Startup circuit for subregulated amplifier

An apparatus for providing a startup voltage for a first subcircuit, comprising:

• a first subcircuit adapted to conduct an operating current drawn from a power supply terminal thereof,
• a second subcircuit having a control node, an output terminal coupled to the power supply terminal of said first subcircuit, an input terminal coupled to an output terminal of said first subcircuit, and having an output voltage responsive to the second subcircuit input signal;
• a comparison circuit responsive to the level of said operating current, having an input node coupled said current source and having an output node coupled to said control node, said comparison circuit having a first output state if the level of said operating current is below a predetermined level, and having a second state if the level of said operating current is above said predetermined level;
• whereby, in response to said first output state of said comparison circuit, the output voltage of said second subcircuit is driven to rise to a level high enough to provide an operating voltage for said first subcircuit, and whereby, in response to said second output state of said comparison circuit, the output voltage of said second subcircuit is independent of a level of the output state of said comparison circuit, thus allowing normal operation of the overall circuit beyond the startup time period.

An apparatus for providing a startup voltage for a first subcircuit, which said first subcircuit receives an operating voltage from the output of a next subcircuit, and wherein an output of said first subcircuit is coupled to an input of said next subcircuit, thereby causing the output voltage of said next subcircuit to be dependent on operation of said first subcircuit, comprising:

• a first subcircuit having a current source providing a substantially constant current of nominal level during normal operation of said first subcircuit;
• a next subcircuit having a control node, an output coupled to a power supply terminal of said first subcircuit, an input terminal coupled to an output terminal of said first subcircuit, and having an output voltage responsive to the next subcircuit input signal and to the state of a signal applied to said control node;
• a comparison circuit responsive to the level of said substantially constant current, having an input coupled to a node in said current source and having an output coupled to said control node of said next subcircuit, said comparison circuit having a first output state if the level of said current is at a substantially nominal level, and having a second state if the level of said current is not at said substantially nominal level;
• whereby said first output state of said comparison circuit, indicative of being in a startup time period and being coupled to said control node of said next subcircuit, substantially forces the output voltage of said next subcircuit to a level high enough to provide nominal operating voltage for said first subcircuit, and whereby said second output state of said comparison circuit, indicative of being past the startup time period, has substantially no effect on the operation of said next subcircuit, thus allowing normal operation of the overall circuit beyond the startup time period.

A method for providing an initial operating voltage for a first subcircuit which receives its nominal operating voltage from the output of a next subcircuit, wherein the output of said first subcircuit is coupled to the input of said second subcircuit, comprising the steps:

• i.) measuring, with a suitable known electronic circuit, the level of tail current Itail of a current source coupled to a first amplifier within said first subcircuit;
• ii) comparing, using a known electronic comparison circuit, said level of tail current Itail with a threshold value;
• iii.) coupling a first signal to a control node in said next subcircuit if said current Itail is below said threshold value, whereby said first signal causes the output voltage of said next subcircuit to be substantially at its nominal value;
• iv.) coupling a second signal to said control node in said next subcircuit if said current Itail is above said threshold value, whereby said second signal has substantially no effect on the output voltage of said second subcircuit;
• whereby the output of said next subcircuit is forced to a substantially nominal voltage level sufficient to power said first subcircuit when said comparing step finds said current Itail to be below said threshold, and whereby said comparing step has substantially no effect on the output voltage of said next subcircuit if said comparing step finds said current Itail to be above said threshold.