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Exp_8_fa09 - Physics 3330 Experiment#8 Fall 2009 Field...

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Physics 3330 Experiment #8 Fall 2009 Field Effect Transistors and Noise Purpose In this experiment we introduce field effect transistors. We will measure the output characteristics of a FET, and then construct a common-source amplifier stage, analogous to the common-emitter bipolar amplifier we studied in Experiment 7. We will also learn to measure amplifier noise, and use our common-source amplifier to measure the thermal noise of a resistor. Introduction As we discussed in Experiment 7, transistors are the basic devices used to amplify electrical signals. They come in two general types, bipolar transistors and field effect transistors (FETs). The input to a FET is called the gate, analogous to the base of a bipolar transistor. But unlike the situation with bipolar transistors, almost no current flows into the gate, and FETs are nearly ideal voltage amplifiers with very high input impedance. In junction FETs (JFETs) the gate is connected to the rest of the device through a reverse biased pn junction, while in metal-oxide-semiconductor FETs (MOSFETs) the gate is connected via a thin insulating oxide layer. Bipolar transistors come in two polarities called npn and pnp, and similarly FETs come in two polarities called n-channel and p-channel. In integrated circuit form, small MOSFETs are ubiquitous in digital electronics, used in everything from simple logic circuits to the 50-million transistor Pentium IV processor chip. Small MOSFETs are also used in some op-amps, particularly when very low supply current is needed, as in portable battery-powered circuits. Small discrete (single) MOSFETs are not normally used because they are extremely fragile. Large discrete MOSFETs are used in all sorts of high power applications, including commercial radio transmitters. JFETs generate very little noise themselves, thus a JFET input op-amp is often the first choice for low-noise amplification. Discrete JFETs are commonly seen in scientific instruments. In this experiment we will study an n-channel JFET (the 2N4416A) with excellent low-noise performance. Like bipolar transistors, JFETs suffer from wide “process spread”, meaning that critical parameters vary greatly from part to part. We will start by measuring the properties of a single device so that we can predict how it will behave in a circuit. Then we will build a common-source amplifier from our characterized JFET, and check its quiescent operating voltages and gain. Noise is an important subject in electronics, especially for scientists who need to construct sensitive instruments to detect small signals. Some experiments are limited by external interference that is not intrinsic to the measuring instrument, but if that can be removed there will still be noise Experiment #8 8.1 Fall 2009
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generated by the measuring electronics itself. The main sources of this noise are usually 1) the thermal noise of resistors, an unavoidable consequence of the equipartition theorem of statistical mechanics, and 2) noise from active components such as transistors.
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