Class A high-fidelity amplifier

A transistor power amplifier for driving a high-fidelity loudspeaker for the reproduction of music without audible distortion, said amplifier comprising

• a complementary-symmetry push-pull output stage comprising a complementary pair of field-effect power output transistors,
• a complementary-symmetry push-pull current amplification stage comprising a complementary pair of bipolar power output transistors,
• said field-effect transistors having greater thermal stability than said bipolar transistors,
• said bipolar transistors having a higher current capability than said field-effect transistors,
• said field-effect transistors each having a source electrode,
• said bipolar transistors each having a base electrode and an emitter electrode,
• an output terminal for connection to a high-fidelity loudspeaker system,
• current-sensing resistor means extending from said source electrodes to said output terminal,
• said resistor means having an impedance of a magnitude to permit sufficient output current to flow from said source electrodes through said resistor means to said output terminal and then through said loudspeaker system to drive the latter,
• first conductive means connecting said current amplification stage bipolar transistor base electrodes to said current-sensing resistor means,
• second conductive means connecting said current amplification stage bipolar transistor emitter electrodes to said output terminal,
• means biasing said output stage field-effect transistors to an active state with a predetermined quiescent current, and
• means biasing said current amplification stage bipolar transistors to a quiescent state approximately at cutoff,
• whereby at lower amplifier power output levels said loudspeaker system is driven solely by said field-effect output stage until the output current flowing through said current-sensing resistor means reaches a predetermined value to apply a signal voltage to said base electrodes, thereby activating the current amplification stage to provide additional current to said output terminal,
• whereby there are provided the combined advantages of the thermal stability of the field-effect transistors and the high current capability of the bipolar transistors.

A transistor power amplifier for driving a high-fidelity loudspeaker for the reproduction of music without audible distortion, said amplifier comprising

• a complementary-symmetry push-pull output stage comprising a complementary pair of power output transistors,
• a complementary-symmetry push-pull current amplification stage comprising a complementary pair of power output transistors,
• said output stage transistors having greater thermal stability than said current amplification transistors,
• said current amplification transistors having a higher current capability than said output stage transistors,
• said output stage transistors each having an output electrode,
• said current amplification transistors each having an input electrode and an output electrode,
• an output terminal for connection to a high-fidelity loudspeaker system,
• current-sensing impedance means extending from said output stage transistor output electrodes to said output terminal,
• said impedance means having an impedance of a magnitude to permit sufficient output current to flow from said output stage transistor output electrodes through said impedance means to said output terminal and then through said loudspeaker system to drive the latter,
• first conductive means connecting said current amplification transistor input electrodes to one end of said current-sensing impedance means,
• second conductive means connecting said current amplification transistor output electrodes to the other end of said impedance means,
• means biasing said output stage transistors to an active state with a predetermined quiescent current,
• whereby at lower amplifier power output levels said loudspeaker system is driven by said output stage until the magnitude of the output current flowing through said current-sensing impedance means reaches a predetermined value to apply a signal voltage to said input electrodes of the current amplification transistors, thereby activating the current amplification stage to provide additional current to said output terminal, and
• whereby there are provided the combined advantages of the thermal stability of the output stage transistors and the high current capability of the current amplification transistors.