Overdrive protection clamp scheme for feedback amplifiers

Abstract

A low-noise, low-distortion clamping scheme includes a bootstrapped voltage clamp and an R.sub.gm current clamp that provide superior overdrive protection when used together in a Class-AB feedback amplifier. The bootstrapped voltage clamp includes a transistor that is connected to a circuit node to be clamped. The transistor's base is bootstrapped to the node to maintain a constant V.sub.be when not clamping, to reduce the adverse effects of the junction capacitance C.sub.je which would normally vary with the node voltage and distort the signal at the node. Two such clamps provide positive and negative voltage limiting. The R.sub.gm current clamp is used in the input stage of a Class-AB feedback amplifier to limit the current through the resistor R.sub.gm that interconnects the current inputs of two transconductance amplifiers whenever the voltage drop across R.sub.gm increases to an unacceptable level. The clamp uses a dependent current source whose output current increases with the magnitude of the current flowing through R.sub.gm and prevents that current from exceeding a predetermined limit. When not clamping, the clamp has a very small effect on the amplifier's input stage. Two R.sub.gm current clamps are typically used to limit current flowing in either direction through R.sub.gm.