Recent Advances in High-Frequency Performance of Feedback Amplifier-Transformer Combinations

The high-frequency limitations inherent in systems dependent upon performance of amplifier-transformer combinations have often restricted ac calibration in the high audio range. Three recent advances in design of feedback amplifiers incorporating transformers make possible significant improvements in bandwidth, linearity, and computing accuracy in this range. The first method compensates for the leakage reactance-output capacitance resonance that frequently constitutes the basic bandwidth limitation in feedback amplifier systems which incorporate output transformers. The second approach introduces resonant stabilization, allowing more loop gain for distortion reduction at specific harmonics. The third technique employs two or more computing amplifier-ratio transformer combinations effectively in parallel. This configuration can yield worst-case computing errors inversely proportional roughly to loop gain raised to a power equal to the number of parallel combinations. Applications of the three techniques in a practical, all-solid-state 20 kHz ac calibration source are described. The output amplifier-transformer combination supplies an external load with 20 VA to 20 kHz and 1 kV, with distortion of 0.03 to 0.05 percent at the high-frequency end. At the same time, parallel sets of computing amplifier-ratio transformer combinations within the source furnish ratio transformer scaling accuracy in division. This, in turn, permits overall linearity which is typically ±0.01 percent to 5 kHz, ±0.02 percent to 20 kHz, over a ten-to-one output amplitude range, expressed in percent of local (rather than full-scale) output voltage.