Enhanced-accuracy semiconductor power amplifier

Abstract

A DC amplifier uses complementary npn and pnp output transistors on n-type and p-type substrates, respectively. The output transistors are in an emitter-follower configuration with no emitter resistor to prevent thermal runaway. Instead, emitter-follower driver-stage transistors are provided on the same substrates as the output transistors to force a reduction in the bias voltage on the output stage when the temperature of an output transistor increases. This circuit prevents thermal runaway and temperature-dependent offsets without emitter resistors, which would increase output impedance, and without feedback from the output stage to the input stage, which would slow the response of the amplifier. Additionally, compensation-network transistors are provided to eliminate offsets resulting from driver- and output-transistor base-to-emitter voltage differences caused not only by temperature differences between the transistors on different substrates but also by manufacturing variations. The compensation-network transistors, like the driver-stage transistors, are on the same substrates as the output transistors. They are connected to provide a compensation voltage that is equal to the difference between the base-to-emitter voltages of transistors on the two substrates. The compensation voltage is used to cancel offsets that would otherwise result from such differences.