# Common-Mode Voltage Control for Mixed-Voltage Applications

Original Publication Date: 2003-Feb-20

Included in the Prior Art Database: 2003-Jun-21

## Publishing Venue

IBM

## Abstract

Disclosed is a circuit topology that translates common-mode voltage between circuits. A common-mode translation between voltage domains alleviates common-mode mismatch between circuits and consequently common-mode noise. Controlling common-mode mismatch is important in Infiniband applications since the Infiniband standard requires a low common-mode noise voltage and allows the common-mode voltage of the output driver to driver to vary. The topology consists of a differential amplifier that has an easily controllable common-mode voltage and a bias circuit that sets the common-mode voltage of the amplifier. The desired common-mode voltage of the amplifier is a weighted average of the common-mode of the circuit driving the amplifier and the circuit being driven by the amplifier. Directly measuring the common-mode voltage of the amplifier’s output and subsequently adjusting the bias current of the amplifier sets the common-mode voltage of the amplifier. The bias current is adjusted until the amplifier's common-mode voltage is the desired weighted average. The common-mode voltage of the amplifier’s output is available on an internal node of the amplifier. Common-mode voltage is defined in terms of a differential signal. A differential signal is “two nodes that have equal but opposite signal excursions around a fixed potential Fairchild Semiconductor, http://www.fairchildsemi.com/products/interface/techfeature.html ). The fixed potential is the common-mode voltage of the differential signal, and a differential signal’s nodes are equal only when the nodes are at their common-mode voltage under normal operating conditions. Generally, the common-mode voltage of an amplifier’s differential output is the voltage at each output node when the differential input nodes are both at their common-mode voltage. Furthermore, the instantaneous common-mode voltage of a differential signal is the average of the two nodes of the differential signal.

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__Page 1 of 5__**Common-Mode Voltage Control for Mixed-Voltage Applications**

Disclosed is a circuit topology that translates common-mode voltage between circuits.

A common-mode translation between voltage domains alleviates common-mode mismatch between circuits and consequently common-mode noise. Controlling common-mode mismatch is important in Infiniband applications since the Infiniband standard requires a low common-mode noise voltage and allows the common-mode voltage of the output driver to driver to vary.

The topology consists of a differential amplifier that has an easily controllable common-mode voltage and a bias circuit that sets the common-mode voltage of the amplifier. The desired common-mode voltage of the amplifier is a weighted average of the common-mode of the circuit driving the amplifier and the circuit being driven by the amplifier. Directly measuring the common-mode voltage of the amplifier's output and subsequently adjusting the bias current of the amplifier sets the common-mode voltage of the amplifier. The bias current is adjusted until the amplifier's common-mode voltage is the desired weighted average. The common-mode voltage of the amplifier's output is available on an internal node of the amplifier.

Common-mode voltage is defined in terms of a differential signal. A differential signal is "two nodes that have equal but opposite signal excursions around a fixed potential (*Fairchild Semiconductor, http://www.fairchildsemi.com/products/interface/techfeature.html*). The fixed potential is the common-mode voltage of the differential signal, and a differential signal's nodes are equal only when the nodes are at their common-mode voltage under normal operating conditions. Generally, the common-mode voltage of an amplifier's differential output is the voltage at each output node when the differential input nodes are both at their common-mode voltage. Furthermore, the instantaneous common-mode voltage of a differential signal is the average of the two nodes of the differential signal.

The differential amplifier in Fig. 1 can be biased such that the common-mode voltage Vcm is controllable. Generally, the common-mode voltage of the differential-pair amplifier is the gate volt-age of its active load. When the input nodes DP and DN are equal and at their common-mode voltage, I1 must equal I2, the node OP must equal the node ON, and I1=I2 = Ibias/2 by symmetry. The voltages OP and ON must both be Vcm since no voltage drop is present across the resistors and no current is drawn by the FET gates. Thus, Vcm is the common-mode voltage of the amplifier.

1

__Page 2 of 5__VBIAS

(W/L)B

^{(W/L)}(W/L)S

S

Ibias

VDD

**Figure 1**: Differential Amplifier Topology

The voltage Vcm is given by the FET current equation I1=I2 = KL*(Vcm-Vtn)2 = Ibias/2. The bias current Ibias is given by Ibias = KB*(VBIAS-VDD-Vtp)2. Consequently, the bias voltage VBIAS can be set such that t...