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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 RL, 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 rp. Since rp 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 Ka




Tubes with high μ have high rp 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 Ka 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.

Last Update: 2010-10-06