Biasing JFETS - Breadboard A Bare-Ass Boost

2013 By Small Bear Electronics LLC
This article explains some basics of how a JFET (Junction Field Effect Transistor) works and how to measure the parameters that make it work as an amplifier. I will walk you through breadboarding a two-stage "Bare-Ass" clean boost with tone control, an extremely useful utility for stage or recording studio. There are other sources for much of the material that I present here, but I sometimes found their language and level of detail hard-to-follow. I will focus on useful basics and practicalities, and I'll do my best to make the theory accessible to other non-engineers.

If you have never used a solderless breadboard, please refer to the intro article on breadboarding before you continue. That How-To covers a lot of basic information and techniques; I presume that you have been through it, have done the demos, understand how the tool works and have started learning to use your multimeter. If you are already familiar with breadboarding, note that the level of detail in this article is meant to guide complete beginners, so please be a little patient.

Looking at the typical structural drawing of an "N-channel" JFET (Fig. 1), it's a bar of N-type silicon with contacts at each end and a junction of P-type material diffused into it. The ends of the channel are called the Drain (D) and Source (S) respectively. The junction is called the Gate (G).

It occurred to me that since we have a p-n junction, the Gate-To-Drain and Gate-To-Source ought to behave as silicon diodes, and so they do (Fig. 2).

So how does the device work for purposes of controlling a voltage or current? Forward biasing by making the gate positive with respect to the source won't buy us anything; current will simply flow as in a normal diode, and the gate junction can be destroyed if forward current gets excessive. But things get interesting when the gate is reverse-biased by making it negative with respect to the source.

A "depletion region" (Fig.3) is created that progressively chokes off current flow through the channel. If the reverse bias is high enough, almost no current flows. A relatively small change in the bias voltage causes a relatively large change in the current through the channel, so the JFET can function as an amplifier or switch.

A Few Notes On Reading A Schematic
In the next few drawings, I'm going to set up to measure some essential parameters of a common JFET. When reading any schematic, it's important to note:
  • Where wires cross in a schematic but Don't connect, you'll see this: 

  • Where wires cross or join in a schematic and Do connect, you'll see this: 

  • The Ground symbol is not a connection to the Earth; it's the reference point from which all voltages in the circuit are measured. All points in the circuit that have or share the Ground symbol are connected. If this is not clear yet, you'll get the idea as we go further.

What's IDss?
I've noted that we don't want the gate to be positive with respect to the source, and reverse-biasing it begins to limit current flow through the channel. So if we put both the gate and the source at ground (Fig. 4), the gate is neither forward-biased nor reverse-biased. IDss is the current through the device when the gate is at the same voltage as the source. Let's set this up on the breadboard and measure IDss for one of the JFETs that we will be using, the J201.

Figure 5 shows the pinout of the device. This is the same for many common ones, but not all, so check your datasheet if you are using something else. For clarity, I have used a few jumpers to spread out the connections in figure 6. Your multimeter should be set to one of its low-current scales, typically 2 ma. or less.

So this device has an IDss of .32 milliamperes, or 320 microamperes. Just for grins I checked the datasheet, and the acceptable range is .2 to 1 ma. (200 to 1000 microamperes). This one meets the spec!
Now let's see how it behaves when we put a reverse bias on the gate as in figure 7.

The 51K resistor and the 50K pot are connected as a voltage divider; since they are roughly equal in resistance, half the battery voltage is dropped across the resistor and half across the pot. The gate of the JFET is connected to the wiper so, as the wiper goes more clockwise (CW), it will see from zero to about .75 volts negative with respect to ground. Figure 8 shows the pot about half-way clockwise and the Drain current considerably lowered. The voltage at which the channel is closed and drain current is minimum is the pinchoff voltage, Vp .

Notes On Doing These Demos...

If you bought the breadboard kit, it includes two devices for which Vp has already been measured by the method shown here. The J201 in the kit  will have a Vp of -1.0 V or smaller. If you are doing a similar demo using your own J201s, be aware that Vp for that part can be as high as -1.5 V. To test parts with a Vp between -.75 V and -1.5 V, leave out the 51K resistor. Other popular devices, like the 2N5457, for example, can have a Vp of -6 volts or greater. For those, use a 9 volt battery rather than the AA penlight.

We're going to want to measure gate voltage against drain current and not have to use two meters, so the meter in the Drain circuit is replaced with a 100 ohm resistor as in figure 9 (Thanks to Runoffgroove!). Now the current in the Drain circuit can be read as a voltage drop across the resistor.

For example, say that .2 ma. (.0002 amp) or thereabouts is flowing in the drain circuit as shown above. By Ohm's law, the voltage drop across the 100 ohm resistor is I * R or .0002 * 100 = .02 volts. And that's what it is when I actually measure it (Fig. 10). Multiplying the voltage reading by 10 expresses the current in ma.

Now I am good to take a series of measurements of gate voltage vs. drain current, raising the gate voltage in equal steps to the point where drain current measures zero (or very small). This voltage is the pinchoff voltage, or Vp. My measurements gave me the table in figure 11. The Vp is -.43 V, which exceeds the minimum -.3 V specified in the datasheet.

Call me obsessive...I actually graphed my data and got a slope like figure 12.

Fig. 11


Bias ID
-0 V .32 ma.
-.02 V .29 ma.
-.04 V .27 ma.
-.06 V .25 ma.
-.08 V .22 ma.
-.1 V .2 ma.
-.12 V .19 ma.
-.14 V .17 ma.
-.16 V .15 ma.
-.18 V .14 ma.
-.2 V .12 ma.
-.22 V .11 ma.
-.24 V .10 ma.
-.26 V .09 ma
-.28 V .07 ma.
-.3 V .06 ma
-.32 V .05 ma.
-.36 V .03 ma.
-.38 V .02 ma.
-.4 V .01 ma.
-.43 V 0 ma.
So why are these parameters so important? Because they allow us to express the "gain" of the device, which for a JFET is transconductance or gm. Transconductance for a JFET (or a tube, for that matter) is the change in current through the device divided by the change in the voltage on the controlling element, in this case the gate. The old unit for expressing transconductance was the mho--ohm spelled backwards. The modern unit is the Siemens.
Working from the data in figure 10, a change in bias of .1 Volt, say from -.1 to -.2, produces a change in current of .08 milliamps (.2 - .12).

.08 /.1 = .8 Millisiemens or .0008 Siemens

Anticipated FAQ #1: Do I really need to know/do all of this in order to build something that works?
A: Yes, If you are using unsorted JFETs And you want to be able to predict the performance of your build. The reason is that JFETs of a given type number vary A Lot in the parameters that I have discussed.
Anticipated FAQ #2: What devices are in the breadboard kit?
A: The breadboard kit includes two devices that are characterized for Vp, IDss and gm. The device for the first stage will be a J201 with a Vp of -.3 to -1 V. The device for the second stage will have Vp of -.9 to -2.0 V, and it may be one of several types depending on available stock and results from sorting.
Creating A Practical Amplifier - Biasing The Gate
If we set the bias on the Gate such that ID is somewhere in the middle of its possible range, the current will be able to properly "swing" up and down in response to an input signal. But it isn't practical to use a potentiometer to do this; unless the resistance of the pot were extremely large, it would load down any input device, and we would lose the high input resistance of the FET that is its big advantage. Figure 13 shows one way that the biasing is typically done, often called "self-biasing".

The resistor from the gate to ground will be a very high value--typically 1 Meg or more. It allows the gate to "see" ground, but no voltage is dropped across it because current doesn't flow into the gate. In the drain circuit, we know that current flows from drain to source. By putting a small resistor RS between the source and ground, the gate becomes effectively negative with respect to the source by the amount of the voltage drop (ID * RS) across the resistor.

Looking at the graph and the data in figure 11, I chose to set the bias at -.18 V, which puts ID at .14 ma. (.00014 amp). By Ohm's law, the required resistance is then: .18 / .00014 or 1285 Ohms.

We could use a 1% resistor to get really close, but that is not necessary for most practical purposes. I set RS at 1.2K, the nearest standard value in 5% tolerance and added the 100 ohm resistor to be able to easily measure ID (Fig. 14). As you can see from the readings in Fig. 15 and Fig. 16, the bias is almost spot-on.

The breadboard kit includes a range of 5% carbon film resistors from which to select a value that sets ID close to half of IDSS.

Setting The Value of Drain Resistor RD
   
We need a resistance in the Drain circuit across which the varying current can develop an output voltage. If we want the voltage at the Drain to be able to swing equally both positive and negative in response to an input signal, it follows that the Drain resistance RD should be such as to put the Drain at roughly half the supply voltage, or about 4.5 volts. It's possible to calculate that resistance (see the Fetzer Valve article at Runoffgroove). However, it works as well for stompbox purposes to insert a trimpot in the Drain circuit and adjust that to set the Drain voltage (Fig. 17, Fig. 18).
Set Up The Input and Output

To make this into a usable stage, we need to add three capacitors: input, output and source bypass (Fig. 19). The purpose of the first two is pretty clear; we need to pass the AC signal voltages in and out while blocking DC from getting into the guitar or following amplifier stages. The source bypass capacitor CS, a 10 mf. electrolytic, provides a signal path around RS. By preventing RS from dropping the signal voltage, the gain of the stage is made much greater. We may not always want that much gain, so a switch is included to connect or disconnect it the capacitor. Fig. 20  is a closeup of the breadboard setup.

Now we need to add input and output jacks for instrument and  amplifier as in Figure 21. Figure 21 shows mono jacks. This type has two contacts: tip and sleeve. Figure 22 shows the relationship between the schematic symbol and the physical item. Figure 23 does the same thing for a stereo jack, which has three contacts: tip, ring and sleeve. The stereo jack shown is a Switchcraft #12B; note that the arrangement of the contacts may be different on other makes.

In a typical pedal build, we would use a stereo jack for the input and connect the ring contact to switch battery power. To keep things simple here, we use only the tip and sleeve. Solder a short length of shielded cable to each jack, making sure that the shield is soldered to the sleeve. (Ordinary insulated wire will work, but shielded cable reduces some of the noise inherent in an open layout like this.) Add bare wire terminations to the ends that go to the breadboard just as you did for the battery snap.

 

The shields are plugged to the ground bus. Input tip goes to the input capacitor, output tip to output capacitor. Connect your gear with the amplifier set to low volume and connect a battery. Things should look like figure 24, and you should hear a definite clean boost. Flip the switch and make sure that you hear the difference in gain with and without the source bypass cap. You can leave the trimpot in place if you want to be able to tweak further later on. It's also your choice as to whether to set it for 4.5 volts or for maximum volume. I prefer to measure the resistance of the trimpot and find the value of an appropriate fixed resistor. For this device, RD at about 34K puts the Drain at 4.5 volts (Fig. 25). I substituted the nearest 5% value, 33K.

The breadboard kit includes both the trimpot and an assortment of resistors in the range of 10K to 100K.

How Much Does It Boost?  
Or to ask it another way: What is the gain of the stage? Even if we stop building here and just add an output level control, we do want to know how much output to expect for a given input signal. Remember that I said that transconductance was the equivalent of gain for a JFET? For this circuit, with the source resistor bypassed, the stage gain is:

 R (ohms)  x  gm (Siemens)

So:  33,000  x  .0008  =  26.4

So for a nominal guitar signal of 10 millivolts (.01 volt), we should see something like 260 millivolts at the output.

 
Measuring The Actual Stage Gain
I took this project as an excuse to build something that I should have made or bought long ago: a signal source with an output of known amplitude. R. G. Keen's Test Oscillator is one good way to go for this. However, I recently started offering a Velleman mini-kit that does a similar job. So I built one, added a DC power jack, RCA output jack, level pot and a rotary switch to select waveform and boxed it. At 1 KHz it gives a choice of square, triangle or faux sine output at up to 100 millivolts nominal (Fig. 26).

I connected the signal source in place of my guitar and used the AC voltage scale of my multimeter to measure the voltage at input and output (Fig. 27 and 28). 7.4 mv. in, 185 mv. out, factor of 25...Whaddya know, the gain formula is about right!

Flipping the switch to take the source bypass out cuts the stage gain by a little less than half (Fig. 29).

   
Add A Tone Stack, With Bypass  
If I only wanted a flat "utility" boost, I could stop right here, add an output level control and start to create a board. But this kind of preamp is way more useful if it has a tone control. We also need the ability to switch that in or out; inserting a tone stack reduces the overall gain, and we might or might not want it in the signal path.

 Figure 30 is the basic schematic for the tone stack that is used in many popular pedals, like the E-H Big Muff and the BOSS DS-1. It's simple and effective, and it gives a good sweep from bass to treble with a single pot. The capacitor and resistor in red form a high-pass filter, and the resistor and cap in blue bleed treble to ground. So what gets through the tone stack depends on the position of the wiper of the tone pot.

I asked Paul Reid (PRR of diystompboxes.com) for help with setting the R-C values of the tone stack, and he was most generous with advice. Especially, he suggested higher resistances to match the input load. He also directed me to the Duncan Tone Stack Calaculator, which is a great tool for experimenting with the profile of this section. Figure 31 shows the schematic with my chosen component values and bypass switch, and Figure 32 shows the added components on the breadboard.

I tested this much by connecting the output jack tip to the output contact of the bypass switch and verifying that the bypass and the tone control worked properly.

How Does The Tone Stack Affect The Boost?
Bring back the multimeter and signal generator: Fig. 33 shows the output voltage from the first stage with the tone stack inserted. It was 185 mv. without the tone stack, and now it is down about one-third because the tone stack is loading the JFET output. Fig. 34 shows the output from the tone stack. Between loading and insertion loss, the tone stack cuts the overall gain by well over half. But that's OK...

Add A Recovery Stage
Why not? The whole thing will still fit on a small board, and a gain of 10, or even less,  for the recovery stage will deliver a substantial fraction of a volt of output--plenty of signal for many studio and stage jobs. Switch the tone stack out and you can have line level or greater. However, we need to pay close attention to selecting a suitable device for Q2.

A basic JFET design rule for avoiding distortion is that the input signal voltage to the Gate must not exceed Vp. I presumed a nominal 10 millivolt input signal to the first stage, but peaks may be several times higher depending on who's playing and how hot the pickups are. Vp for the first-stage device was .43 V. Now figure input peaks of 30-50 millivolts and overall gain of 10 through the tone stack...Vp needs to be much higher for the second stage. So I sorted a bunch of parts and came up with one that had these specs:

IDss = .72 ma.    Vp = .936 V   gm  = .875 ms

This device will boost about as much as the other one, but it will tolerate a much larger input signal. Once again, I measured the bias voltage that would set the Drain current to 50% of IDss, or .36 ma. For this device it was .296 V, so the Source resistance RS  =  E/I =  .296 / .00036  =  822 ohms. I stuck an 820 ohm resistor in there, and Ohm's Law rules again (Fig. 35, Fig. 36):

The breadboard kit contains an assortment of resistors for biasing the source.

Now for the Drain load resistor RD.  I'm going to shoot for a gain of 10 with the source resistor bypassed. So:

AV (symbol for voltage gain) = R (ohms)  x  gm  So 10  =  R (ohms)  x  gm   Solving for RD:  10 / .000875 = about 11.4K. I'm going to try 11K and see what the input and output voltages look like.

If you are following these procedures on your breadboard, you can either:

  • Do the arithmetic above based on the gain you choose and the gm of your device to get a value for RD, or
  • Insert a trimpot, set it to a value that puts 4.5 Volts on the Drain, measure the resistance and substitute a fixed resistor.

I'm putting a 1 Meg level pot at the input to give me control of the signal level going into the recovery stage (Fig. 37, Fig. 38).

Figure 39 shows what I saw at the output with a little over 7 mv. in, both source capacitors connected and the tone stack bypassed. I don't know that anyone would use the circuit this way. However, the gain is there, and it can be throttled back at any of the control points depending on your needs.

Paul Reid suggests inserting a "pad" that equalizes the input to the second stage when the tone stack is bypassed (Fig. 40). He suggests 300K to 470K on top and 100K to 220K on bottom depending on how much attenuation turns out to be needed. I did not breadboard this, but I will leave room for it when laying out a board.

Final Tweeks and Changes
Figure 41 is what I intend to design a board for. The 10K resistor at the input prevents damage to the FET in case the booster is connected to a very high output source, and the pulldown resistor at the output prevents DC leakage into the stage following. If the volume pad is not wanted, the points in red are connected. If the volume pad is added, the wiring is in green.

Where To Go From Here

In the article that will follow this one, I will show how to wire the circuit on perfboard and create a finished pedal in the Bare Box #1. I hope you enjoyed learning to use the breadboard, and that you will use your new skills to hack into other designs and try out your own ideas.

 
While I extend a  furry, bearish Thank You! to both Paul Reid and R. G. Keen for their suggestions and support, any mistakes are mine. Comments and suggestions are welcome at smallbearelec@ix.netcom.com.
 

A Parts List

Here is everything needed to do all of what I have described, including the LED demo in the intro article.

Quantity Description SBE Stock List SKU
  Resistors - All 1/4-watt 5% Carbon Film  
1 100 Ohms 0900, 0901, etc.
1 10K  
1 12K  
1 82K  
1 100K  
1 1 Meg  
1 Source Resistor Bag: 240 ohms to 1.5K (17 values)  
1 Drain Resistor Bag: 2K to 100K (41 Values)
  Potentiometers and Trimmers  
1 100 K Linear 1005A
1 1 Meg Audio 1005A
1 100K Cermet Trimmer 1015
     
  Capacitors  
1 .0039 mf. 50 Volt Polyester Film 1101B or 1150
2 .022 mf. 50 Volt Polyester Film  
1 .01 mf. 50 Volt Polyester Film 1101B or 1150
2 .1 mf. 50 Volt Polyester Film  
2 10 mf. 16 Volt Radial Electrolytic 1400
1 47 mf. 16 Volt Radial Electrolytic  
     
  Transistors and Diodes  
2 Selected JFETs  
     
  Wire and Tubing  
  Bare Tinned Copper Wire, #22 or #24 0509
  Insulated Tinned Copper Wire, #22 or #24 0508M
  Shielded Cable 0510
  1/16" heat shrink 0500
     
  Jacks, Fittings, Switches  
1 Mono Jack, Switchcraft #11 0600
1 Stereo Jack, Switchcraft #12B 0602
1 9-Volt Battery Snap 0619
2 Sub-miniature SPST Toggle Switch 0222
1 Sub-miniature DPDT Toggle Switch 0223
     
  Tools  
1 Breadboard or Breadboard Strip 2700, 2700A, 2700B
1 Multimeter 2701, 2701A