Amplifier Classification

Amplifiers can be divided into classes, depending upon the mode of operation. A class A amplifier is one in which the grid bias and alternating grid voltage are such that anode current flows continuously. In a class B amplifier the grid bias is almost equal to the cut-off value, so that plate current is nearly zero when no exciting grid voltage is applied. When full alternating grid voltage is applied, plate current flows for approximately one-half of each cycle. A class C amplifier has a grid bias greater than the cut-off value, so that the plate current is zero when no alternating grid voltage is applied and it flows for appreciably less than one-half of each cycle when an alternating grid voltage is applied. These classes are illustrated in Fig. 105, in which the alternating plate current, plate voltage, grid voltage, and grid current are shown with the steady or average values which are, respectively, I_B, E_B, E_c, and I_G. Relative plate and grid voltage amplitudes for these three types of amplifiers are shown in Fig. 105, and other properties are summarized in Table XI.

Fig. 105. Amplifier voltages and currents.

Table XI. Amplifier Classes

Class A amplifiers are characterized by comparatively high no-signal anode current. Usually the grid never swings positive. Anode current remains comparatively constant, when averaged over a whole a-c cycle. In class B amplifiers, the grid is biased at a greater negative potential so that current is nearly cut off in the absence of a signal. Positive swings of grid voltage result in anode current being drawn; this causes a dip in the residual voltage on the plate of the amplifier. Negative grid swings cause no plate current to flow but do cause a positive plate voltage swing. In class C amplifiers, the grid is biased more negatively still, with the result that plate current flows for less than half a cycle, and mostly when the plate voltage on the tube is at a relatively low value. Grid current in this class of amplifier reaches values comparable to the plate current. Output voltage wave form is maintained by a tuned plate circuit.

Operation may sometimes be improved by the use of two tubes connected push-pull, as shown in Fig. 106.

Fig. 106. Push-pull amplifier.

This is the most common connection for class B amplifiers; also, it is frequently used in class A amplifiers. Intermediate between class A and class B amplifiers are those known as class AB with grid bias and efficiency intermediate between class A and class B amplifiers. Such amplifiers are further subdivided into class AB₁ and class AB₂. Class AB₁ amplifiers draw no grid current, but the bias voltage is somewhat higher than the class A value and the plate current may be discontinuous during the cycle when grid signal is applied. Class AB₂ amplifiers draw grid current but are not biased as close to cut-off as class B amplifiers. Both class AB₁ and AB₂ amplifiers are commonly used with the push-pull connection.

Tube properties such as plate resistance r_p, amplification factor μ, and mutual conductance g_m may be calculated from data published for each tube in the form of characteristic curves. Operating conditions such as plate- and grid-voltage swings, power output, plate dissipation, and efficiency also are found from these curves. Theoretical discussions of such data may be found in books on amplifiers.