Physics 3330 Experiment #8 Fall 2005 Experiment #8 8.1 Fall 2005 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 excel in the low-noise department, and 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.Experiment #8 8.2 Fall 2005 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 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. To introduce the subject of noise we will first measure the noise of the lock-in amplifiers we have in the lab. Next, we will reduce the effects of this noise by using our JFET common source amplifier as a low-noise pre-amplifier for the lock-in. Finally, we will use our JFET amplifier to measure the thermal noise of a resistor. Readings 1. Sections 3.01-3.10 of H&H introduce FETs and analog FET circuits. You might find that this is more than you want or need to know about FETs. In that case, you could read instead Application Note AN101 “An Introduction to JFETs” from Vishay/Siliconix. There is a link to it on our course web site. You will also find on our web site the data sheet for the 2N4416A, the n-channel JFET we will be using. 2. Amplifier and resistor noise is discussed in H&H sections 7.11-7.22. Read at least Sections 7.11 and 7.12. 3. (Optional) If you are on campus you can read the original 1934 papers on resistor thermal noise by Johnson and Nyquist. See the links on our web site.Experiment #8 8.3 Fall 2005 Theory JFET CHARACTERISTICS The schematic symbol shown in Fig. 8.1a is used for the n-channel JFET. For the p-channel version the arrow points the other way and all polarities discussed below would be reversed. The three leads are gate (G), drain (D), and source (S). The path from drain to source through which the output current normally flows is called the channel. In many ways, the gate, drain and source are analogous to the base, collector and emitter of an npn transistor. However, in normal operation the gate voltage is always below the source voltage (this keeps the gate pn junction reverse biased) and almost no current flows out of the gate. The voltage of the gate relative to the source (VGS) controls how much current flows from drain to source through the channel. For more detail, look at the Output Characteristics graphs on page 7.3 of the 2N4416A data sheet. The drain-to-source current (ID) is plotted versus the drain-to-source voltage (VDS) for various values of the controlling gate voltage (VGS). The JFET is normally operated with VDS greater than about 3 V, in the “saturation region” where the curves have a small slope. In this region the current is nearly constant, independent of VDS, but controlled by VGS. Thus the JFET can be viewed as a voltage-controlled current source. The gain of a JFET is described by the transconductance gfs: GSDfsVIgδδ≡ which tells us how much change in drain current results from a small change in the gate voltage. This quantity is called a conductance because it has the dimensions of inverse ohms, and a transconductance because the current and voltage are not at the same terminal. In the SI system one Figure 8.1 a) N-channel JFET b) Measuring Output Characteristics VGI D (aVDS -9 V (bG S D GS D 10 kΩ 5 kΩExperiment #8 8.4 Fall 2005 inverse ohm is called a Siemans (S). According to the data sheet, gfs is guaranteed to be between 4.5 and 7.5 mS at VGS =0 for VDS =15V. Like the β of a bipolar transistor, gfs is not really a constant, and it has large process variations. The transconductance only varies a little with VDS as long as VDS is greater than about 3V. The dependence on VGS is more rapid (see the plot on the data sheet). The transconductance is maximum at VGS =0, where the best noise performance also occurs. Other