Transformer-Coupled Amplifiers

Amplifier circuits in which transformers are used can be represented by a circuit similar to that of Fig. 103(a).

Fig. 103. (a) Transformer-coupled amplifier; (b) equivalent circuit.

Here a triode is shown with a voltage e_c impressed upon the grid, which comprises the grid bias (a constant negative direct voltage) and a superimposed alternating voltage e_g. Anode voltage E_B is supplied from some source through the primary of the transformer, across which appears an alternating voltage e_p. The secondary of the transformer is connected to a load Z_L. Under certain conditions, which will be defined below, this circuit may be simplified to that of Fig. 103 (b). A fictitious alternating voltage μe_g is impressed on the circuit, where μ is the tube amplification factor. Internal tube resistance Z_G is in series with the load Z_L, which is reflected by the transformer to the proper value in the primary circuit for tube operation. That is, Z_L in Fig. 103(b) is equal to that in Fig. 103(a) only if the transformer has a 1:1 ratio. For any turns ratio, the quotient of two Z's is equal to the (turns ratio)² as in equation 7. Note that the winding resistances are regarded as zero, so that, in the absence of a grid signal, full voltage E_B appears on the plate of the vacuum tube.

Alternating voltage μe_g causes voltage e_p to appear across the load Z_L. The voltage e_p is not μ times e_g but is related by the following equation:

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Although transformer-coupled amplifiers are used sometimes for voltage amplification, they are used mostly where power output is required of the amplifier and where a good reproduction of the grid voltage is required in the plate circuit.