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JFETS: How They Work, How to Use Them
May 1969 Radio-Electronics Article

May 1969 Radio-Electronics

May 1969 Radio-Electronics Cover - RF Cafe[Table of Contents]

Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

Last Fall I posted part 2 of this Radio-Electronics article first because I did not yet have the May issue that contained part 1. So, if you read "JFET's - Put Last Month's Theory to Work" and have been waiting with bated breath for part 1, you may breathe easily again; here it is. Author Thomas Haskett enthusiastically introduces readers to the junction field effect transistors (JFET) as a more natural replacement than the bipolar junction transistor BJT) for vacuum tubes because of JFET (and other varieties of the FET) operational parameters being much more those of tubes than a BJT. Regarding his conversion, Haskett refers to himself as a "die-hard 'fire-bottle'" man - a term with which I am not familiar. My assumption is that "fire-bottle" is a slang name for vacuum tubes because of how hot they get, and they glow orange like a fire in a glass bottle. A Google search on the term turns up a lot of results for a guitar amplifier overdrive gizmo called the Fire Bottle, but that is probably not it. Here is one reference to tubes as "fire bottles" (and here). If you have a link to a definitive "fire bottle" definition, please let me know.

JFETS: How They Work, How to Use Them - Cover Story

JFETS: How They Work, How to Use Them, May 1969 Radio-Electronics - RF CafeField-effect transistors have revolutionized solid-state circuit design. Find out why in this fact-filled series - Part 1

By Thomas R. Haskett

In all electronics, not many devices have been so long awaited or so enthusiastically welcomed as the Field-Effect Transistor (FET). Many die-hard fire-bottle men (like me) have been reluctant to use transistors except where they clearly did a better job than tubes, or where size and portability were major considerations.

Conventional transistors, unlike vacuum tubes, don't provide good input-output isolation. Their internal feedback causes distortion and requires complex circuits to deliver fidelity. Further, drift is a problem, and ordinary transistors are highly susceptible to zapping (burnout).

The FET has overcome many of the disadvantages of the conventional transistor, and tube lovers who learn about FET's are happy once again. Perhaps you don't realize that at this moment nearly three dozen different FET's can be had for less than $2.50 each - many for only a buck. They'll do most of the jobs of vacuum tubes and many of conventional transistors. In some cases, they'll do better jobs than any other amplifying devices.

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Fig. 1 - Large current flows when gate is open. JFET pictorial at a, schematic is at b.

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Fig. 2a,-b - Low negative gate bias reduces the current flowing between drain and source.

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Fig. 3-a,-b - Increasing gate bias enlarges fields and pinches off the drain current.

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Fig. 4 - JFET's are of two types: a - symmetrical and (b) nonsymmetrical.

Where FET's Have an Edge

The first thing most people seem to learn about the FET is that its high impedance - like a tube - rather than low impedance - like a conventional transistor. Typical FET input impedance (Z) is 100 megohms, with an output Z of 100,000 ohms.

Why such high impedance? The FET is a voltage-controlled device - like a tube - rather than a current-controlled device - like an ordinary transistor. That is, input voltage controls output current. Because of the FET's high impedance, it can be used in vacuum-tube circuit designs with little change in components. For one thing, you don't have to use electrolytics as coupling capacitors, and you can drive an FET directly from a high-impedance microphone or phono cartridge.

As a dc amplifier, the FET has a zero temperature coefficient. Unlike a tube or ordinary transistor, it does not drift.

As a general-purpose amplifier, the FET has the high power gain of a vacuum tube and less noise than either tube or transistor. Of course, it has no filament or heater, and power requirements are small.

As a vhf mixer, the FET has less noise and cross-modulation than either a tube or an ordinary transistor. This is due to the FET's nearly perfect square-law transfer characteristic. Output current varies with the square of the input voltage; hence the FET produces only second harmonics. Tubes and conventional transistors produce higher-order harmonics, causing more interference.

Other FET pluses include nearly constant current-output characteristic (flat drain-current/drain-voltage curves); almost completely unilateral gain function (nearly perfect input-output isolation), and it can be used as a voltage-controlled variable resistor (attenuator) when operated at low supply voltages.

One unfortunate disadvantage of today's FET's is that they cannot produce much output power. The present limit seems to be a few hundred milliwatts. Some manufacturers, however, are working on power FET's. Such devices would be welcome, and would probably replace ordinary transistors in output stages.

How the FET Works

There are two basic FET types; this article deals only with the first - the Junction FET. (The other type will be covered in a later article.) A basic JFET is shown in Fig. 1-a. It is made up of a bar of n-type semiconductor material with two p-type junctions diffused in its sides. The p-type junctions are connected together and called the gate. At one end of the n-type bar is a terminal called the source, which is connected to the negative pole of the battery. (This terminal is the source of current.) At the other end of the bar is a terminal called the drain, connected to the positive pole of the battery. (This terminal drains off current.) Thus current flows from the negative battery terminal, through the source and the bar (or channel), out the drain and to the positive battery terminal. The bar acts like a resistor, for it limits current flow. The schematic symbol for this basic n-channel JFET is in Fig. 1-b. (It is called an n-channel JFET because the bar, or channel, is made of n-type material.)

In Fig. 2-a, you see the same hookup as before, except that now another battery (with lower voltage than the first) has been connected between the gate and source. Note that the gate is made negative with respect to the source. As shown, little current flows from source to drain. Why? Because the reverse voltage bias applied to the gate causes electric fields, or depletion zones, to form around the p-type gate junctions. These fields oppose and de-crease current flow through the channel, or bar.

If the gate-to-source voltage is increased enough, as in Fig. 3-a, current flow from source to drain stops entirely. This level of gate-to-source potential is known as pinchoff or cutoff voltage. Thus the bar of semiconductor material has become a variable resistor.

Usually JFET's are made with symmetrical geometry. The symbol for such a device is shown in Fig. 4-a. Thus it makes no difference which end of the channel you call the drain or which you call the source - current can flow in either direction. On the other hand, some manufacturers make nonsymmetrical JFET's. The symbol for such a device is shown in Fig. 4-b. In this type of JFET, drain and source terminals are not interchangeable.

Operating Characteristics

A bipolar junction transistor uses both majority and minority current carriers. In an npn version, for instance, most of the electrons flow from emitter to collector; they are the majority carriers. Holes flow the other way, and are the minority carriers. In a pnp bipolar, everything is just the reverse. Bipolars are also called injection transistors, because of electron-hole injection at the base.

Note: With five exceptions, all devices listed are furnished in epoxy cases. The five Dickson IFET's are furnished in TO-18 metal cases. In these devices, the gate is internally connected to the case.

The FET, on the other hand, is a unipolar device. It uses only majority carriers - electrons in the n-channel version, holes in the p-channel type. (During manufacture, impurity atoms are mixed into the channel in such density that only one type of current carriers is available.) And because the gate junction is reverse-biased, neither holes nor electrons are injected into the channel.

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Fig. 5 - Typical curves (a) for JFET, (b) for bipolar transistor, (c) pentode.

All three types of amplifying devices mentioned above employ some type of input bias. The gate of a FET is biased by a voltage with respect to the source. The base of a bipolar transistor is biased by a voltage with respect to the emitter. And the grid of a vacuum tube is biased by a voltage with respect to the cathode. Note that a JFET is normally on until reverse-biased at the gate; a bipolar transistor is normally off until forward-biased at the base, and a vacuum tube is normally on until reverse-biased at the grid.

Characteristic curves for each of the three devices under discussion appear in Fig. 5. The JFET drain-voltage/drain-current curves for various values of base bias are in Fig. 5-b. Bipolar transistor collector-voltage/collector-current curves for various values of base bias are in Fig. 5-6. Pentode vacuum-tube plate-voltage/plate-current curves for various values of grid bias are in Fig. 5-c.

Look at Fig. 5-a again and you'll see one important aspect of JFET operation. As drain-to-source voltage increases from zero, current flows through the channel. Channel current is approximately proportional to drain-source voltage - up to a point. This portion of the curve is known as the ohmic region, because the channel resistance is varied linearly by the current flowing through it. It's also called the triode region because the curve looks like that of a triode tube. (Near the bottom of the ohmic region of the curve, channel resistance is several megohms in a typical JFET.)

But at the knee of the curve, the current has become so great that it sets up a reverse bias between the gate junctions which acts the same as external gate bias - it depletes the channel. The curve flattens out because additional increase in drain-source voltage has little effect on drain current. (The channel resistance has decreased to about 1000 ohms.)

Since this action resembles pinchoff voltage at the gate, this portion of the curve is known as the pinchoff region. The channel is saturated with current flow, so this part of the curve is also known as the saturation region. And since the curve is flat, like that of a pentode, this area is also called the pentode region.

By operating the JFET in the pinchoff region, with drain-source (Vds) voltage between about 6 and 12 (referring to Fig. 5-a), small changes in gate voltage (Vgs) produce large changes in drain current (Id). Thus the JFET is useful as an amplifier.

Beyond the pinchoff region is the avalanche or breakdown region. It's similar to the breakdown region in a bipolar transistor. If total device power dissipation (PD) is not limited to a safe value, the JFET will be destroyed.

Comparison with Other Amplifiers

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Fig. 6-a - N-channel JFET requires negative gate bias. b-Npn bipolar transistor requires positive base bias and triode tube (c) needs negative grid bias.

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Table I - Low-Cost JFET's

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Fig. 7 (above) - Polarities of p-channel JFET (a) and pnp transistor (b) are opposite from those in Fig. 6.

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Fig. 8 (top right) - Basing diagrams for the JFET's listed in Table I, page 25.

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Fig. 9-a, -b (right) - Symbols used for dual-gate symmetrical JFET's. Non-symmetrical dual-gate JFET uses c.

The similarity between the JFET, the bipolar transistor and the vacuum tube, when each is used as a simple af amplifier is shown in Fig. 6. In each case, input is through coupling capacitor CC1 to the FET gate, the bipolar base and the tube grid.

The FET gate is biased (with respect to the source) by the voltage drop across RS the source dropping resistor (Fig. 6-a). The tube grid is also biased by the same method, by the voltage drop across cathode resistor RK (Fig. 6-c). The bipolar transistor is biased by the drop across voltage divider RB1-RB2 between the collector supply and ground (Fig. 6-b). (Some bias also occurs because of emitter resistor RE, but this resistor is also used for stability.)

In each case a bypass capacitor is used across the lower element - CS across the FET source resistor; CE across the bipolar emitter resistor, and CK across the tube cathode resistor. This bypass capacitor allows its associated resistor to act as a voltage dropper for dc, but not for ac.

Output is taken, in each example, across load resistor RL - which is called RD for the FET, RC for the bipolar and RP for the tube. Output is through coupling capacitor CC2.

Now you've seen the similarities of the FET, the bipolar transistor and the vacuum tube. What about differences? Well, FET's are also made. with p-type channels (see Fig. 7). Everything here is just the opposite from the n-channel version above. The channel is p-type material, and the gate junction is n-type material. Drain voltage is negative and source voltage (ground) is positive. The gate is biased positive with respect to the source. This is similar to a pnp bipolar transistor, as Fig. 7-b shows, where the collector is negative, the emitter positive and the base biased positive with respect to the emitter.

Of course, there is no such thing as a p-channel vacuum tube. If you make the plate negative with respect to the cathode the tube simply won't conduct.

There are some differences between n- and p-channel FET's. For one thing, the mobility of electrons is greater than that of holes. Since electrons are the majority current carriers -which is all that FET's use - in n-channel devices, they are better than their p-channel counterparts in several respects. The n's have more gain (for a given input capacitance) than p's. They also have lower on resistance and lower noise figures. As a matter of fact, n-channel JFET's have the best low-frequency noise figures of any amplifying device.

All these benefits aren't achieved without a drawback or two. Like all semiconductor devices, JFET's are manufactured with a rather wide spread of specifications. Table I lists 35 low-cost JFET's and Table II explains their parameters. Nearly all are subject to some variation from one device to the next (of the same type number). For this reason, minimum and! or maximum values are shown. Base terminals for the listed JFET's are in Fig. 8.

Two parameters aren't shown in Table I because they are somewhat similar for most of the devices listed. The first is the resistance of the channel: rds(ON). or drain-to-source on resistance, with gate shorted to source, at 1 kHz. The values range from about 100 to 3000 ohms, depending on the device, and are useful when designing a voltage-controlled attenuator. The off resistance is much greater, of course.

The other parameter not shown in Table I is the noise figure, usually a rating of spot noise at one frequency. Values range from about 0.1 to 4 dB, but this depends pretty much on the specified frequency, which can be 1 kHz for audio frequency JFET's and 100 MHz for vhf devices.

Dual-Gate JFET's

Just as multigrid vacuum tubes are useful for mixing two signals, dual-gate or tetrode JFET's are used as mixers, converters, and in age service. Schematic symbols in current use are in Fig. 9; (a) and (b) being symmetrical and (c) nonsymmetrical. No dual-gate JPET's are listed in Table I because there aren't any which cost less than $2.50.

There it is, the complete JFET operating theory. Next month we'll put this theory to practice. We'll show you how to build a JFET preamp for a ceramic microphone and a simple JFET audio oscillator.


Table II

Important JFET Parameters and What They Mean

BVDGO - Drain-to-gate breakdown voltage, with source open. A maximum rating, this voltage must not be exceeded or the device will be permanently damaged (unless power dissipation is limited to a safe level).

BVGSS - Gate-to-source breakdown voltage, with drain shorted to source. Also a maximum rating which must not be exceeded or else permanent damage results (unless power dissipation is held to a safe level).

PD - Total device power dissipation at ambient temperature of 25°C (77°F). This absolute maximum rating must not be exceeded or the device will be destroyed.

IDSS - Drain-to-source current, with gate shorted to source (zero gate voltage). Also called pinchoff or saturation current. This rating indicates the maximum useful current which can flow through an output load resistor. Exceeding the maximum IDSS rating causes avalanche in the device, resulting in permanent damage (unless power dissipation is held to a safe level).

yfs - Small-signal, common-source, forward transadmittance (sometimes called gfs-transconductance) at 1 kHz. This is the key dynamic figure of merit for FET's, for it indicates how much gain is available.

VP or VGS(OFF) - Gate-to-source pinchoff (or cutoff) voltage. That value of gate bias voltage which reduces drain-to-source current to a very small amount (usually somewhat greater than the lass leakage-current value).

Giss -Input (gate-to-source or common-source) capacitance at 1 MHz. (Also called Cgss and measured with drain shorted to source.) This rating is particularly important in rf and switching applications.

IGSS - reverse-bias gate-to-source leakage current, with drain shorted to source. This leakage current, which is usually low (due to the reverse-biased gate junction) determines input impedance, which is usually high (several hundred megohms ).

 

 

Posted April 30, 2019

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