TRANSFORMER COUPLING CIRCUITS 



615 



importance that the conductance component as well as the capacitance 

 component of the input impedance of the second tube be kept to a 

 minimum if maximum amplification is to be obtained. If the second 

 tube in Fig. 1 is operated as a negative grid bias detector the ratio of 

 E2'E may be made considerably larger than if it is operated as an 

 amplifier. The application of a large negative grid bias and the use 

 of a by-pass condenser in the plate circuit of the tube to improve 

 its modulation efficiency usually results in a very small input capaci- 

 tance and conductance. It is therefore sometimes desirable in multi- 

 stage amplifiers to place the selective circuits between the last amplifier 

 tube and the detector tube. With the advent of the shielded-grid 

 tube having the characteristic of low input capacitance and conduct- 

 ance, transformers of this type are particularly adapted to high 

 frequency amplifiers such as used for the intermediate frequency 

 amplifier of a superheterodyne receiver. 



III. Practical Applicatioxs 

 Fig. 3 shows the transmission characteristic of a simple transformer 

 coupling circuit when operating from a balanced resistance of 1000 

 ohms into the grid circuits of push-pull amplifying tubes. Stability 



^' 



Fig. 3^Typical transmission characteristic of a transformer coupling circuit. 



was more important in this particular case than maximum voltage 

 step-up so that a fixed condenser C2 was added externally across the 



