Jfet is usually a depletion mode device (active device when no gate bias is applied). which makes them still very useful in simple discrete circuits – in particular – automatic gain feedback mechanisms (AGC).
This technology is used in some of the still popular operational amplifiers like opa827 and lf353 which are now old and related family members. the device’s main functions are lower noise and higher bandwidth-gain products.
Junction field effect transistors combine several advantages of conventional and bipolar transistors. some of which are mentioned below:
Advantages of JFET
- Operation depends on majority carrier flow only, therefore, unipolar device (one type of carrier). on the other hand in ordinary transistors both majority carriers and minorities take part in conduction and, therefore, ordinary transistors are sometimes called bipolar transistors. vacuum tubes are another example of a unipolar device. & quot;
- it is simpler to make, smaller in size, rugged in construction and has a longer life and higher efficiency. easier to make in ic form and also lower space requirement.
- it has a high input impedance (of the order of 100 m q), because its input circuit (gate to source) is reversed bias, and hence allows a high isolation level between the input and the output circuit. However, the input circuit of ordinary transistors is forward biased and, therefore, ordinary transistors have low input impedance.
- it carries a very small current because the gate is reversed biased and, therefore, operates like a hollow tube in which the grid control (corresponding to the gate in jfet) carries very small current and the input voltage controls the output current. this is the reason that jfet is basically a voltage driven device (ordinary transistors are devices that are operated current because the input current controls the output current.)
- regular transistors use current to their base to control large currents between the collector and the emitter whereas in the jfet voltage at the terminal gate (base) is used to control the current that flows (the current between the channel and the source). thus the usual transistor gain is characterized by current gain while the jfet gain is characterized as transconductance (drainage current ratio and gate-source voltage).
- jfet has no junction like a regular transistor and conduction is through a large material current carrier (type-n or p-type semiconductor material) that does not pass through the junction. then the sound attached to the tube (due to high temperature operation) and the regular transistor (due to transition junction) is not present in the jfet.
- it is relatively immune to radiation. .
- has a negative temperature resistance coefficient and, therefore, has better thermal stability.
- It has high power and, therefore, the necessity of employing the driver phase is eliminated.
- it shows there is no offset voltage at zero current drain and, therefore, makes chopper a very good signal.
- has the characteristics of the squared law and, therefore, is very useful in radio and tv receiver tuners.
- it has got high frequency response.
The main drawback of jfet is
- the gain-bandwidth product is relatively small compared to conventional transistors.
- greater susceptibility to damage in its handling.
- jfet has low voltage gains because of small transconductance .
- costlier when compared to bjt’s.
