Common-mode biasing

Common-mode biasing#

If an amplifier shows natural two-port behavior, the common-mode port voltages need to be defined. This is because the floating ports of a natural two-port have an infinite common-mode impedance, and any common-mode current caused by external noise sources or temperature variations may drive the common-mode port voltage outside its desired operating range. In practice, this operating range will be limited by a power supply voltage or by physical breakdown voltages of circuit components in the amplifier.

AC coupling#

Fig. 313 shows a concept with a common-mode port impedance of zero for both ports. The input common-mode voltage has been set to ground level and the output common-mode level has been set to \(V_{cm}\) with respect to the ground.

../_images/trafoIsolationCMcontrol.svg

Fig. 313 Balanced transformer coupled amplifier with zero input common-mode impedance and output common-mode impedance, zero input common-mode voltage and its output common-mode voltage set tos \(V_{cm}\).#

Alternatively, one may consider the use of brute-force techniques as they have been demonstrated for the biasing of the input port of the operational amplifier (see section Deriving bias quantities from the power supply).

DC-coupled floating port amplifiers#

There are four techniques for defining DC common-mode voltages of floating ports:

  1. Brute force methods

    ../_images/bruteForceCMbias.svg

    Fig. 314 Brute-force common-mode biasing of the input port and the output port of a natural two-port.#

    We speak of brute force techniques if the common-mode voltages are fixed by simply connecting the port terminals to a voltage source by means of an impedance that allows for DC transfer. Fig. 314 shows the principle with resistors. The common-mode input resistance of this circuit equals \(R_{ci}\) and the common-mode output resistance \(R_{co}\). The common-mode input voltage and the common-mode output voltage have been set to \(V_{cmi}\) and \(V_{cmo}\), respectively. Brute force fixing of the common-mode input voltage of a floating port affects the differential-mode impedance of that port. In order to keep the possible adverse effects on the signal processing quality as small as possible, the sum of the two brute force impedances should be much larger than the differential driving point impedance. \sidenote [][-1cm]{The driving point impedance of a port is the in-circuit measured impedance at that port.}

  2. Feed forward techniques

    Feed forward techniques make use of the existing nonzero common-mode transfer of an amplifier or an amplifier stage.

  3. Fig. 315 combines brute force common-mode biasing of the input port of a two-port with feed forward biasing of its output port. An example of a feed forward biasing of the output port of a balanced transresistance amplifier has been shown in Fig. 316. The common-mode output voltage of this circuit is fixed by both \(I_{cm}\) and \(V_{cmi}\):

    \[V_{ocm}=V_{cmi}+I_{cm}R_{f}.\]
    ../_images/feedForwardCMbias.svg

    Fig. 315 Feed forward common-mode biasing of the output port of a two-port with a finite, nonzero common-mode transfer.#

  4. Local common-mode feedback

    Local common-mode feedback is a technique in which the common-mode voltage or current of a port is measured, compared with a reference, and controlled by adding a common-mode current or voltage at the same port. This is shown in Fig. 317.

    The common-mode input impedance of this circuit equals \(1/G_{cm}\) while its differential-mode impedance is infinity. Fig. 318 shows the concept with two transimpedance amplifiers.

    ../_images/localFeedbackCMbias.svg

    Fig. 317 Principle of common-mode biasing of a floating port with the aid of local feedback.#

  5. Over-all common-mode feedback

    With over-all common-mode feedback, a common-mode output quantity is measured and controlled by a common-mode input quantity. This is only possible if the two-port has a nonzero common-mode transfer from the controlling quantity to the controlled quantity.

    Fig. 319 shows an arrangement in which the common-mode output voltage of a two-port is controlled by its common-mode input current. These techniques will be discussed in more detail in the volume about transistor-level design.

../_images/overAllCMfeedback.svg

Fig. 319 Common-mode biasing of a floating port with the aid of over-all common-mode feedback. The common-mode output voltage is measured, compared with a set point, and contolled by inserting a common-mode input current. The two-port needs to have a nonzero common-mode transresistance.#

~\vfill

Contents