Physics 3330 Experiment #7 Fall 2005 Experiment #7 7.1 Fall 2005 Bipolar Transistor Amplifiers Purpose The aim of this experiment is to construct a bipolar transistor amplifier with a voltage gain of minus 25. The amplifier must accept input signals from a source impedance of 1 kΩ and provide an undistorted output amplitude of 5 V when driving a 560 Ω load. The bandwidth should extend from below 100 Hz to above 1 MHz. Introduction An electrical signal can be amplified using a device which allows a small current or voltage to control the flow of a much larger current from a dc power source. Transistors are the basic device providing control of this kind. There are two general types of transistors, bipolar and field-effect. The difference between these two types is that for bipolar devices an input current controls the large current flow through the device, while for field-effect transistors an input voltage provides the control. In this experiment we will build a two-stage amplifier using two bipolar transistors. In many practical applications it is better to use an op-amp as a source of gain rather than to build an amplifier from discrete transistors. A good understanding of transistor fundamentals is nevertheless essential. Because op-amps are built from transistors, a detailed understanding of op-amp behavior, particularly input and output characteristics, must be based on an understanding of transistors. We will learn in Experiments #9 and #10 about digital electronics, including logic circuits and microcontrollers. These integrated circuits are also made from transistors. In addition to the importance of transistors as components of op-amps, digital circuits, and an enormous variety of other integrated circuits, single transistors (usually called “discrete” transistors) are used in many circuit applications. They are important as interface devices between integrated circuits and sensors, indicators, and other devices used to communicate with the outside world. High-performance amplifiers operating from DC through microwave frequencies use discrete transistor “front-ends” to achieve the lowest possible noise. Transistors are generally much faster than op-amps. The device we will use this week has a gain-bandwidth product of 300 MHz. The three terminals of a bipolar transistor are called the emitter, base, and collector (Figure 7.1). A small current into the base controls a large current flow from the collector to the emitter. The current at the base is typically one hundredth of the collector-emitter current. Moreover, the large current flow is almost independent of the voltage across the transistor from collector to emitter. This makes it possible to obtain a large amplification of voltage by taking the output voltage from a resistor in series with the collector. We will begin by constructing a common emitter amplifier,Experiment #7 7.2 Fall 2005 which operates on this principle. A major fault of a single-stage common emitter amplifier is its high output impedance. This can be cured by adding an emitter follower as a second stage. In this circuit the control signal is again applied at the base, but the output is taken from the emitter. The emitter voltage precisely follows the base voltage but more current is available from the emitter. The common emitter stage and the emitter follower stage are by far the most common bipolar transistor circuit configurations. Figure 7.1 Pin-out of 2N3904 and 1 k trimpotEBCEBCcwccwwiperccwwipercw Readings 1. The basic theory for this experiment is covered in Horowitz and Hill, Chapter 2. H&H give an excellent introduction to practical bipolar circuit design without unnecessary mathematics. The most important sections are 2.01–2.03, 2.05, the first page of 2.06, 2.07, 2.09–2.12, and the part of 2.13 on page 84 and 85. Have a look at Table 2.1 and Figure 2.78 for a summary of the specifications of some real devices. 2. A data sheet for the 2N3904 transistor is posted on our course web site. 3. (Optional) Diefenderfer Sections 8.1-8.6. 4. (Optional) Bugg gives a brief account of the solid state physics behind transistor operation in Chapter 9. In Chapters 17 and 18 he discusses transistor circuit design in the language of h-parameters and hybrid-π equivalent circuits, i.e. with more mathematical detail than is normally required for circuit design. You will probably find the chapter in H&H more useful.Experiment #7 7.3 Fall 2005 Theory CURRENT AMPLIFIER MODEL OF BIPOLAR TRANSISTOR From the simplest point of view a bipolar transistor is a current amplifier. The current flowing from collector to emitter is equal to the base current multiplied by a factor. An npn transistor operates with the collector voltage at least a few tenths of a volt above the emitter voltage, and with a current flowing into the base. The base-emitter junction then acts like a forward-biased diode with a 0.6 V drop: VB ≈ VE + 0.6V. Under these conditions, the collector current is proportional to the base current: IC = hFE IB. The constant of proportionality is called hFE because it is one of the "h-parameters," a set of numbers that give a complete description of the small-signal properties of a transistor (see Bugg Section 17.4). It is important to keep in mind that hFE is not really a constant. It depends on collector current (see H&H Fig. 2.78), and it varies by 50% or more from device to device. If you want to know the emitter current rather than the collector current you can find it by current conservation: IE = IB + IC = (1/hFE + 1) IC. The difference between IC and IE is almost never important since hFE is normally in the range 100 – 1000. Another way to say this is that the base current is very small compared to the collector and emitter currents. Figure 7.2 shows the two main bipolar transistor circuits we will consider. In the emitter-follower stage the output (emitter) voltage is always 0.6V (one diode drop) below the input (base) voltage. A small signal of amplitude δV at the input will therefore give a signal δV at the output, i.e. the output just “follows” the input. As we will see later, the advantage of this circuit that it has high input impedance and low output impedance. In the common emitter stage of figure 7.2b, a small signal of amplitude δV at the input will again give a signal δV at the emitter. This will cause a varying current of amplitude δV /RE to flow Figure 7.2