Soldering On (Part 4)
MOSFETs and more
More vital information for constructors — Tim Edwards introduces FETs and UJTs.
In the October issue we looked at bipolar transistors. This month we will deal with the other common transistors, namely the field effect and unijunction transistors (FETs and UJTs).
Firstly the FETs, of which there are two main types; junction gate FETs or JFETs and insulated gate FETs or metal oxide semi-conductor FETs (IGFETs or MOS-FETs). Fig 1 shows the symbols for JFETs. Note the relative polarities of the device connections and that these vary for n and p channel transistors. Field effect transistors are so called because the current through them is controlled by an electric field produced by applying a voltage between gate and source. This field 'pinches' the current so changing the amount of current that the device can pass as the gate-source voltage (Vgs) changes.
In the ideal FET there is no current flow through the gate connection because the gate-source PN junction is reverse biased. In the MOSFET the gate is separated from the rest of the device by an insulating layer of silicon oxide. So FETS are voltage controlled devices and one advantage over bipolar transistors becomes immediately clear. They have a very high input resistance ie, they don't load the signal source appreciably. Fig 2 shows a practical common source amplifier using an n-channel JFET. The gate is required to be negative with respect to the source terminal. This is achieved by the inclusion of the source resistor and choosing the correct ratio of values for the gate bias resistors.
Circuits like this usually give a fairly low value of voltage gain due to the device characteristics, but are useful for the first stage of an amplifier where high impedance is a benefit. They also exhibit much lower noise figures than equivalent bipolar devices. Fig 3 shows the symbols for n and p channel MOSFETs and Fig 4 illustrates a MOSFET amplifier. Notice that with this type of FET, although the device is n channel, the gate voltage is positive with respect to the source in order to make the device function. With the n channel JFET the gate voltage is negative.The action of the device is essentially the same, but with the MOSFET, a zero gate voltage gives a minimum current through the transistor. With the JFET the maximum current flows with zero gate voltage.
The unijunction transistor is a completely different device. It should more properly be called a double based diode. Fig 5 shows a circuit for a UJT oscillator. Note that the terminals are now called base 1, base 2 and emitter (b1, b2 and e). The channel between b1 and b2 normally behaves as a high resistance and because of the potential developed across it, the emitter-base one junction remains reverse biased and therefore non-conducting. When the voltage at the emitter rises to 0.4-0.8Vbb (where Vbb is the voltage between b1 and b2), the e-b1 junction becomes forward biased. The emitter current then rises rapidly. This characteristic is the reason for the UJT often being used as a capacitor discharge device, as in the relaxation oscillator.
Feature by Tim Edwards
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