III Answer the following questions:

1. What is an electronic amplifier?

2. Where is it used in?

3. How can amplifiers be divided into?

IV Explain the following phrases as they have been used in the text. Translate them into Ukrainian:

an “open loop” performance, common source, common gate, common drain, common plate, slew rate.

V Choose the correct variant from those in brackets:

1. A generator (is, be, are) a machine that converts mechanical energy (in, on, into) electrical energy.

2. All dc motors (have, has, have to) a stationary field member and a rotating armature member.

3. There are (numerous, much, many) types of control systems.

4. The field coils in this (kind, type, sort) of generator have a high resistance.

5. The bearing housing (are made, is made) of cast iron.

VI Translate the text in writing

In recent years, magnetic amplifiers have found considerable application in automatic a.c. and d.c. motor control. As is well known, whenever current is absent in the control winding of the magnetic amplifier or in its exciting winding (which operate with direct current), the a.c. main or load winding will exhibit considerable inductive reactance.

As the flow of direct current is increased in the control winding, the core of the magnetic amplifier will saturate and lead to a decrease in the inductive reactance of the a.c. winding.

In magnetic amplifiers, low power input (applied to the d.c. winding) will be sufficient to considerably change the power available at the amplifier output. The magnetic amplifier can, hence, provide high power gains, reaching values of the order of tens of thousands. Another valuable feature of such apparatus is their extremely high reliability.

To attain a high power gain, as in the case of rotating amplifiers, use is often made of an additional winding to provide self-excitation, This winding is also called the positive feedback winding and may be either series- or parallel-connected.

Unit 12 Function

I Read and memorise the following words:

1. stage – стадія, фаза

2. respond – реагувати

3. precise – точний

4. actuate – приводити в рух

5. feed back – зворотний зв’язок

II Read and translate the text:

Other amplifiers may be classified by their function or output characteristics. These functional descriptions usually apply to complete amplifier systems or sub-systems and rarely to individual stages.

• A servo amplifier indicates an integrated feedback loop to actively control the output at some desired level. A DC servo indicates use at frequencies down to DC levels, where the rapid fluctuations of an audio or RF signal do not occur. These are often used in mechanical actuators, or devices such as DC motors that must maintain a constant speed or torque. An AC servo amp can do this for some ac motors.

• A linear amplifier responds to different frequency components independently and does not generate harmonic distorion or intermodulation distortion. A nonlinear amplifier does generate distortion.

• A wideband amplifier has a precise amplification factor over a wide range of frequencies, and is often used to boost signals for relay in communications systems. A narrowband amp is made to amplify only a specific narrow range of frequencies, to the exclusion of other frequencies.

• An RF amplifier refers to an amplifier designed for use in the radio frequency range of the electromagnetic spectrum, and is often used to increase the sensitivity of a receiver or the output power of a transmitter.

• An audio amplifier is designed for use in reproducing audio frequencies. This category subdivides into small signal amplification, and power amps which are optimised for driving speakers, sometimes with multiple amps grouped together as separate or bridgeable channels to accommodate different audio reproduction requirements.

• A special type of amplifier is widely used in instruments and for signal processing, among many other varied uses. These are known as operational amplifiers, (or op-amps). This is because this type of amplifier is used in circuits that perform mathematical algorithmic functions, or "operations" on input signals to obtain specific types of output signals. A typical op-amp has differential inputs (one "inverting", one, "non-inverting" relative to the output) and one output. An idealised op-amp has the following characteristics:

• Infinite input impedance (so as to not load circuitry it is sampling as a control input)

• Zero output impedance

• Infinite gain

• Zero propagation delay

The performance of an op-amp with these characteristics would be entirely defined by the (usually passive) components forming a negative feedback loop around it, that is, the amplifier itself has no effect on the output.

Today, op-amps are usually provided as integrated circuits, rather than constructed from discrete components. All real-world op-amps fall short of the idealised specification above -but some modern components have remarkable performance and come close in some respects.