Pre-text exercises. 1. Pronounce the following words paying attention to the letter combi­nations:

1. Pronounce the following words paying attention to the letter combi­nations:

th: they, there, that, without, thus, other, thousand, through, thought, fifth

sh: show, short, relationship, should, finish, sheet, shape, shift

ch: change, which, such, research, characteristic, chemis­try, charge, cheap, check, choice, choose

II. Memorize the pronunciation of the following words:

diagrams, terminals, linear, transfer, domain, amplitude, response, tangent, impedance, successive, attenuation, cycle, delay, ratio, zero

III. Read and translate the text: “AMPLIFIERS”

To " amplify" means to make bigger or enlarge. An amplifier is a piece of electrical equipment for making signals bigger. In Fig. 2 the amplifier is shown as a " black box" with two terminals on the left and two on the right. In diagrams signals move from left to right where possible, so the input signal is connected across the two terminals on the left, and the output is taken from across the two terminals on the right. The box in Fig. 2 is a four terminal device with two ports. A great many amplifiers are three terminal devices as shown in Fig.3, but they are still two port. If an input signal is perfectly reproduced at the output of the amplifier, changed only by being bigger, then the amplifier is " linear". This is shown in Fig. 4. The straight line which gives the relationship between the input and the output signal is called the transfer characteristic for the device, a linear amplifier in this case. The ratio of the amplitude of the output signal to that of the input signal is called the gain. For a linear amplifier the gain is constant, that is, the tangent 9 is a constant. A delay, it is shown in Fig. 4. If all parts of the output signal are delayed by the same amount, this is not bad. Fig. 5 shows an amplifier which must operate under certain conditions in order to be linear.

The input signal amplitude must not be greater than A and must operate about the point P on the transfer characteristic. Point P is the operating point for the amplifier. If the operating point moves, because of changes in temperature or power supply levels, or if the input signal A is too great, then the output signal will be distorted.

So far amplifiers have been described by the use of waveforms, that is, in the time domain. They can also be completely described by their response to sinusoids, that is, in the frequency domain. An amplifier that is linear has the frequency domain characteristics of constant gain for sinusoids over the frequencies of interest and phase delays proportional to their frequencies. Fig. 6 should make this statement clear. When phase delay is proportional to frequency, then the time delay At is constant for all the parts of the signal in the time domain. It is not possible to have the ideal gain and phase characteristics shown in Fig, 6 over the whole frequency range.

Amplifiers are named by the frequency range over which they operate. Amplifiers which have the required characteristics at zero frequency and higher are called d.c. amplifiers. A v.f. amplifier is for voice frequen­cies, 300 to 3, 300 cycles per second (Hertz). A Hi-Fi(high fidelity) amplifier might have a range from 20 to 25, 000 Hz.

An amplifier may have only one active element in it to change the power from its power" supply into enlarged signal wanted. This is called a single stage amplifier. If more amplification is needed, amplifiers can be connected in tandem. For instance, an amplifier with four active elements providing four successive stages of amplification is a four stage amplifier. A power amplifier is designed to transfer the most power to its load. If the load is resistive, then Zout is designed to be equal to R. The gain of a power amplifier is the ratio of the output power (transferred to the load) to that provided at its input. This ratio is written in a special way. For example, if the gain of an amplifier is 100, then this is written first as 10² the 2 is then multiplied by 10, and the result written as 20 db; db means decibel (pronounced " dee-bee", the word " decibel" is rarely used in conversation and " bel" is never heard).

Some amplifiers are designed to be non-1inear. For example, when playing a record a magnetic pick-up head has a poor frequency response at the " low end" (of its frequency range). This is shown in Fig. 7. In order to correct this, signal levels or amplitudes in the low end of the frequency range are emphasized by a non-linear amplifier. When an amplifier is provided only for emphasis to correct the signal, and is then followed, in tandem, by a linear amplifier, then pre-emphasis is being used. The opposite of gain is attenuation. This can be shown graphically as negative gain if a db scale is used, or as a gain less than one if a linear scale is used.