电子学1---**General Contextual Information (Left Column):** circuit. In our opinion that is unnecessarily complicated and unintuitive. Not only does circuit behavior tend to be revealed to you as something that drops out of elaborate equations, rather than deriving from a clear understanding in your own mind as to how the circuit functions; you also have the tendency to lose sight of which parameters of transistor behavior you can count on and, more important, which ones can vary over large ranges. In this chapter we will build up instead a very simple introductory transistor model and immediately work out some circuits with it. Soon its limitations will become apparent; then we will expand the model to include the respected Ebers-Moll conventions. With the Ebers-Moll equations and a simple 3-terminal model, you will have a good understanding of transistors; you won’t need to do a lot of calculations, and your designs will be first-rate. In particular, they will be largely independent of the poorly controlled transistor parameters such as current gain. Some important engineering notation should be mentioned. Voltage at a transistor terminal (relative to ground) is indicated by a single subscript (C, B, or E): VC is the collector voltage, for instance. Voltage between two terminals is indicated by a double subscript: VBE is the base-to-emitter voltage drop, for instance. If the same letter is repeated, that means a power-supply voltage: VCC is the (positive) power-supply voltage associated with the collector, and VEE is the (negative) supply voltage associated with the emitter. **Section Title:** 2.01 First transistor model: current amplifier **Introductory Text for Rules:** Let’s begin. A transistor is a 3-terminal device (Fig. 2.1) available in 2 flavors (npn and pnp), with properties that meet the following rules for npn transistors (for pnp simply reverse all polarities): **Transistor Properties/Rules (Right Column):** 1. The collector must be more positive than the emitter. 2. The base-emitter and base-collector circuits behave like diodes (Fig. 2.2). Normally the base-emitter diode is conducting and the base-collector diode is reverse-biased, i.e., the applied voltage is in the opposite direction to easy current flow. 3. Any given transistor has maximum values of IC, IB, and VCE that cannot be exceeded without costing the exceeder the price of a new transistor (for typical values, see Table 2.1). There are also other limits, such as power dissipation (ICVCE), temperature, VBE, etc., that you must keep in mind. 4. When rules 1–3 are obeyed, IC is roughly proportional to IB and can be written as * **Formula:** IC = hFEIB = βIB * **Explanation:** where hFE, the current gain (also called beta), is typically about 100. Both IC and IE flow to the emitter. Note: The collector current is not due to forward conduction of the base-collector diode; **Chart/Diagram Descriptions:** **Figure 2.1:** * **Type:** Transistor symbols and physical packages. * **Title:** Figure 2.1 Transistor symbols, and small transistor packages. * **Main Elements:** * **NPN Transistor Symbol:** A standard NPN transistor circuit symbol, showing terminals labeled "collector", "base", and "emitter". The emitter has an arrow pointing outwards from the base. Label "npn" is below the symbol. * **PNP Transistor Symbol:** A standard PNP transistor circuit symbol, showing terminals labeled "C" (collector), "B" (base), and "E" (emitter). The emitter has an arrow pointing inwards towards the base. Label "pnp" is below the symbol. * **TO-5 / TO-18 Package:** A cylindrical metal can package with three leads emerging from the bottom. The leads are labeled "C", "B", "E". Text labels "TO-5" and "TO-18" are next to it. * **TO-92 Package:** A black, rectangular plastic package with three leads emerging from the bottom in a linear arrangement. The leads are labeled "E", "B", "C" from left to right. Text label "TO-92" is next to it. **Figure 2.2:** * **Type:** Internal diode models representing an ohmmeter's view of a transistor's terminals. * **Title:** Figure 2.2 An ohmmeter’s view of a transistor’s terminals. * **Main Elements:** * **NPN Transistor Internal Model:** Shows three terminals: "B" (Base), "C" (Collector), and "E" (Emitter). Two diodes are depicted: one connecting B to E (anode at B, cathode at E) and another connecting B to C (anode at B, cathode at C). Current arrows indicate "IB" flowing into B and "IC" flowing into C. * **PNP Transistor Internal Model:** Shows three terminals: "B" (Base), "C" (Collector), and "E" (Emitter). Two diodes are depicted: one connecting E to B (anode at E, cathode at B) and another connecting C to B (anode at C, cathode at B). Current arrows indicate "IB" flowing out of B and "IC" flowing out of C.