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Home Electronic Devices The TransformerCoupled Amplifier  
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The TransformerCoupled AmplifierAuthor: E.E. Kimberly In Fig. 2713 the output voltage at AB for any particular platecircuit 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 = N_{2}/N_{1}. 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 highpermeability 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 stepup 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} is
Tubes with high μ have high r_{p} and are generally unsatisfactory for transformer coupling. The amplification characteristic of a transformercoupled circuit is much the same as that of Fig. 2717, 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.


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