Active-Error Feedback and Its Application to a Specific Driver Circuit

A short discussion of the advantages and disadvantages of active-error feedback in amplifier design is given. Such feedback can yield all the advantages of ordinary negative feedback without gain reduction and is particularly suitable for use in reducing the distortion of individual amplifier stages. Active-error feedback is applied to a cathode follower by amplifying the difference between its input and output voltages, then adding the amplified error to the output. The resulting driver has very low output impedance and low distortion and is especially useful for driving the grid of an output tube far positive. A direct-coupled version of the circuit using ordinary miniature tubes had an output impedance of 5.6 ohms and could supply several hundred milliamperes of positive current. The theory of the circuit agrees with experiment, and the distortion of the driver when driving an output tube grid to the diode line is found to be far less than that of an ordinary cathode-follower driver.