Pentode Amplifiers

Tetrode tubes have an additional grid between anode and control grid to reduce the grid-to-anode capacitance. This additional grid is known as the screen grid and is operated at a positive potential with a-c bypass to reduce the grid-to-anode capacitance. The chief drawback to this type of tube is that the anode voltage swing is limited to the difference between the anode voltage and screen voltage. This disadvantage is overcome by the addition of a third grid known as the suppressor, which removes this limitation and allows large anode voltage swings down to the diode line of the tube. Sometimes the third electrode is connected internally to the cathode.

Similar characteristics are obtained with the so-called beam tubes, which are tetrodes with special screen-grid spacings. Figure 126 shows 6L6 beam tube plate characteristics, with a typical load line of 2,500 ohms.

Fig. 126. Plate characteristics of type 6L6 beam tube.

As a single-side amplifier, such a tube is likely to have large distortion because of the uneven spacing of constant-grid-voltage lines. Distortion is reduced in a push-pull amplifier, especially for high power output. Plate resistance r_p is very high in pentodes and beam tubes, of the order of 10 times the load resistance.

Pentodes are essentially constant-current devices. The value of load impedance is thus an indication of the output voltage, at least for low frequencies. Response of a low-frequency transformer-coupled pentode amplifier can be taken from Fig. 118.

At high frequencies, leakage inductance of the transformer intervenes between the pentode and its load, so that the primary voltage and secondary or load voltage are not identical. In Fig. 127 the change of output voltage for a constant grid voltage at high frequencies is shown.

Fig. 127. Pentode frequency response with pi-filter output circuit.

In this figure, the equivalent circuit is a pi-filter, which is desirable for pentode transformers, and is approximated when the transformer ratio is 1:1. Harmonic content of pentodes is high, especially in single-side amplifiers. Large phase angle and low load impedance produce undesirable distortion. It is best to use values of X_N/R₂ greater than 2 in Fig. 118 at the lowest frequency to avoid distortion.

Semiconductor amplifiers known as transistors have emitter, collector, and base electrodes; these are analogous, respectively, to grid, plate, and cathode in a vacuum tube. Emitter and collector currents are of the same order of magnitude in grounded-base transistors, but collector impedance is much larger than emitter impedance. To match impedances, transformer coupling is often used between stages of transistor amplifiers. Junction transistors resemble pentode amplifiers in having nearly constant collector current over a large range of collector voltage. Hence junction transistor transformer operation closely resembles that of pentode vacuum-tube transformers, and the foregoing discussion is generally applicable to both.