Audio Frequency Amplification

Distant Reception.

For the reception of stations 20 m. or more from a main broadcasting station the amplification must be still further increased. This can be accomplished in either the high frequency or in the low frequency portion of the receiver. It is better to achieve the desired result by increasing the high frequency amplification, for the two following reasons : (I) the response of the rectifier falls off rapidly when the strength of the incoming signals sinks below a certain level; (2) increased ampli fication must be accompanied by augmented selectivity, and the requisite electrical circuits can be incorporated in the high fre quency portion of the receiver only.

As the amplification is increased, so also is the tendency of the receiver to generate spurious oscillations, particularly in the high frequency portion. In designing an instrument for distant recep tion, particular attention must be paid to the elimination of such effects. Moreover, it is imperative to screen each radio fre quency circuit to prevent electromagnetic and electrostatic coup ling, and to accomplish the same object, the radio frequency must be kept out of the audio frequency circuit3. A very fruitful source of self-oscillation at radio and at audio frequency is to be found in the means used to supply filament and anode current to the valves. A battery, whether it be an accumulator or a dry cell, has an internal resistance which is by no means negligible when the voltage amplification runs into thousands. By virtue of this resistance, the relatively large alternating current in the valve preceding the rectifier causes a voltage drop which is applied to other radio frequency amplifying valves at the beginning of the amplifier. This causes enhanced amplification which usually cul minates in self-oscillation, thereby putting the receiver out of ac tion. A similar argument applies to the audio frequency portion of the receiver. To avoid this "coupling" due to the battery, it is abso lutely essential to use a special system whereby the battery feed is electrically filtered or purged of the alternating current. The golden rule is that no alternating current, either of radio or of audio frequency, must pass through the battery. All alternating current is, therefore, bye-passed from the neighbourhood of the positive terminal of the high tension battery, to the negative terminal. In series with the first battery tapping point is con nected an inductive resistance of low self-capacity, while across its outer terminal and the negative pole of the battery a condenser of large capacity is shunted. The resistance offers great imped ance to the currents of radio frequency, whilst the condenser bye-passes them with ease. Hence the alternating current through the battery is negligible. A similar procedure is adopted for the audio frequency feed where an audio frequency choke is used in place of a radio frequency choke. Both choke and condenser

must have much greater values than those for the radio fre quency filter.

So far as the filament battery is concerned, the question of filter arrangements is much less acute. In this instance, it is usually adequate to connect the positive battery terminal via a large condenser to earth or to the metal screen of the receiver. Where very short waves of so metres are used, it is expedient to put a choke in one or in both filament legs. In a complete receiver designed on the above principles the centre tapped frame aerial is followed by two stages of high frequency amplification using screened grid valves with tuned anode coupling, the coils being of toroidal construction and having substantially no external magnetic field. The remainder of the circuit is identical with that in fig. 2. As a limiting case, three high frequency stages can be used, but great care must be exercised to obtain adequate screen ing of the various components. The side of the frame aerial in this case should not exceed about 12 in. in length, since the amplification is so enormous that signals are drowned by noise.

Degree of Amplification.

A few remarks on the amplifi cation obtained in modern receiving sets may be of interest. There are two methods of expressing amplification :—(r) by the ratio of the input voltage to the power valve to that induced in the aerial by the electromagnetic waves; (2) by the ratio of the electrical power in the loud-speaker to the power received by the aerial. The power amplification is enormously greater than the voltage amplification and is sometimes cited for purposes of advertisement. It is more usual in scientific circles, however, to consider the voltage amplification. Neglecting interaction of various components in a receiver, the total amplification is the product of the amplification per stage. With any type of valve the gain per stage is limited because of the ever present possibility of self-oscillation. Modern screened valves can be used to secure a stable gain of from 3o to 4o per stage over the wave range 300 to 600 metres. Taking, therefore, the magnification per stage of the last mentioned receiver we obtain the following approxi mate figures : ist radio frequency 2nd radio frequency ÷30, rectifier ÷ 20, 1st audio frequency ÷7o. The product of these is 126,00o. This, however, must be divided by 4 to allow for the audio frequency voltage modulation of the transmitter being only of the radio voltage on the aerial. The amplification due to the frame has been neglected. It may be estimated at about 6o, although higher values can be obtained at the expense of stability. Taking the modulation factor and the frame into ac count, we get a resulting factor of 20, so that the total mag nification is just short of 2 million-fold.