Operational amplifier circuit with slew rate enhancement

An operational amplifier with self-adjusting performance enhancement, comprising:

• a FET differential amplifier having a differential input for receiving a differential input signal, a bias input for receiving a bias current, and output means for providing an output signal, said FET differential amplifier being operative to produce said output signal by amplifying the received differential input signal, wherein a characteristic of said output signal is limited according to said bias current, and
• means for monitoring a characteristic of the differential input signal and adjusting said bias current according to said characteristic of the differential input signal when said characteristic of the differential input signal exceeds a predetermined threshold,
• wherein the FET differential amplifier includes:
• first and second input terminals for receiving said differential input signal,
• a current source node,
• first and second FET transistors, each having a source, gate and drain, the gates being coupled to the first and second input terminals respectively, the sources being coupled to said current source node, and the drains each having an output impedance, each FET transistor being operative to modulate a drain current according to its transconductance and the voltage between the gate and source,
• first bias means coupled to said current source node for supplying a first bias current to the current source node for biasing the first and second FET transistors, and
• a current mirror having first and second ports connected to the drain of said first FET transistor and the drain of said second FET transistor respectively for receiving the modulated drain currents and current biasing the first and second FET transistors substantially under small signal conditions and generating an interstage signal according to received modulated drain currents,
• wherein the output means of said FET differential amplifier comprises amplifier means having an input connected to a primary one of said first and second ports for receiving said interstage signal, an output, and compensation means coupled between the output and the input of the amplifier means for compensating said FET differential amplifier, said compensation means limiting the transition speed of said output signal according to the compensation thereof and current supplied to said current source node,
• said monitoring and adjustment means comprising means for enhancing the transition speed of said output signal by providing more current to said current source node when the amplitude of said differential input signal exceeds a given threshold.

A self-adjusting amplifier comprising:

• a differential common source amplifier, comprising a pair of common source connected FETs, with the gates of said pair of common source connected FETs providing a differential input for receiving a differential input signal therebetween,
• current source means,
• the sources of said FETs being coupled to said current source means for receiving a bias current, and the drains providing a differential output for producing a differential output signal, wherein a characteristic of said differential output signal is affected according to said bias current and a characteristic of the differential input signal, and
• means for monitoring said characteristic of the differential input signal and adjusting said bias current when said characteristic of the differential input signal exceeds a given threshold, said monitoring means comprising:
• first and second sensing FETs having gates coupled to respective gates of said common source connected FETs of the differential common source amplifier and each of said sensing FETS having a source providing an output signal directly related to the voltage at its gate, and
• first and second current injection means for providing enhanced current to the common source node of said common source connected FETs, said first and second current injection means being controlled by respective output signals provided from the sources of said first and second sensing FETs.

An operational amplifier with self-adjusting performance enhancement, comprising:

• a FET differential amplifier having a differential input for receiving a differential input signal, a bias input for receiving a bias current, and output means for providing an output signal, said FET differential amplifier being operative to produce said output signal by amplifying the received differential input signal, wherein a characteristic of said output signal is limited according to said bias current, and
• means for monitoring a characteristic of the differential input signal and adjusting said bias current according to said characteristic of the differential input signal when said characteristic of the differential input signal exceeds a predetermined threshold,
• wherein the FET differential amplifier comprises first FET transistors provided with channels having first predetermined width-to-length ratios to effect respective drain currents for given gate-to-source voltage drops,
• said monitoring and adjustment means comprising second FET transistors coupled to said differential input and provided with channels having second predetermined width-to-length ratios to effect respective drain currents for given gate-to-source voltage drops, and
• said monitoring and adjustment means including means for providing additional current to said first FET transistors when the amplitude of the signal as presented to said second FET transistors exceeds a predetermined threshold determined according to the ratio between said first predetermined width-to-length ratios and said second predetermined width-to-length ratios.