A compact circuit topology for the realization of a bipolar class-HB output stage operating down to 0.9 V is presented. This compact topology is obtained by using the same path for the signal as well as for the definition of the bias current in the output transistors. The output amplifier is used to drive a 64- Omega load, representing the series connection of two telephones in a headphone set. An output signal of 600 mV/sub p-p/ across the 64- Omega load at a supply voltage of 1.3 V shows a total harmonic distortion plus noise better than -60 dB. The amplifier uses 0.14-mm/sup 2/ chip area and is realized in a 2.5- mu m BiCMOS process including vertical p-n-p transistors.
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