Overdrive protection clamp scheme for feedback amplifiers

An overdrive-protection clamp scheme suitable for use with a feedback amplifier, comprising:

• a feedback amplifier, comprising:
• a first transconductance amplifier having a first voltage input, a first current input and a first pair of current outputs,
• a second transconductance amplifier having a second voltage input, a second current input and a second pair of current outputs, said first and second pairs of current outputs operably summed at a summing node and said first and second current inputs interconnected with a resistor R.sub.gm, and
• an output stage connected to said summing node and producing an output voltage that is fed back to said second voltage input, said output voltage varying with an input voltage presented at said first voltage input when said amplifier is not overdriven;
• at least one V.sub.be -bootstrapped voltage clamp, each comprising:
• a bipolar transistor having an emitter connected to said summing node and a base that is bootstrapped to said summing node so that said transistor's base-emitter voltage remains about constant as the voltage at said node varies and said amplifier is not overdriven, and
• a comparison circuit connected to turn on said transistor when said summing node voltage exceeds a predetermined voltage to clamp said summing node at said predetermined voltage;
• and
• at least one current clamp for limiting current through said resistor R.sub.gm interconnecting said current inputs, each of said at least one current clamps comprising a dependent current source connected to one terminal of said resistor R.sub.gm, said current source dependent on the current flowing through said resistor and limiting said current through said resistor to a predetermined value.

A V.sub.be -bootstrapped voltage clamp suitable for clamping a circuit node to prevent said node from exceeding a predetermined clamp voltage, comprising:

• a comparison circuit having first and second inputs, said first input connected to a reference voltage and said second input connected to a level-shifting circuit connected to a circuit node, and an output that in a first state varies with said circuit node when the voltage at said second input is less than the voltage at said first input and in a second state is a fixed voltage when the voltage at said second input is greater than the voltage at said first input,
• a first transistor having a current circuit and a control input that modulates said current circuit, said control input connected to receive said comparison circuit output and said current circuit connected to said circuit node, such that when said comparison circuit output is in said first state said first transistor is bootstrapped to said circuit node reducing the adverse effects of said first transistor's junction capacitance, and when said comparison circuit output is in said second state and the voltage drop between said control input and said circuit node is sufficiently large said first transistor is turned on and clamps said node at a predetermined clamp voltage, and
• a level-shifting circuit connected between said circuit node and said comparison circuit, said level-shifting circuit receiving the voltage at said node as an input, level-shifting said voltage and presenting said level-shifted voltage to the second input of said comparison circuit, such that the voltage at said first transistor's control input is about equal to the voltage at said node when said comparison circuit output is in said first state.