Transformer Coupling Circuits for High-Frequency 



Amplifiers 



By A. J. CHRISTOPHER 



This article deals with the use of transformer type of coupling circuits 

 in high-frequency amplifiers to transmit efficiently voltages or currents 

 between certain limiting frequencies while attenuating those above and 

 below the limiting frequencies. The similarity of these coupling circuits 

 to band-pass filters is shown and the conditions to be satisfied in order 

 that they may act as such are covered. Means of obtaining uniformly 

 high amplification over relatively wide frequency bands are explained. 

 Typical conditions under which these coupling circuits have been employed 

 and factors affecting their performance are discussed. 



I. Introduction 



THE 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 



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