Transistor power amplifier with negative output impedence

The response of electromechanical transducers can often be greatly improved by proper electrical termination that compensates for their coil impedance. The results of a detailed analysis are presented for two practical transistor amplifier configurations which produce negative output impedance. A push-pull power amplifier, based on this analysis, uses the bias stabilizing elements of the output stage to obtain the necessary positive current feedback. The three-stage circuit delivers, without a transformer, 12 watts into 8 ohms or 8 watts into 16 ohms. The output impedance is adjustable from plus 1 ohm to either minus 7 or minus 14 ohms. This amplifier has been used successfully in an all-transistor stereophonic high-fidelity system.