Voltage Buffer Feed-Forward Circuit for High Slew Rate Gain
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Solution Overview
Problem
Existing diamond buffer amplifiers face limitations in achieving high slew rate and current gain at high frequencies due to low values of βnpn and βpnp, which restrict the effectiveness and efficiency of the amplifier circuitry.
Innovation Solution
The implementation of adjustable current source circuits with feed-forward capacitors and current follower transistors, which provide a high-speed short circuit feed-forward path between the emitter and collector of input transistors, reducing the need for compensation capacitance and enhancing the stability and speed of the amplification circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If constant current sources are used in diamond buffer amplifier, then circuit simplicity is maintained, but current gain and slew rate are limited due to low β values
Solution Approach 1:
The amplifier is divided into two independent push-pull sections (NPN and PNP sides) that can be optimized separately. Each side has its own current source and transistor pair, allowing independent optimization of current gain and slew rate for each polarity without compromising the other side's performance.
Solution Approach 2:
The patent changes the current source configuration from simple constant current sources to more complex regulated current sources with higher current capability. This parameter change enables the circuit to overcome the limitations of low β values by providing sufficient base current to output transistors, thereby achieving high current gain and improved slew rate.
2Productivity
If higher output current is desired with low β values, then current sources I1 and I2 must be increased, but total quiescent current increases
Solution Approach 1:
The patent implements dynamic current regulation where the current sources adapt their output based on the operating conditions. The regulated current sources can provide high current when needed for maximum output while maintaining lower quiescent current during normal operation, thus resolving the contradiction between output current capability and power consumption.
3Speed
If parasitic base capacitances of output transistors are charged by currents I1 and I2, then slew rate is determined by these currents, but high slew rate requires high current capability
Solution Approach 1:
The regulated current sources are designed to be ready to deliver high current immediately when needed, rather than relying on gradual charging. The circuit architecture allows the current sources to pre-position the necessary charge and be ready to rapidly charge or discharge the base capacitances of output transistors, achieving high slew rate without continuously maintaining high current capability.
Data Source
AI summary
An amplifying circuit which may be useful in a diamond buffer amplifier or operational amplifier includes an input transistor including an emitter, a collector, and a base coupled to receive an input voltage. An adjustable current source circuit is coupled between a first reference voltage and the emitter of the input transistor. A current source is coupled between a second reference voltage and the collector of the input transistor. An isolation resistor has a first terminal coupled to an output terminal of the adjustable current source circuit and a second terminal coupled to the emitter of the input transistor. A current follower circuit is coupled between the collector of the input transistor and an input terminal of the adjustable current source circuit. A feed-forward capacitor is coupled between the collector of the input transistor and the first terminal of the isolation resistor.