US 5,528,192 A · Bi-mode circuit for driving an output load

Abstract

A bi-mode circuit for driving an output load selectively couples the output load to a supply voltage source or to a low discharge voltage source such as ground using switches which are controlled by an input buffer in response to an input signal. A high input signal closes a first switch to provide a biasing current to first and second current amplifiers to turn on a first output transistor which couples the output load to the low reference voltage to discharge the output load. Conversely, a low input signal closes a second switch to provide the biasing current to a third current amplifier to turn on a second output transistor which couples the output load to the supply voltage source. When the input signal becomes high, rapid pulldown of a capacitive output load is achieved using a high internal pre-drive current provided by the first and second current amplifiers, in a first mode of operation. As the output voltage decreases, turnoff of the first output transistor is prevented by switching to a second mode when the output voltage drops below a threshold value as determined by a comparator. The comparator effectively closes a third switch and thereby a loop which bypasses the first current amplifier to prevent the drive current from escaping the first current amplifier, so that the second current amplifier continues to amplify the biasing current to maintain the first output transistor on.

Claims35

Dependent claims

Apparatus for driving an output load in response to an input signal and in reference to a low voltage source, comprising:

a transistor coupled between the output load and the low voltage source, wherein the transistor couples the output load to a low voltage source to pull down the output load when biased by a current;
means for biasing the transistor into conduction under the control of the input signal comprises means providing a biasing current, a first current amplifier for amplifying the biasing current, and a second current amplifier for amplifying the biasing current from the first amplifier and applying it to the transistor;
and
means responsive to the voltage at the output load for bypassing the first current amplifier and amplifying the biasing current with the second current amplifier to the exclusion of the first current amplifier when the voltage at the output load falls below a predetermined level.

1 more element

Apparatus for driving an output load in response to an input signal, comprising the combination of:

means providing a biasing current;
a first current amplifier;
an input stage which is responsive to the input signal for applying the biasing current to the first current amplifier when the input signal has a high value, the first current amplifier being biased by a voltage at the output load to amplify the biasing current except when the voltage at the output load is less than a reference voltage;
a second current amplifier coupled to the first current amplifier and operative to amplify said biasing current amplified by the first current amplifier;
a first transistor coupled between the output load and a low voltage source and coupled to be biased by biasing current amplified by the second current amplifier;
means for comparing the voltage at the output load with the reference voltage;
and
a control stage which is responsive to the means for comparing for biasing the second current amplifier to the exclusion of the first current amplifier to amplify the biasing current whenever the voltage at the output load becomes less than the reference voltage.

5 more elements

Apparatus for driving a capacitive output load, comprising the combination of:

a capacitive output load;
discharge means coupled to discharge the capacitive output load in response to a further amplified biasing current;
a second current amplifier coupled to the discharge means to provide the further amplified biasing current;
a first current amplifier coupled to the second current amplifier to provide an amplified biasing current to the second current amplifier in response to a biasing current at an input of the first current amplifier;
means for biasing the second current amplifier in response to an input signal, even when a voltage at the capacitive output load falls below a reference value;
wherein the means for the second current amplifier includes means for coupling the biasing current to the input of the first current amplifier to amplify the biasing current and to provide the amplified biasing current to the input of the second current amplifier when the voltage at the capacitive output load is above the reference value and means for bypassing the first current amplifier to provide the biasing current to the second current amplifier when the voltage at the capacitive output load is below the reference value;
wherein the biasing current is provided by a bias current source, the first current amplifier comprises a first transistor having a base terminal coupled to the bias current source and having emitter and collector terminals, and a first resistor coupled between the base and emitter terminals of the first transistor, the emitter terminal of the first transistor being coupled to the second current amplifier, and the means for bypassing comprises a second transistor having a base terminal coupled to the bias current source, an emitter terminal coupled to the collector terminal of the first transistor and a collector terminal coupled to the second current amplifier.

4 more elements

Apparatus for receiving a bias signal and for providing a controlled biasing signal for driving an output load having a load signal, the biasing signal dependent upon the load signal of the output load, the load signal being variable across a predetermined reference value, the apparatus comprising:

a switch coupled between the output load and the input of the first signal amplifier and controllable between a first state and a second state, the switch being in the first state upon the load signal being below the reference value and in the second state upon the load signal being above the reference value;
a first signal amplifier having an input for receiving the bias signal and an output, the first signal amplifier being coupled to the switch such that the first amplifier is shunted between its input and output by the switch upon the switch being in the first state;
a second signal amplifier having an input coupled to the output of the first amplifier, the second signal amplifier having an output for providing the controlled biasing signal;
and
discharge means coupled between the output of the second signal amplifier and the output load to discharge the output load in response to the controlled biasing signal.

2 more elements

Apparatus for receiving a bias signal and for providing a controlled biasing signal to a biased device having an input and an output, the controlled biasing signal derived from the bias signal and dependent upon a load signal produced by a load having a port, the load signal being available at the port of the load, the load signal being variable across a predetermined reference value, wherein the output of the biased device is coupled to the port of the load, the apparatus comprising:

a switch circuit coupled between the load and the input of the first signal amplifier and controllable between a first state and a second state, the switch circuit being in the first state upon the load signal being below the reference value and in the second state upon the load signal being above the reference value;
a first signal amplifier having an input for receiving the bias signal and an output, the first signal amplifier being coupled to the switch such that the first amplifier is bypassed by the switch upon the switch being in the first state;
a second signal amplifier having an input coupled to the output of the first signal amplifier, the second signal amplifier having an output for coupling the controlled biasing signal to the biased device input.

Dependencies are inferred from the claim text.

Bi-mode circuit for driving an output load

Life of this patent

Bi-mode circuit for driving an output load

Life of this patent

Abstract

Claims35