High gain, high speed comparator operable at low current
View Patent ↗A comparator includes a differential pair of transistors providing a first amplification stage and receiving inverting and non-inverting input signals. An output transistor is coupled to the differential pair of transistors providing a second amplification stage and transitioning the output signal state when the non-inverting input signal is larger than the inverting input signal. The output node of one of the differential pair of transistors is connected to an input node of a current-tail transistor. The output node of the other differential transistor is connected to an input node of the output transistor. The other nodes of the differential pair of transistors are connected to each other and are coupled to an output node of the current-tail transistor. The output nodes of the differential pair of transistors and an output node of the output transistor are each coupled to a separate current generator that may include a complex impedance element.
1. A comparator comprising
a differential pair of transistors for (a) providing a first stage of amplification and (b) receiving inverting and non-inverting input signals,
a current-tail transistor coupled to the differential pair of transistors and configured to have a current flow that is substantially equal to the sum of the currents flowing in each of the differential pair of transistors,
an output transistor having a grounded node, the output transistor coupled to the differential pair of transistors for (a) providing a second stage of amplification and (b) transitioning an output signal from a low level state to a high level state, when the non-inverting input signal is higher than the inverting input signal, and
a feedback transistor coupled to the current-tail transistor for establishing the currents flowing in each of the differential pair of transistors,
wherein an output node comprising either a drain or a collector of one of the differential pair of transistors is directly connected to an input node of the current-tail transistor, and
an output node of the other one of the differential pair of transistors is connected to an input node of the output transistor.
2. The comparator of claim 1 wherein
input nodes of the differential pair of transistors receive the inverting and non-inverting input signals,
emitter or source nodes of the differential pair of transistors are connected to each other forming connected nodes, and
the connected nodes are coupled to an output node of the current-tail transistor.
3. The comparator of claim 2 wherein
a grounded node of the current-tail transistor and the grounded node of the output transistor are connected to each other.
4. The comparator of claim 3 wherein
respective output nodes of the differential pair of transistors, an output node of the output transistor and an output node of the feedback transistor are each coupled to a separate current generator.
5. The comparator of claim 4 wherein
each current generator includes a field effect transistor (FET) coupled to each respective output node, and
the FET is biased by a current flowing in the feedback transistor.
6. The comparator of claim 4 wherein
the current generators provide currents of i 1 and i 2 to the output nodes, respectively, of the differential pair of transistors, and
a current of i 3 is provided to the output node of the current-tail transistor, the current i 3 having substantially the following relationship:
i 1+ i 2= i 3.
7. The comparator of claim 6 wherein
the current generators provide a current of i 5 to the output node of the output transistor, and
the current of i 5 mirrors the current of i 3 .
8. The comparator of claim 5 wherein
an input node of the feedback transistor is connected to the input node of the current-tail transistor.