Transformer Coupling Circuits for High-Frequency Amplifiers

T HE designer of high-frequency amplifiers is often confronted with the problem of obtaining, with a given number of amplifying tubes and coupling circuits in cascade, maximum voltage amplification over a predetermined band of frequencies, and high attenuation to all voltages outside of the desired band of frequencies. To secure a large voltage amplification the most convenient, economical and practical arrangement for coupling the various stages of the amplifier is by means of the step-up transformer. By adding condensers in parallel with one or more of the windings of the transformer a frequency discrimination characteristic is obtained which can be controlled to a large extent by the proper choice of the transformer constants and the tuning capacitances. In the case of the usual type of transformer coupling with the secondary winding tuned to resonance at a given frequency the voltage amplification for a single stage depends on the resistance of the secondary winding, the conductance of the grid circuit of the second tube and the size of the tuning condenser. With proper choice of the transformer constants very large amplification can generally be obtained over a relatively narrow band of frequencies. To obtain high amplifications over wider frequency bands other factors must be taken into consideration. Possibly the most important of these factors is the impedance of the circuit into which the secondary winding operates. In the case of an interstage transformer with the secondary winding connected directly to the grid circuit of a three-element tube this impedance depends on the electrode capacities of this tube, the amplification factor, the plate impedance and the impedance connected to the 608