Low-voltage low-power variable gain amplifier
In high speed communication applications, e.g., optical communication, a variable gain amplifier is used for input signal amplitude normalization or for linear equalization. Traditionally a bipolar Gilbert multiplier circuit is used. When moving towards a low-power application, a modified circuit topology is implemented to reduce the minimum supply voltage requirement of the variable gain amplifier while ensuring that bias current levels remain substantially the same and achieving the same current switching capacity as the traditional circuit. As a result, the power consumption of the circuit can be greatly reduced. The modified circuit topology combines the amplifier and gain transistors and achieves gain programming using a voltage difference of two pairs of floating voltage sources.
1. A low-voltage low-power variable gain amplifier, the amplifier comprising:
first differential voltage nodes following differential input voltages at differential input nodes based on a first fixed voltage change;
first current branches driving differential output nodes;
second current branches paired with respective first current branches to divide, according to a coefficient, a bias current and a difference current representative of the differential input voltages;
first floating voltage sources, each one of the first floating voltage sources coupled between respective one of the first differential voltage nodes and respective one of first current branches to provide a first predetermined voltage change; and
second floating voltage sources, each one of the second floating voltage sources coupled between respective one of the first differential voltage nodes and respective one of second current branches to provide a second predetermined change in the same direction as the first predetermined voltage change;
wherein the first predetermined voltage change and the second predetermined voltage change correspond to the first fixed voltage change in magnitude but in an opposite direction and a difference between the first predetermined voltage change and the second predetermined voltage change sets the coefficient to provide variable gain of the amplifier.
2. The amplifier of claim 1 , wherein:
the first current branches and the second current branches comprises bipolar transistor devices; and
the amplifier operates properly with a supply voltage as low as 1.5 Volts.
3. The amplifier of claim 1 , wherein the amplifier has a minimum supply voltage requirement comprising one base-emitter voltage, compliance voltage of a current source generating the bias current, half of peak-to-peak voltage at the differential input nodes, and half of peak-to-peak voltage at the differential output nodes.
4. The amplifier of claim 1 , wherein:
the first differential voltage nodes are emitter nodes of emitter followers following respective differential input voltages; and
the first fixed voltage change is one base-emitter voltage.
5. The amplifier of claim 4 , wherein:
the first predetermined voltage change and the second predetermined voltage change are set to substantially offset the one base-emitter voltage in the opposite direction.
6. The amplifier of claim 1 , wherein:
the coefficient controls an amount of the bias current and the difference current representative of the differential input voltages flowing through the first current branches and a remaining amount of the bias current and the difference current representative of the differential input voltages flowing through the second current branches to achieve the variable gain of the amplifier.
7. The amplifier of claim 1 , wherein:
each one of the first current branches comprises a bipolar transistor whose base is driven by a respective first floating voltage source; and
each one of the second current branches comprises a bipolar transistor whose base is driven by a respective second floating voltage source.
8. The amplifier of claim 1 , further comprising:
a first node at the emitters of transistors in a first one of the first current branches and a first one of the second current branches;
a second node at the emitters of transistors in a second one of the first current branches and a second one of the second current branches; and
a resistance between the first node and the second node;
wherein voltage difference of the differential input voltages is replicated at the first node and the second node through the first and second floating voltage sources and the first and second current branches, and the difference current is the voltage difference replicated at the first and second node divided by the resistance.
9. The amplifier of claim 1 , further comprising:
second differential voltage nodes replicating a voltage difference of the differential input voltages through the first differential voltage nodes, the first and second floating voltage sources, and the first and second current branches;
wherein the difference current is generated by the replicated voltage difference across a resistance between the second differential voltage nodes.
10. The amplifier of claim 1 , wherein:
the difference current is generated by a replicated voltage difference across a resistance; and
the replicated voltage difference is based on (1) a first one of the differential input voltage shifted by a fixed amount comprising the first predetermined voltage change and the second predetermined voltage change, and (2) a second one of the differential input voltage shifted by the same fixed amount.
11. The amplifier of claim 1 , wherein:
the difference current is independent from the difference between the first predetermined voltage change and the second predetermined voltage change.
12. The amplifier of claim 1 , wherein:
the difference between the first predetermined voltage change and the second predetermined voltage change is on the order of a hundred millivolts.
13. A low-voltage low-power variable gain amplification method comprising:
driving a base of a first transistor with a first voltage following a first one of differential input pair shifted by a first predetermined voltage shift, wherein the first transistor is coupled to a first one of a differential output pair;
driving a base of a second transistor with the first voltage shifted by a second predetermined voltage shift;
driving a base of a third transistor with a second voltage following a second one of a differential input pair shifted by the first predetermined voltage shift, wherein the third transistor is coupled to a second one of a differential output pair;
driving a base of a fourth transistor with the second voltage shifted by the second predetermined voltage shift;
generating a difference current based on a voltage difference of the differential input pair; and
dividing, between the first transistor and the second transistor and between the third transistor and the fourth transistor, a bias current and the difference current based on a difference between the first predetermined voltage shift and the second predetermined voltage shift.
14. The method of claim 13 , wherein:
the difference between the first predetermined voltage shift and the second predetermined voltage shift corresponds to a gain coefficient of a variable gain amplifier.
15. The method of claim 13 , wherein:
generating the difference current comprises generating the difference current independent from the difference between the first predetermined voltage shift and the second predetermined voltage shift.
16. The method of claim 13 , further comprising:
replicating the voltage difference of the differential input pair using a same set of voltage shifts for both (1) a first path from the first one of the differential input pair to emitters of the first transistor and the second transistor, and (2) a second path from the second one of the differential input pair to emitters of the third transistor and the fourth transistor.
17. The method of claim 16 , wherein the same set of voltage shifts comprises the first predetermined voltage shift, and the second predetermined voltage shift.
18. The method of claim 13 , wherein:
the first predetermined voltage shift and the second predetermined voltage shift are in an opposite direction of a voltage shift from the first one of the differential pair to the first voltage.
19. The method of claim 13 , further comprising:
converting collector currents of the first transistor and the third transistor to voltages at the differential output pair using output resistors.
20. A low-voltage low-power variable gain amplifier comprising:
a first input buffer following a first one of differential inputs;
a second input buffer following a second one of the differential inputs;
an output stage replicating a voltage difference of the differential inputs and converting the replicated voltage difference to a difference current independent of a gain setting;
a first gain setting stage controlling the output stage comprising a first floating voltage source and a second floating voltage source for shifting an output of the first input buffer by a first predetermined voltage change and a second predetermined voltage change respectively; and
a second gain setting stage controlling the output stage comprising a third floating voltage source and a fourth floating voltage source for shifting an output of the second input buffer by the first predetermined voltage change and the second predetermined voltage change respectively;
wherein a difference between the first predetermined voltage change and the second predetermined voltage change sets the gain setting being applied to a bias current and the difference current to provide a variable gain of the amplifier.
21. The low-voltage low-power variable gain amplifier of claim 20 , wherein the first predetermined voltage change and the second predetermined voltage change are set to substantially offset, in the opposite direction, a base-emitter voltage change in the first input buffer and in the second input buffer.
22. The low-voltage low-power variable gain amplifier of claim 20 , wherein:
the replicated voltage difference is based on (1) the first one of the differential inputs shifted by a fixed amount comprising the first predetermined voltage change and the second predetermined voltage change, and (2) the second one of the differential inputs shifted by the same fixed amount.
23. The low-voltage low-power variable gain amplifier of claim 20 , wherein the difference current is generated by the replicated voltage difference across a resistance between differential voltage nodes having the replicated voltage difference.