Many analog circuits are designed to be self-biased in order to provide independence from the variations in the supply voltage that energizes the circuit. However, such self-biased circuits often have a stable state in which zero current flows in the circuit even when the power supply voltage is nonzero. This occurs because at the instant the power is supplied, the currents in the bipolar transistors in the circuits are in the picoampere range. As is known, the current gain of these transistors at very low current levels is often less than unity. As a result, even with the power supply voltage at a nonzero value, the self-bias circuit is usually unable to drive itself out of the zero-current state. Accordingly, unless precautions are taken, the circuit may remain in this undesired zero-current condition, and consequently, the circuitry which it biases is also undesirably nonfunctional.
To avoid the zero-current conditions, separate start-up circuits are usually used with the self-biased circuits to inject an initial current into the circuits. Since self-biased circuits with zero-current and nonzero-current operating points have sufficient regenerative or positive feedback to bring the biasing currents in the circuit to the desired nonzero-current operating point, the initial injection of a relatively small current is sufficient to bring the biasing circuit to the desired operating point.
After the regenerative action of the self-biased circuit has initiated the movement of the biasing currents to the desired operating level, the starting circuit is no longer needed. Also, the start-up circuits must not interfere with the normal operation of the steady state biasing circuit currents once they have reached the desired operating point. Accordingly, start-up circuits are typically designed to automatically disconnect themselves from the biasing circuit as the desired operating point is reached.
While the start-up circuits found in the prior art generally provide sufficient starting current for the biasing circuit and adequately switch themselves out of the circuit once the desired operating point has been reached, they have several limitations. The start-up circuit is usually a series resistor-diode combination connected between the positive and negative supply terminals. The series resistor-diode combination is connected to the biasing circuit by another diode. The starting current flows through the connecting diode into the biasing circuit until it is biased off by the current buildup in the bias circuit. However, the use of a series resistor-diode combination, even when switched out of the circuit, results in the resistor dissipating power. This factor especially hurts the performance of a micropower circuit. Furthermore, the resistors used with this technique are typically of a large value and are implemented in integrated circuit form as an epitaxial resistor or a series of pinch resistors. Because of the large value and high breakdown voltage necessary for the series resistor, a large area on the chip is needed for this circuitry, even though it only used when power is first applied.
Accordingly, it is the principal object of the present invention to eliminate the power dissipated by a start-up circuit after it has injected the necessary starting current to a self-bias circuit.
It is another object of this invention to eliminate the large area on an integrated circuit die required by the resistive element of a start-up circuit.
A further object of this invention is to provide a relatively inexpensive start-up circuit.
Still another object of this invention is to provide a start-up circuit of the type which can be easily fabricated in integrated circuit form.
Still another object of this invention is to provide a start-up circuit which does not affect the steady-state values of the currents in a biasing circuit to which it is connected after it has supplied a starting current to that circuit.