Operational amplifiers and charge pumps are well known in the prior art. An operational amplifier is used to perform a wide variety of linear and digital functions. A charge pump is used to create a negative voltage level from a positive voltage level. Thus, one common technique is to use a charge pump in combination with an operational amplifier, thereby allowing a user to provide only a single supply signal. Through this combination, the single supply signal, when transformed via the charge pump, provides both a positive and a negative supply to the operational amplifier. The charge pump and the operational amplifier, however, are found in separate packages and thus require the user to couple them together for implementation.
In addition to the above, when implementing an operational amplifier with a charge pump, a problem occurs from a ripple effect inherent in the negative voltage level created in the charge pump. The ripple effect is created by an oscillator within the charge pump, causing voltage spikes to appear on the negative voltage level. Prior art operational amplifiers have limited capability to avoid this ripple effect. Thus, this ripple effect is often introduced into the operational amplifier circuitry, decreasing the performance capabilities of the circuit.
Another goal often required by the user of an operational amplifier is to provide an output of zero volts since this is a known reference voltage. However, prior art single supply operational amplifiers can only provide an output within one to two diode voltages of zero volts. Many techniques have been used to create this zero volt output of the operational amplifier, but these techniques often degrade other circuit characteristics. For example, by using bipolar circuitry, a zero volt output can be obtained by sacrificing linearity and low distortion at the output of the operational amplifier. The resultant high distortion and nonlinearity may cause inaccuracies throughout an implemented system. As another example, CMOS technology can produce a zero volt output, but sacrifices high output current and low output impedance from the operational amplifier. These sacrifices are often undesirable because many applications require high output current to drive highly capacitive loads. Thus, prior art single supply operational amplifiers cannot provide a zero volt output without affecting other circuit characteristics.
From the foregoing, it may be appreciated that a need has arisen for an operational amplifier able to provide a zero volt output without sacrificing linearity, low distortion, high output current, or low output impedance. A need has also arisen to combine a charge pump and an operational amplifier into a single device and isolate the ripple effect of the negative voltage level created in the charge pump from the operational amplifier.