Method for controlling an output stage of a bipolar micro-power rail-to-rail amplifier

A method for controlling a bipolar output stage that comprises first and second output transistors of opposite conductivities having bases, emitters that are respectively connected to first and second supply voltage terminals, and collectors that are connected at an output terminal and conduct respective output currents, the method comprising:

• connecting a load between said output terminal and said first supply voltage terminal;
• applying a drive voltage signal to the base of said first output transistor to adjust its output current thereby imbalancing said output currents and producing a load current at said output terminal;
• generating a gain current in response to said drive voltage signal;
• supplying said gain current by feeding it back in a regenerative feedback path so that said gain current can vary over a predetermined range while idling at a lower end of said range;
• and
• modulating a control voltage at the base of said second output transistor by a) adjusting a first voltage as a logarithmic function of said gain currents b) adjusting a second voltage as a linear function of said gain current, and c) summing said first and second voltages to provide said control voltage at the base of said second output transistor so that said control voltage changes approximately logarithmically for gain current at a lower end of said range and changes progressively more linearly as said gain current increases so that the second output transistor's output current is approximately a linear function of said gain current at the lower end of the range and becomes progressively closer to an exponential function of said gain current as said gain current increases thereby further imbalancing said output currents to increase the gain of said load current.

A method for controlling a bipolar output stage that comprises first and second output transistors of opposite conductivities having bases, emitters that are respectively connected to first and second supply voltage terminals, and collectors that are connected at an output terminal and conduct respective output currents, said output stage conducting a total idle current, the method comprising:

• connecting a load between said output terminal and said first supply voltage terminal;
• applying a drive voltage signal to the base of said first output transistor to adjust its output current thereby imbalancing said output currents and producing a load current at said output terminal;
• generating a gain current in response to said drive voltage signal;
• supplying said gain current by feeding it back in a regenerative feedback path so that when the output stage is idling the gain current idles at a low level and when the output stage is driven the gain current is supplied at a high level that exceeds the output stage's total idle current;
• and
• modulating a control voltage at the base of said second output transistor as a function of said gain current to control its output current thereby further imbalancing said output currents to increase the gain of said load current.