Audio Frequency Amplification

Receiving Instruments.

For broadcast reception, receiving instruments can be divided broadly into two classes : (r) those in which the incoming radio waves are amplified and selected at their original frequency, thereafter being rectified and amplified again at audible frequencies; ( 2) those in which the incoming waves are first amplified and selected at their original frequency, after which they are rectified and the frequency is reduced to an intermediate value, being still inaudible or supersonic before being rectified a second time and amplified at audio frequency. A receiver of the latter type is known as a supersonic heterodyne. Using ordinary three-electrode valves or triodes, it is possible to get a large overall magnification and a high degree of selectivity more readily by the supersonic method than by a method where all the radio amplification is effected without frequency change. Nevertheless, the quality of reception with a "straight" high fre quency amplifier is usually superior to that obtained with a super sonic heterodyne. By suitable design there is reason to believe that the difference in quality could be made negligible. However, the recent introduction of the screened tetrode, this being a valve in which the electrostatic capacity—which is mainly responsible for self-oscillation in high or in low frequency amplifiers—between anode and grid is reduced to a small value, will make the design of the supersonic heterodyne more facile. A diagrammatic repre sentation of a "straight" circuit is shown in fig. 1.

Although in 1928 the majority of receivers on the market which were suitable for broadcast work used ordinary triodes, we shall consider the more modern receivers in which screened grid valves (see THERMIONIC VALVE) are incorporated in the high frequency stages. A receiver suitable for use a few miles from a main broadcasting station is illustrated diagrammatically in fig. 2. Here an open aerial is used, this being flatly tuned, since selectivity is unimportant with very strong signals and flat tuning prevents the higher audible frequencies being appreciably attenuated. The aerial can be either directly connected to the grid and filament of the rectifier or via a coupled circuit. The latter method is shown. After rectification by what is known as the "anode bend" method, which utilizes the curved portion of the valve character istic, the signals consist of two components (a) high or radio fre quency, (b) low or audio frequency. The former are undesired in the output and must be removed or filtered out. With this end in view, an inductance, L, of small self-capacity, is inserted in the anode circuit of V1, whilst a small condenser, C, is connected from the anode of V1 to the negative pole of the battery. The radio voltage change occurs on the anode of and the inductance, L, chokes it back so that it is bye-passed by C and little or no radio enters the audio frequency amplifying circuits. The audio fre quency causes a voltage change across a tapped resistance, r, which is passed on to the grid of V2 by means of the condenser and resistance known as the grid leak. The resist

ance r is tapped to control the output intensity. Valve V2 is coupled to the power or output valve V3 by means of an iron-cored trans f ormer T, or by another resistance capacity unit. The loud-speaker is associated with valve V3 either directly or in conjunction with a filter circuit or a transformer. The filter circuit, as shown by the inductance L2 and condenser C2, prevents the anode feed current from the battery to the valve—which is usually fairly large—from passing through the loud-speaker windings, and it also prevents the loud-speaker current from passing through the battery.

To secure a uniform response from the amplifying system over a wide frequency band—apart from the loud-speaker—the various valves and their associated components must be specially selected and properly designed. For example, if C1 and are made too small, those frequencies below the middle of the pianoforte will be so weak as to be inaudible. The same result will occur if the inductance of the primary winding of transformer T is too small, or if the internal resistance of valve V2 is too large. Also, the bye pass condenser C must not be too large, or some of the higher audible frequencies will be bye passed in addition to the radio. This bye-pass condenser and the equivalent grid-to-filament capacity of V2 are responsible for reducing the upper audible fre quencies above 3,00o cycles. Moreover, if the transformer T amplifies the upper frequencies more than the lower, there will be a degree of compensation. Over compensation can be readily averted by augmenting the value of C. Thus transformer T should have a rising characteristic as shown in fig. 3.

When an open aerial, either indoors or outside, is impracticable, a loop or frame aerial can be used for receiving the local broad casting station. Where the receiver is quite near the transmitter, an open aerial can be replaced by a frame aerial. For reception at an increased distance from the transmitter, it will be necessary to augment the valve amplification when a frame aerial is used. This can conveniently be effected by adding a stage of high frequency amplification in the form of a screened grid valve. Thus we must modify fig. 2 by adding a screened valve prior to the rectifier and replacing the open aerial by a tuned frame aerial whose sides can be from 18 inches to 3o inches long. The result is shown in fig. 4. Here it will be seen that the frame is tapped at the centre and only half of it connected across the grid and filament of the valve. This artifice is not always required for local reception, but is a useful artifice for distant reception. When a powerful station, which it is desired to exclude, causes trouble, this mid point con nection is often helpful in reducing the interference.