126. Resistance Coupling.-Figure 88 shows what is called a resistance coupled circuit. It consists of resistance in the plate circuit of the first tube and a resistor in the grid circuit of the second tube. The plate of the first tube is connected to the grid of the second tube by means of a condenser. The action of this circuit is as follows:-When an alternating voltage is impressed upon the grid of the first tube a pulsating direct current will flow in the plate circuit of the tube. This pulsating current can be considered as being made up of two distinct parts, one a steady direct current and the other an alternating current. The direct current part is known as the "DC component" and the alternating current part is known as the "AC component." The AC component is the part we are interested in because it represents the sound current and is the only part which is amplified in vacuum tubes. When the pulsating direct current flows in the resistor of the plate circuit of the first tube a pulsating voltage is produced across the resistor; this voltage like the current is made up of two parts or components. The AC component of the voltage. causes alternating current to flow through the condenser connecting the plate of the first tube to the grid of the second tube. This current flowing through the resistor in the grid circuit of the second tube produces an alternating voltage across it, which is impressed on the grid of the second tube.

The advantage of resistance coupling is that a wide range of frequencies can be amplified equally. The disadvantage is that a great deal of energy is used up in the resistor and a high direct voltage is required for the plate circuit. This type of coupling is particularly adaptable for use with tubes having a high amplification.

127. Impedance Coupled Amplifiers.-This type of coupling utilizes an inductive impedance instead of resistance in the plate circuit. Otherwise it is similar to resistance coupled circuits. Since the inductance offers a low resistance to direct current and a high impedance to alternating current, it will give an equal voltage amplification of the AC voltage while requiring a less DC voltage on the plate than is necessary in resistance coupled circuits. Its disadvantage is that equal amplification cannot be obtained over as wide a range of frequencies as is possible with resistance coupling.

128. Transformer Coupled Amplifiers.-Transformer coupling is the most common method of coupling vacuum tubes. Figure 89 shows a circuit using transformer coupling. The primary of a transformer is connected in the plate circuit between the plate and the plate voltage supply. If an alternating current is impressed on the grid of the tube, a pulsating current will flow in its plate circuit.

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