Multistage or Cascade Amplifiers

Author: J.B. Hoag

Whenever it is necessary to increase the amplification by an amount greater than can be accomplished with a single-stage amplifier, several stages are connected one after another. The various methods commonly used to couple one tube to the next will now be discussed briefly. For the time being we shall omit as many of the voltage supplies as possible in order to focus attention upon the coupling units. Later we shall study the more complicated complete circuits.

Fig. 13 F. A simple two-stage resistance-coupled or d.c. amplifier

The amplifier of Fig. 13 F is called a resistance-coupled or d.c. amplifier because its coupling consists of a single resistance, R, and because the circuit can amplify not only alternating but also direct voltages applied to the input terminals. Were it not for the potential E_C, the voltage on the grid of tube 2 would be the same as that on the plate of tube 1. This amounts to the high positive value given by the battery E_B minus the IR drop in the coupling resistor R. If the grid of tube 2 were allowed to operate at a high positive potential, the plate current of this tube would be extremely large, a heavy grid current would flow and the tube would probably burn out. E_C must be greater than (E_B-IR) by such an amount as to bias tube 2 to the middle of the straight portion of its characteristic curve.

Fig. 13 G. Resistance-capacitance coupling

In the circuit of Fig. 13 G, which is called a resistance-capacitance (R-C) coupled amplifier, the d.c. plate potential of the first tube is kept off the grid of the second tube by means of the coupling condenser C_g. The use of C_g alone, however, would insulate or " float" the grid of the second tube. Electrons which reach this grid accumulate, except for a slight leakage over the outer surface of the tube, and build up a negative C-bias of erratic unstable amount. To avoid this, the high resistance grid leak R_g is added.

It will be recalled that the d.c. voltage across the plate of a tube is less than that of the B-voltage supply by an amount equal to the d.c. voltage drop in the plate resistor. An economy can be effected by using a low-resistance, high-inductance coil in place of the resistor, as in the impedance-resistance-coupled amplifier of Fig. 13 H.

Fig. 13 H. Impedance coupling

The d.c. loss in the resistance of the coil will be small, yet a fluctuation in the plate current will set up a comparatively high voltage across the coil; and this will be impressed through C_g and R_g, upon the grid of the second tube. But the amount of this voltage will be different for different frequencies of the voltage sent into the amplifier, and frequency distortion will occur. This may be an advantage or a disadvantage, according to the use to which the amplifier is to be put.

Fig. 13 I. Transformer coupling for audio frequencies

Fig. 13 J. Tuned transformer coupling for radio frequencies

The advantage of stepping up the voltage with an inter-tube transformer, as in Figs. 13 I and 13 J, has made the transformer-coupled amplifier one of the most commonly used. Here again, however, the reactive elements of the transformer cause the step-up voltage to have different values for different input frequencies. In Fig. 13 I, the iron core of the audio-frequency transformer is indicated by the parallel vertical lines between its primary and secondary. In Fig. 13 J, the primary and secondaries are tuned by means of condensers so as to amplify a given frequency (together with those in the immediate neighborhood) to a high value, to the exclusion of all others. This type of coupling is used in radio-frequency amplifiers.

In addition, more elaborate combinations of coils, condensers, and transformers are sometimes used in the coupling circuit.