Low-voltage low-power variable gain amplifier

Patent metadata

Field	Value
Publication number	US-2017194911-A1
Application number	US-201614989203-A
Country	US
Kind code	A1
Filing date	Jan 6, 2016
Priority date	Jan 6, 2016
Publication date	Jul 6, 2017
Grant date	—

Abstract

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.

First claim

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.

Assignees

• Analog Devices Global

Inventors

• Aksin Devrim

Classifications

• H03F2203/45112

  the biasing of the differential amplifier being controlled from the input or the output signal · CPC title

• H03F3/4508 Primary

  using bipolar transistors as the active amplifying circuit (H03F3/45278 takes precedence) · CPC title

• H03F1/0211 Primary

  with control of the supply voltage or current · CPC title

• H03G3/30

  in amplifiers having semiconductor devices · CPC title

• H03F2203/45701

  the LC comprising one resistor · CPC title