Interesting experiments: some possibilities of the field effect transistor. Scheme, description

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Interesting experiments: some possibilities of the field effect transistor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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It is known that the input resistance of a bipolar transistor depends on the load resistance of the cascade, the resistance of the resistor in the emitter circuit and the base current transfer coefficient. Sometimes it is relatively small, making it difficult to match the stage with the input signal source. This problem completely disappears if you use a field-effect transistor - its input resistance reaches tens and even hundreds of megaohms. To get to know the field effect transistor better, do the suggested experiments.

A little about the characteristics of the field effect transistor. Like the bipolar, the field has three electrodes, but they are called differently: gate (similar to the base), drain (collector), source (emitter). By analogy with bipolar field-effect transistors, there are different "structures": with a p-channel and an n-channel. Unlike bipolar ones, they can be gated in the form of a pn junction and with an insulated gate. Our experiments will concern the first of them.

The basis of the field-effect transistor is a silicon plate (gate), in which there is a thin area called a channel (Fig. 1a). On one side of the channel is the drain, on the other - the source. When the positive transistor is connected to the source, and the negative terminals of the GB2 power battery (Fig. 1, b) are connected to the drain, an electric current appears in the channel. The channel in this case has a maximum conductivity.

Entertaining experiments: some possibilities of a field-effect transistor

It is worth connecting another power supply - GB1 - to the source and gate terminals (plus to the gate), as the channel "narrows", causing an increase in resistance in the drain-source circuit. The current in this circuit immediately decreases. By changing the voltage between the gate and the source, the drain current is regulated. Moreover, there is no current in the gate circuit, the drain current is controlled by an electric field (that is why the transistor is called a field effect transistor), created by the voltage applied to the source and gate.

The above applies to a transistor with a p-channel, but if the transistor is with an n-channel, the polarity of the supply and control voltages is reversed (Fig. 1, c).

Most often, you can find a field-effect transistor in a metal case - then, in addition to the three main conclusions, it may also have a case terminal, which, during installation, is connected to a common wire of the structure.

One of the parameters of the field-effect transistor is the initial drain current (Is beginning), i.e., the current in the drain circuit at zero voltage at the transistor gate (in Fig. 2, a the variable resistor slider is in the lower position according to the circuit) and at a given supply voltage.

If you smoothly move the resistor slider up the circuit, then as the voltage at the transistor gate increases, the drain current decreases (Fig. 2, b) and, at a voltage determined for a given transistor, will drop to almost zero. The voltage corresponding to this moment is called the cutoff voltage (UZIots).

Entertaining experiments: some possibilities of a field-effect transistor

The dependence of the drain current on the gate voltage is quite close to a straight line. If we take an arbitrary increase in the drain current on it and divide it by the corresponding increase in the voltage between the gate and the source, we get the third parameter - the slope of the characteristic (S). This parameter is easy to determine without removing the characteristics or searching for it in the directory. It is enough to measure the initial drain current, and then connect, say, a galvanic cell with a voltage of 1,5 V between the gate and the source. Subtract the resulting drain current from the initial one and divide the remainder by the cell voltage - you will get the slope of the characteristic in milliamps per volt.

Knowledge of the features of the field-effect transistor will complement the acquaintance with its stock output characteristics (Fig. 2, c). They are removed when the voltage between the drain and the source changes for several fixed gate voltages. It is easy to see that up to a certain voltage between the drain and the source, the output characteristic is non-linear, and then, over a significant voltage range, it is almost horizontal.

Of course, a separate power supply is not used in real designs to supply bias voltage to the gate. The bias is formed automatically when a constant resistor of the required resistance is included in the source circuit.

And now pick up several field-effect transistors of the KP103 (with p-channel), KP303 (with n-channel) series with different letter indices and practice determining their parameters using the diagrams given.

Field effect transistor - touch sensor. The word "sensor" means feeling, sensation, perception. Therefore, we can assume that in our experiment, the field-effect transistor will act as a sensitive element that reacts to touching one of its outputs.

In addition to the transistor (Fig. 3), for example, any of the KP103 series, you will need an ohmmeter with any measurement range. Connect the ohmmeter probes in any polarity to the drain and source terminals - the ohmmeter needle will show a small resistance of this transistor circuit.

Entertaining experiments: some possibilities of a field-effect transistor

Then touch the shutter release with your finger. The ohmmeter needle will deviate sharply in the direction of increasing resistance. This happened because the induction of electric current changed the voltage between the gate and the source. The channel resistance increased, which was recorded by the ohmmeter.

Without removing your finger from the gate, try touching the source terminal with another finger. The ohmmeter needle will return to its original position - after all, the gate turned out to be connected through the resistance of the arm section to the source, which means that the control field between these electrodes has practically disappeared and the channel has become conductive.

These properties of field-effect transistors are often used in touch switches, buttons and switches.

Field effect transistor - field indicator. Change the previous experiment a little - bring the transistor with the gate terminal (or body) as close as possible to the mains socket or the wire of a working electrical appliance included in it. The effect will be the same as in the previous case - the ohmmeter needle will deviate in the direction of increasing resistance. It is understandable - an electric field is formed near the outlet or around the wire, to which the transistor reacted.

In a similar capacity, a field-effect transistor is used as a device sensor for detecting hidden electrical wiring or a wire break in a New Year's garland - at this point, the field strength increases.

Holding the transistor-indicator near the mains wire, try turning the appliance on and off. A change in the electric field will be recorded by an ohmmeter needle.

Field effect transistor - variable resistor. After connecting the bias voltage adjustment circuit between the gate and the source (Fig. 4), set the resistor slider to the lower position according to the diagram. The ohmmeter needle, as in previous experiments, will record the minimum resistance of the drain-source circuit.

Entertaining experiments: some possibilities of a field-effect transistor

By moving the resistor slider up the circuit, you can observe a smooth change in the ohmmeter readings (increase in resistance). The field effect transistor has become a variable resistor with a very wide range of resistance changes, regardless of the value of the resistor in the gate circuit. The polarity of connecting the ohmmeter does not matter, but the polarity of switching on the galvanic cell will have to be changed if an n-channel transistor is used, for example, any of the KP303 series. Field effect transistor - current stabilizer.

To conduct this experiment (Fig. 5), you will need a DC source with a voltage of 15 ... 18 mA, yes field effect transistor.

Entertaining experiments: some possibilities of a field-effect transistor

First, set the resistor slider to the lower position according to the diagram, corresponding to the supply of a minimum supply voltage to the transistor - about 5 V, with the values \u2b\u3bof the resistors R1 and R1,8 indicated in the diagram. By selecting the resistor R2,2 (if necessary), set the current in the drain circuit of the transistor to 5 ... 15 mA. By moving the resistor slider up in the circuit, observe the change in drain current. It may happen that it generally remains the same or increases slightly. In other words, when the supply voltage changes from 18 to 1 ... 2 V, the current through the transistor will be automatically maintained at a given level (by resistor RXNUMX). Moreover, the accuracy of maintaining the current depends on the initially set value - the smaller it is, the higher the accuracy. An analysis of the stock output characteristics shown in Fig. XNUMX will help confirm this conclusion. XNUMX, in.

Such a cascade is called a current source or current generator. It can be found in a wide variety of designs.

Author: B.Ivanov

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