The impedance of a capacitor is directly proportional to the frequency and the capacitor effectively blocks ac voltage. False Coupling capacitors can be approximated as ac shorts. True The ac collector voltage of a commonemitter transistor circuit is inverted 360° out of phase with the input voltage. False A bypass capacitor is similar to a coupling capacitor because it appears open to ac and shorted to dc. False False The ac equivalent circuit model called the Ebers-Moll looks like a T on its side and is sometimes called a T model. True The common-emitter amplifier can be recognized by noting that the emitter is at ac ground. True The common-collector amplifier has its ___________at ac ground. collector The impedance of a capacitor is ___________ proportional to the frequency. inversely Integrated circuit means all components like resistors, capacitors, and transistors are created and connected on a chip, a piece of semiconductor material. True A __________ capacitor is used to connect an ac signal into an amplifier without disturbing its operating point. Coupling Distortion is undesirable in high-fidelity amplifiers because it changes the sound of voice and music The reactance of a coupling capacitor at dc voltage is _________. True. Infinite One way to reduce distortion in a transistor amplifier is to keep the base ac input voltage large. The ac output voltage of an amplifier divided by the ac input voltage is referred to as the amplifier’s ___________ gain. voltage ___________is the term that describes components such as resistors, capacitors, and transistors that are separately inserted and connected to get a final circuit. Discrete The stretching and compressing of alternate half cycles at the output of a transistor amplifier is called _______________. what kind of circuit? A)discrete B)integrated C)chat D)protection Distortion is undesirable in high-fidelity amplifiers because A)it changes the sound of voice and music B)it amplifies the sound of voice and music C)it trips a circuit breaker D)cannot be heard in either speaker Distortion In transistor amplifier notation, lowercase letters and subscripts are used for _____________ quantities. Ac In a well-designed coupling circuit, the coupling capacitor approximates A)an ac open B)a dc short C)a dc open D)an ac short The ac output measured at the collector of a common-emitter amplifier is A)in phase with the input voltage B)180° out of phase with the input voltage C)270° out of phase with the input voltage D)90° out of phase with the input voltage The ac base-emitter voltage divided by the ac emitter current equals A)ac resistance B)dc resistance C)reactance D)impedance ) C What is an amplifier called when its collector is at ac ground? A)common-emitter B)common-collector C)common-base When all components are simultaneously created and connected on a piece of semiconductor material, it is referred to as Coupling capacitors prevent the ac source and load resistance from A)changing the amplifier gain B)changing the operating point C)changing the supply voltage D)changing the input resistance SEC. 9-1 BASE-BIASED AMPLIFIER Good coupling occurs when the reactance of the coupling capacitor is much smaller than the resistance at the lowest frequency of the ac source. In a basebiased amplifier, the input signal is coupled into the base. This produces an ac collector voltage. The amplified and inverted ac collector voltage is then coupled to the load resistance. SEC. 9-2 EMITTER-BIASED AMPLIFIER Good bypassing occurs when the reactance of the coupling capacitor is much smaller than the resistance at the lowest frequency of the ac source. The bypassed point is an ac ground. With either a VDB or a TSEB amplifier, the ac signal is coupled into the base. The amplified ac signal is then coupled to the load resistance. SEC. 9-3 SMALL-SIGNAL OPERATION The ac base voltage has a dc component and an ac component. These set up dc and ac components of emitter current. One way to avoid excessive distortion is to use small-signal operation. This means keeping the peak-to-peak ac emitter current less than one-tenth of the dc emitter current. SEC. 9-4 AC BETA The ac beta of a transistor is defined as the ac collector current divided by the ac base current. The values of the ac beta usually differ only slightly from the values of the dc beta. When troubleshooting, you can use the same value for either beta. On data sheets, hFE is equivalent to β dc, and hfe is equivalent to β . SEC. 9-5 AC RESISTANCE OF THE EMITTER DIODE The base-emitter voltage of a transistor has a dc component VBEQ and an ac component vbe. The ac base-emitter voltage sets up an ac emitter current of ie. The ac resistance of the emitter diode is defined as vbe divided by ie. With mathematics, we can prove that the ac resistance of the emitter diode equals 25 mV divided by dc emitter current. SEC. 9-6 TWO TRANSISTOR MODELS As far as ac signals are concerned, a transistor can be replaced by either of two equivalent circuits: the π model or the T model. The π model indicates that the input impedance of the base is β r'e. SEC. 9-7 ANALYZING AN AMPLIFIER The simplest way to analyze an amplifier is to split the analysis into two parts: a dc analysis and an ac analysis. In the dc analysis, the capacitors are open. In the ac analysis, the capacitors are shorted and the dc supply points are ac grounds. SEC. 9-8 AC QUANTITIES ON THE DATA SHEET The h parameters are used on data sheets because they are easier to measure than r' parameters. The r. parameters are easier to use in analysis because we can use Ohm’s law and other basic ideas. The most important quantities are the data sheet are hfe and hie. They can be easily converted into >β and r'e. AC Current Gain: This refers to the amplification capability of a transistor or other active electronic component in an alternating current (AC) circuit. It's usually represented by the symbol "hfe" or "β" and indicates the ratio of small changes in collector current to small changes in base current when the device is operating with an AC signal. AC Emitter Resistance: In a transistor amplifier circuit, the AC emitter resistance represents the dynamic variation of the emitter resistor due to AC signals. It affects the gain and frequency response of the amplifier by influencing the input and output impedances. AC Equivalent Circuit: An AC equivalent circuit simplifies a complex electronic circuit, usually containing active components like transistors or amplifiers, into an equivalent circuit that models its behavior for AC signals. This simplified circuit retains essential characteristics relevant to AC analysis. AC Ground: In AC circuits, the AC ground is a reference point for measuring voltages and currents. It's typically a low-impedance point connected to the circuit's common terminal or the earth ground. It ensures a consistent reference for AC signals. AC Short: An AC short, also known as a short circuit, occurs when two points in a circuit that are not supposed to be connected together become electrically connected. This usually results in unintended and potentially damaging flow of current. Bypass Capacitor: A bypass capacitor, often called a decoupling capacitor, is used in electronic circuits to provide a low-impedance path for high-frequency AC signals while blocking DC signals. It helps to reduce noise and stabilize the voltage levels at specific points in the circuit. CB Amplifier: A Common Base (CB) amplifier is a type of transistor amplifier configuration. In a CB amplifier, the base terminal of the transistor is the common terminal for both the input and output signals. This configuration provides high voltage gain but relatively low current gain. CC Amplifier: A Common Collector (CC) amplifier is another transistor amplifier configuration. In a CC amplifier, the collector terminal is the common terminal for both the input and output signals. It offers unity voltage gain (or slightly less) but high current gain. CE Amplifier: A Common Emitter (CE) amplifier is a commonly used transistor amplifier configuration. In a CE amplifier, the emitter terminal is the common terminal for both the input and output signals. It provides high voltage gain and moderate current gain. Coupling Capacitor: A coupling capacitor is a capacitor used in electronic circuits to transmit AC signals between different stages while blocking DC signals. It allows the AC component of a signal to pass through while preventing any DC bias from affecting the following stage. DC Equivalent Circuit: A DC equivalent circuit is a simplified representation of an electronic circuit when analyzing its behavior under direct current (DC) conditions. It involves replacing active components like transistors with their biasing arrangements and resistances to model the circuit's static characteristics. Distortion: Distortion in electronics refers to any undesired alteration or deformation of a signal's waveform. It occurs when the output signal does not accurately reproduce the input signal. Distortion can introduce various types of signal degradation, such as harmonic distortion and nonlinear distortion. Ebers-Moll Model: The Ebers-Moll model is a mathematical model that describes the behavior of bipolar junction transistors (BJTs). It explains the transistor's current-voltage relationships and how the currents flow between its terminals under different biasing conditions. π Model: The π (pi) model is a common smallsignal equivalent circuit model used to analyze and design bipolar junction transistor (BJT) amplifiers. It simplifies the BJT's behavior for small variations around its operating point, making it easier to analyze in AC conditions. a. Zero Small-Signal Amplifiers: Small-signal amplifiers are electronic circuits designed to amplify weak AC signals while maintaining linearity. These amplifiers operate within a limited range of input signals where they can be approximated as linear devices, allowing for simplified analysis using linear circuit theory. d. Average Superposition Theorem: The superposition theorem is a principle used in linear circuit analysis. It states that in a linear circuit with multiple sources, the response (e.g., voltage or current) at a particular point can be determined by considering each individual source separately and then summing up their effects. 4. In a bypass circuit, the top of a capacitor is T Model: The T model is another small-signal equivalent circuit model used to analyze and design bipolar junction transistor (BJT) amplifiers. It's an alternative representation to the π model, offering different insights into the transistor's behavior for AC signals. Voltage Gain: Voltage gain is a measure of how much an amplifier increases the amplitude of an input voltage signal to produce a larger output voltage signal. It's usually expressed as a ratio or in decibels (dB) and provides insight into an amplifier's amplification capability. c. Minimum b. Maximum 3. A coupling capacitor is a. A dc short b. An ac open c. A dc open and an ac short d. A de short and an ac open a. An open b. A short c. An ac ground d. A mechanical ground 5. The capacitor that produces an ac ground is called a a. Bypass capacitor b. Coupling capacitor c. DC open d. AC open 6. The capacitors of a CE amplifier appear to be a. Open to ac 1. For de, the current in a coupling circuit is a. Zero b. Maximum b. Shorted to de c. Open to supply voltage d. Shorted to ac c. Minimum 7. Reducing all de sources to zero is one of the steps in getting the d. Average a. DC equivalent circuit 2. The current in a coupling circuit for high frequencies is b. AC equivalent circuit c. Complete amplifier circuit b. Base-emitter voltage d. Voltage-divider biased circuit c. Collector current 8. The ac equivalent circuit is derived from the original circuit by shorting all d. AC base voltage a. Resistors b. Capacitors c. Inductors d. Transistors 9. When the ac base voltage is too large, the ac emitter current is a. Sinusoidal b. Constant c. Distorted d. Alternating 10. In a CE amplifier with a large input signal, the positive half cycle of the ac emitter current is a. Equal to the negative half cycle b. Smaller than the negative half cycle c. Larger than the negative half cycle d. Equal to the negative half cycle 11. AC emitter resistance equals 25 mV divided by the a. Quiescent base current b. DC emitter current c. AC emitter current d. Change in collector current 12. To reduce the distortion in a CE amplifier, reduce the a. DC emitter current 13. If the ac voltage across the emitter diode is 1 mV and the ac emitter current is 100 μA, the ac resistance of the emitter diode is a. 1 Ω b. 10 Ω c. 100 Ω d. 1 k Ω 14. A graph of ac emitter current. ^versus ac base-emitter voltage applies to the a. Resistor b. Emitter diode c. Collector diode d. Power supply 15. The output voltage of a CE amplifier is a. Amplified b. Inverted c. 180° out of phase with the input d. All of the above 16. The emitter of a CE amplifier has no ac voltage because of the a. DC voltage on it b. Bypass capacitor c. Coupling capacitor d. Load resistor 17. The voltage across the load resistor of a capacitor-coupled CE amplifier is a. DC and ac b. DC only c. AC only d. Neither de nor ac 18. The ac collector current is approximately equal to the a. AC base current b. AC emitter current c. AC source current d. AC bypass current 19. The ac emitter current times the ac emitter resistance equals the a. DC emitter voltage b. AC base voltage c. AC collector voltage d. Supply voltage 20. The ac collector current equals the ac base current times the a. AC collector resistance b. DC current gain c. AC current gain. d. Generator voltage 21. When the emitter resistance Re doubles, the ac emitter resistance a. Increases b. Decreases c. Remains the same d. Cannot be determined.
