy distortion. I'm planning on making a couple of pedals based on this design, and experimenting with different tone control circuits to put in between and afterward to tweak the tone.
I've recently designed a 12v amp that I think sounds pretty good. There's more info about it at http://www.riemer.us/mufu-12v. I've attached the schematic. Keep in mind that I'm still a novice, and I'm on a low budget. I think, based on some Multisim simulations of the source follower stage that this can output 2W without much distortion and about 4 watts if driven to square wave distortion. Computer simulation is no substitute for real world measurements, but I don't yet have a working osciloscope. Well, let me know what you think. I hope to learn from any criticisms and I hope this might be useful for others.