It would however be improper to round off the series without also admitting that bipolar transistors are only part of the story. There is another family of transistors which have analogous circuit configurations to their bipolar cousins but work in a completely different way: the Field Effect Transistors, or FETs. That said, the FET is a fascinating device. Join us as we take an in-depth look at their operation, and how and where you might use one.
|Published (Last):||14 March 2019|
|PDF File Size:||18.45 Mb|
|ePub File Size:||1.76 Mb|
|Price:||Free* [*Free Regsitration Required]|
JFETs are low-power devices with a very high input resistance and invariably operate in the depletion mode, i. Most JFETs are n-channel rather than p-channel devices. Two of the oldest and best known n-channel JFETs are the 2N and the MPF, which are usually housed in TO92 plastic packages with the connections shown in Figure 1 ; Figure 2 lists the basic characteristics of these two devices.
All practical circuits shown here are specifically designed around the 2N, but will operate equally well when using the MPF The JFET can be used as a linear amplifier by reverse-biasing its gate relative to its source terminal, thus driving it into the linear region. Three basic JFET biasing techniques are in common use. Suppose that an I D of 1mA is wanted, and that a V GS bias of -2V2 is needed to set this condition; the correct bias can obviously be obtained by giving Rs a value of 2k2; if I D tends to fall for some reason, V GS naturally falls as well, and thus makes I D increase and counter the original change; the bias is thus self-regulating via negative feedback.
In practice, the V GS value needed to set a given I D varies widely between individual JFETS, and the only sure way of getting a precise I D value in this system is to make Rs a variable resistor; the system is, however, accurate enough for many applications, and is the most widely used of the three biasing methods.
This system thus enables I D values to be set with good accuracy and without need for individual component selection. Similar results can be obtained by grounding the gate and taking the bottom of Rs to a large negative voltage, as in Figure 4 b.
The third type of biasing system is shown in Figure 5 , in which constant-current generator Q2 sets the I D , irrespective of the JFET characteristics. This system gives excellent biasing stability, but at the expense of increased circuit complexity and cost. In the three biasing systems described, Rg can have any value up to 10M, the top limit being imposed by the volt drop across Rg caused by gate leakage currents, which may upset the gate bias.
When used as linear amplifiers, JFETs are usually used in either the source follower common drain or common-source modes. Figure 7 shows a source follower with offset gate biasing. Overall voltage gain is about 0. C2 is a bootstrapping capacitor and raises the input impedance to 44M, shunted by 10pF.
Figure 8 shows a hybrid JFET plus bipolar source follower. Offset biasing is applied via R1-R2, and constant-current generator Q2 acts as a very high-impedance source load, giving the circuit an overall voltage gain of 0.
Note then if the high effective value of input impedance of this circuit is to be maintained, the output must either be taken to external loads via an additional emitter follower stage as shown dotted in the diagram or must be taken only to fairly high impedance loads. Figure 9 shows a simple self-biasing common source amplifier; RV1 is used to set a quiescent 5V6 across R3.
Figure 10 shows a simple self-biasing headphone amplifier that can be used with headphone impedances of 1k0 or greater. It has a built-in volume control RV1 , has an input impedance of 2M2, and can use any supply in the 9V to 18V range. Figure 11 shows a self-biasing add-on pre-amplifier that gives a voltage gain in excess of 20dB, has a bandwidth that extends beyond kHz, and has an input impedance of 2M2.
It can be used with any amplifier that can provide a 9V to 18V power source. Figures 12 and 13 show circuits of these types. Q1 is used as a source follower, with its gate grounded via the R1 to R4 network and is offset biased by taking its source to -4V via R5; it consumes about 1mA of drain current. In Figure 14 , R6-RV2 and Q1-R5 act as a Wheatstone bridge network, and RV2 is adjusted so that the bridge is balanced and zero current flows in the meter in the absence of an input voltage at Q1 gate.
Any voltage applied to Q1 gate then drives the bridge out of balance by a proportional amount, which can be read directly on the meter. To use the Figure 14 circuit, first trim RV2 to give zero meter reading in the absence of an input voltage, and then connect an accurate 0. Repeat these adjustments until consistent zero and full-scale readings are obtained; the unit is then ready for use.
In practice, this very simple circuit tends to drift with variations in supply voltage and temperature, and fairly frequent trimming of the zero control is needed. Drift can be greatly reduced by using a zener-stabilized 12V supply.
Figure 15 shows an improved low-drift version of the JFET voltmeter. Q1 and Q2 are wired as a differential amplifier, so any drift occurring on one side of the circuit is automatically countered by a similar drift on the other side, and good stability is obtained.
The circuit is set up in the same way as that of Figure With the values shown, the circuit cycles at a rate of once per 20 seconds, i. When a large negative control voltage is fed to Q1 gate, the JFET acts like a near-infinite resistance and causes zero signal attenuation, so the circuit gives high overall gain but, when the gate bias is zero, the FET acts like a low resistance and causes heavy signal attenuation, so the circuit gives an overall signal loss.
Intermediate values of signal attenuation and overall gain or loss can be obtained by varying the control voltage value. The circuit can accept input signal levels up to a maximum of mV RMS Q1 and R4 are wired in series to form a voltage-controlled attenuator that controls the input signal level to common emitter amplifier Q2, which has its output buffered via emitter follower Q3. Thus, the output level stays fairly constant over a wide range of input signal levels; this characteristic is useful in cassette recorders, intercoms, and telephone amplifiers, etc.
In this case, Q1 acts like an electronic switch that is wired in series with R1 and is gated on and off at a 1kHz rate via the Q2-Q3 astable circuit, thus giving the DC-to-AC conversion. To prevent this, connect a DC input and then trim RV1 until the output is just on the verge of decreasing; once set up in this way, the circuit can be reliably used to chop voltages as small as a fraction of a millivolt.
Part 3 of 4. Field-Effect Transistors FETs are unipolar devices, and have some big advantages over bipolar transistors. This four-part series takes a close look at FETs, their basic operating principles, and practical ways of using them. Need to brush up on your electronics principles? These multi-part series may be just what you need! Everything for Electronics.
Forum Blogs Feedback Techforum Newsletter. Part 4 of 4. Popular Stories Wirespondence! Learning Electronics Need to brush up on your electronics principles? Figure 2. Self-biasing source follower. Source follower with offset biasing. Simple self-biasing common-source amplifier. Simple headphone amplifier. General-purpose add-on pre-amplifier. Common-source amplifier with offset gate biasing.
The RadioBoard Forums
JFETs are low-power devices with a very high input resistance and invariably operate in the depletion mode, i. Most JFETs are n-channel rather than p-channel devices. Two of the oldest and best known n-channel JFETs are the 2N and the MPF, which are usually housed in TO92 plastic packages with the connections shown in Figure 1 ; Figure 2 lists the basic characteristics of these two devices. All practical circuits shown here are specifically designed around the 2N, but will operate equally well when using the MPF The JFET can be used as a linear amplifier by reverse-biasing its gate relative to its source terminal, thus driving it into the linear region. Three basic JFET biasing techniques are in common use. Suppose that an I D of 1mA is wanted, and that a V GS bias of -2V2 is needed to set this condition; the correct bias can obviously be obtained by giving Rs a value of 2k2; if I D tends to fall for some reason, V GS naturally falls as well, and thus makes I D increase and counter the original change; the bias is thus self-regulating via negative feedback.
FET Principles And Circuits — Part 2
The RadioBoard Forums Forums for radio builders and experimenters. Beginners and Experts Welcome! Skip to content. Quick links. The datasheet says the centre leg is the gate. Not so on these. This leads me to the assumption that they are not "genuine" manufactured s, but something that has specs nearer so they branded them as such.
2N3819 JFET. Datasheet pdf. Equivalent
Article : Andy Collinson Email :. The FET is a voltage controlled device and has a very high input impedance. A smaller input voltage controls a larger output current. This property is called transconductance. To produce the common source characteristic curves the circuit was setup as shown in Figure 1. Using LTspice a DC sweep was performed.
Introduction to 2n3819