Questions for Review
1. An amplifier is an electronic instrument used to boost, or increase, audio signals from sources such as tuners, tape decks and CD players, in order to create sound. A voltage amplifier increases voltage levels by a multiplicative factor called the "gain." A current amplifier increase the power of audio signals in order to drive loudspeakers.

2. Amplifiers are necessary in hi-fi systems because the signals from audio sources such as phonographs and tape decks are too weak to cause significant enough motion of speakers that generates audible sound.

3. Hi-fi is all about ac signals, i.e. signals that are comprised of complex combinations of sinusoidally varying voltages over the entire audio frequency spectrum.

4. Total harmonic distortion refers to the power in frequencies that are unwanted harmonics of some fundamental. For example, suppose a recording has a whistler creating musical tones that are nearly pure fundamentals. Harmonic distortion in the amplifier would create additional frequencies that were integer multiples of the whistled fundamental frequency, thereby altering the timbre of the sound. Intermodulation distortion arises due to nonlinear effects in amplifiers, whereby a series of frequencies that are related to two fundamental tones are generated. For example, imagine two whistlers generating fundamentals at 1,000 Hz and 1,500 Hz, respectively. Intermodulation distortion would create frequencies that were multiples of their difference, 500 Hz, such as 500 Hz, 2000 Hz, 2500 Hz, etc. Obviously these additional frequencies would alter the reproduction of the intended audio signal.

5. A diode is a two-terminal device that conducts large currents in one direction only. It is comprised of adjacent layers of p-type and n-type semiconductors. When a dc voltage source is connected across the diode such that the p-type side is "biased" at least 0.6 volts higher in potential than the n-type side, large positive current will be able to flow from p-type to n-type, and through the circuit in which the diode is connected.

6. An operational amplifier is a differential amplifier with "+" and "-" inputs, as well as a single output. It's "open loop gain" is A(v₊ - v_-), where A is a constant of around 10,000 - 100, 000, depending on the type of operational amplifier. In other words, its output signal is proportional to the difference between the two input signals. It is constructed on a wafer of silicon in which different regions of doped semiconductor create transistors that are configured in such a way as to make a differential amplifier.

7. Amplifiers introduce noise because they provide gain to input signals, and invariably there will be fluctuations in currents and voltages due to the thermal motion of charge carriers at the operating temperature of the amplifier. Because they have to dissipate heat, this temperature is significantly above room temperature and leads to random motion of charge carriers that create random fluctuations in output voltages and currents.

8. The damping factor in an amplifier is defined as the ratio: (speaker impedance)/(amplifier output impedance). When an audio signal terminates, the oscillations of the speaker will continue unless the speaker is "damped," i.e. unless its circuitry is such that the oscillations quickly die away. If the speaker impedance is significantly larger than the amplifier output impedance, this ensures that the remaining energy will rapidly dissipate as heat in the speaker's internal resistance.

9. The role of the feedback resistor in an operational amplifier is to provide a signal that is opposite the input signal in phase, thereby reducing the input a bit and maintaining a stable output. A resistor provides a frequency-independent connection between the output and "-" input of the operational amplifier, but a capacitor is a frequency-dependent impedance device which doesn't easily pass low frequency currents. The gain of an op-amp depends on the ratio of the feedback impedance to the input impedance. Since the feedback impedance will be very small at high frequencies, the output will approach zero. At low frequencies, since the capacitor cannot transmit dc current, it will charge up to a voltage that is proportional to the voltage at the input averaged over time. Such a circuit is called an "integrator."

10. Negative feedback maintains stability in an amplifier by reducing the impact of fluctuations that occur due to temperature fluctuations, for example. If some change in the operating parameters of the amplifier causes the gain to momentarily increase, for example, the output signal would increase and this would cause the voltage at the inverting input to increase, thereby reducing the output, and negating the effect of the change in operating parameter.

Exercises
1. C. IM rating
2. C. 8 times louder because the S/N is 30 dB less, so the noise is 30 dB greater, 10 dB/doubling
3. B. Damping factor
4. A. the LOUDNESS control
5. B. total harmonic distortion
6. A. a flat response
7. C. 15 - 25,000 Hz +/- 0.4 dB
8. A. 24 k/0.8 k = 30
9. A. increase
10. B. 30 dB...a factor of 1000 is 3 powers of 10, or 30 dB
11. C. the gain is 1000, so the feedback resistor must be 1,220 k ohms
12. A. the part of the output signal that is fed back is 180 degrees out of phase with the input
13. A. gain increases with increasing feedback resistance
14. B. 8 ohm/0.1 ohm = 80, the damping factor
15. B. damping factor is inversely proportional to the amplifier resistance, so X's is 10 times smaller
16. A. amplifier X has a higher signal to noise ratio, so it's a better amplifier than Y, based on this criterion alone
17. A. 7.5/(15x10^-6) = 500,000
18. B. Total Harmonic Distortion - the square wave has the sine wave frequency plus all odd harmonics
19. A. reverse-biased
20. E. all of the above must be included: speaker resistance, frequency range, THD and number of channels driven
21. B. 250 x 125 x 10^-6 Volts = .03125 Volts