The Transformer-Coupled Amplifier

Author: E.E. Kimberly

In Fig. 27-13 the output voltage at AB for any particular plate-circuit alternating current is

If a transformer is substituted for R_L, the secondary output voltage will be higher than the primary voltage by the factor a = N₂/N₁. For effective use of plate signal current it is essential that the primary input impedance be larger than r_p. Since r_p is always several thousand ohms, the primary coil must have many turns and the core must be of high-permeability iron. Since the secondary coil has about the same volume of copper as the primary coil, the maximum number of secondary turns is limited by the size of the smallest commercially available wire and the step-up ratio is limited accordingly. Ratios between 3:1 and 10:1 have been commonly used in the audio range of frequencies. Any attempt to increase the turns ratio by decreasing the number of primary turns causes a decrease in input impedance and, when carried too far, results in less rather than more secondary voltage.

The voltage across the primary of the transformer is

The amplification factor K_a

Tubes with high μ have high r_p and are generally unsatisfactory for transformer coupling.

The amplification characteristic of a transformer-coupled circuit is much the same as that of Fig. 27-17, but at higher frequencies there is a peaking of K_a because of resonance between the transformer leakage reactance and the distributed capacitance of its windings. The higher amplification factor is obtained at the expense of fidelity of reproduction of the input signal.