Embodiments of this invention relate to methods of compensating, and, more particularly, in at least one embodiment, to analog delay lines and methods using differential amplifiers.
Differential amplifiers are commonly used for a variety of purposes, including for delay stages in analog delay lines. In such analog delay lines, a plurality of differential amplifiers are coupled in series from a first differential amplifier to a last differential amplifier. A differential signal applied to differential inputs of the first differential amplifier may be successively delayed as it propagates through each of the differential amplifiers. As a result, a differential signal that is produced at differential outputs of the last differential amplifier may have a phase or timing that is delayed relative to the differential signal that is applied to the input of the first differential amplifier.
In some cases, it may be desirable for the differential signal coupled through the differential amplifiers of an analog delay line to have an amplitude that is as small as possible. A signal with a small amplitude may cause transistors used in the differential amplifier delay stages to consume less power than would be consumed by propagating a differential signal through the differential amplifiers having a larger amplitude. Further, a differential signal having a smaller amplitude may cause the magnitude of a delay provided by an analog delay line to be less sensitive to changes in a supply voltage used to power the analog delay line. For example, in its extreme example, a differential signal having a peak-to-peak amplitude equal to the difference between two supply voltages may cause the delay to vary substantially with the magnitude of the supply voltages since the differential signal propagating through the delay line may transition between the two supply voltages.
In practice, it may be difficult to set the amplitude of the differential signal to a minimum value that will propagate through the analog delay stages. Even if a differential signal having a small amplitude is applied to the first differential amplifier, if the differential amplifiers used as the delay stages have too much gain, the amplitude of the differential signal propagated through the delay line may progressively increase until it reaches a level that may result in excessive power consumption and sensitivity to power supply voltage changes. On the other hand, if the gain of the differential amplifier is too small, the differential signal propagated through the delay line may progressively decrease until it disappears. It may therefore be desirable to control the gain of differential amplifiers used in analog delay lines and other circuits to a fairly precise value.
The gain of one commonly used differential amplifier using a pair of differential input transistors is proportional to the transconductance of the transistors, which is often abbreviated as “gm.” More specifically, in many such amplifiers, the gain is the product of gm and the impedance of a load, which may be a transistor or resistance. If the load is a resistance that does not change with such factors as semiconductor process variations, supply voltage variations, or temperature variations, the gain of a differential amplifier may be proportional to only the gm of the differential input transistors. If the gm of the differential input transistors can be precisely controlled so that the differential amplifier has a substantially unity gain, the signal propagated through an analog delay line using such differential amplifiers as its delay stages may have a minimum amplitude, thereby allowing the delay line to consume relatively little power and to be substantially insensitive to power supply and other variations.
Unfortunately, it may be difficult to control the gain of differential amplifiers in the presence of process, supply voltage and temperature variations. As a result, it may be necessary to apply a differential input signal to the first stage of an analog delay line having an amplitude that is large enough to ensure propagation through the delay line assuming worst case gains of the differential amplifier stages. The result may be a less than ideal power consumption and supply voltage susceptibility of the delay line.
An analog delay line 10 is shown in
As is well-known in the art, the gain “G” (i.e., the ratio of the differential output voltage Out-OutF to the differential input voltage In-InF) of the differential amplifiers 12 is given by the formula:
G=vin*gm*R∥Rt [Equation 1],
where Vin is the amplitude of the differential input signal, i.e., In-InF, gm is the transconductance of the input transistors 16, 18, R is the combined resistance of the load resistances 24, 26, Rt is the source-to-drain resistance of the transistors 16, 18, and R∥Rt is the parallel resistance of R and Rt. In practice, the combined resistances 24, 26 may be much smaller than the source-to-drain resistance of the transistors 16, 18 so that R∥Rt may essentially be equal to R. The transconductance gm is, in turn, a function of the current drawn through the input transistors 16, 18 by the current sink 20. As shown in
An example of a prior art compensating circuit 40 that may be used as the compensating circuit 34 is shown in
In operation, the magnitude of the feedback voltage Vf corresponds to the magnitude of the supply voltage Vcc less a voltage drop across the load 46. Insofar as the voltage drop across the load 46 is directly proportional to the current through the load 46, and hence the current drawn by the transistor 44, the magnitude of the feedback voltage Vf may be inversely proportional to the current drawn by the transistor 44. The current drawn by the transistor 44 may, in turn, be proportional to the transconductance gm of the transistor 44 and the voltage at the output of the differential amplifier 48. Thus, if the transconductance gm of the transistor 44 increases, the current drawn by the transistor 44 will increase, thereby decreasing the magnitude of the feedback voltage Vf so that it is less than the magnitude of the reference voltage Vr. As a result, the voltage at the output of the differential amplifier 48 will decrease, thereby reducing the voltage applied to the gate of the transistor 44. The current drawn by the transistor 44 will then decrease to increase the magnitude of the feedback voltage Vf. If the closed loop gain through the transistor 44 and differential amplifier 48 is sufficiently high, the voltage applied to the gate of the transistor 44 will be reduced to a level that causes the current through the load 46 to decrease sufficiently such that the feedback voltage is again substantially equal to the magnitude of the reference voltage Vr. The compensating circuit 40 operates in a similar manner in response to a decrease in the transconductance gm of the transistor 44. Thus, the magnitude of the current drawn by the transistor 44 is maintained substantially constant, i.e., at the current that causes the feedback voltage Vf to be substantially equal to the reference voltage Vr. The compensating circuit 40 may therefore ensure that the current drawn by the transistor 44 is substantially insensitive to changes in the transconductance gm of the transistor 44 resulting from various factors such as process variations in fabricating the transistor 44, the supply voltage Vcc, and the temperature of the transistor 44.
The transconductance gm of the transistor 44 may change in the same manner as the transconductance gm of a transistor used as the transistor 30 (
Although the compensating circuit 40 may be able to make the analog delay line 10 substantially insensitive to changes in the transconductance gm of the current sink transistors in the differential amplifiers 12, it may not compensate for changes in the transconductance gm of the input transistors 16, 18 used in the differential amplifiers 12. However, an embodiment of a compensating circuit 50 shown in
As further shown in
In operation, it is assumed that the gains of differential amplifiers used in an analog delay line should be unity so that the amplitude of a signal propagating through the delay line can be controlled with some degree of precision. If the gain is significantly greater than unity, a signal coupled though a delay line may progressively increase in magnitude until the signal transitions between Vcc and ground. Conversely, if the gain is significantly less than unity, a signal coupled though a delay line may progressively decrease in magnitude until it is no longer being propagated through the delay line. The voltage gain of the bias voltage generator 54 is inherently unity since voltage gain is defined as the ratio of the output voltage to the input voltage, and the differential output voltage Vout-VoutF is equal to the differential input voltage Vout-VoutF. As explained above, the gain G of the bias voltage generator 54 is given by the formula:
G=Vin*gm*R∥Rt [Equation 1],
where Vin is the amplitude of the differential input signal, i.e., Aout-AoutF, gm is the transconductance of the transistors 56, 58, R is the combined resistance of the loads 64, 66, Rt is the output resistance of the transistors 56, 58, and R∥Rt is the parallel resistance of R and Rt, which, as also explained above, may be essentially equal to R if the resistance of the loads 64, 66 is made substantially greater than the output resistance of the transistors 56, 58. With a unity gain (G=1), and insofar as Vout=Vin Equation 1 can be rewritten as:
Vout=1/[gm*R∥Rt] [Equation 2].
If the resistance of the loads 64, 66 is made substantially greater than the output resistance of the transistors 56, 58, Equation 3 can be reduced to:
Vout=1/[gm*R] [Equation 3].
Thus, the differential output voltage Aout-AoutF can be set by selecting an appropriate resistance of the loads 64, 66 and gm of the transistors 56, 58, which can be adjusted by varying the current drawn by the current sink transistor 62.
In operation, the differential-to-single converter 68 provides a feedback voltage FB that corresponds to the differential output voltage of the bias voltage generator 54, i.e., Aout-AoutF. The differential amplifier 70 then adjusts the current drawn by the current sink transistor 62 to adjust the gm of the input transistors 56, 58 until the magnitude of the feedback voltage FB is substantially equal to the magnitude of the reference voltage SwingRef. Thus, since the feedback voltage FB corresponds to the differential output voltage of the bias voltage generator 54, i.e., Aout-AoutF, the gain of the bias voltage generator 54 may be automatically adjusted so that the gain is substantially unity at a specific differential output voltage. However, there may initially be no voltage difference between the voltages applied to the respective gates of the transistors 56, 58 (i.e., the differential input voltage may be zero) or the applied voltage may be of the wrong polarity. A start circuit 74 may therefore be provided to initially provide (e.g., apply, supply, output, etc.) a differential voltage to the gates of the transistors 56, 58 that ensures that the magnitude of Aout is greater than the magnitude of AoutF and of the correct polarity so that Aout is greater than AoutF. The magnitude of the voltage is not critical since the input/output voltages Aout, AoutF will automatically adjust to the correct values so that the gain of the bias voltage generator 54 is substantially unity.
An embodiment of a differential-to-single converter 80, which may be used as the differential-to-single converter 68 shown in
In operation, if the magnitude of the feedback voltage FB may be indicative of, and varies inversely with, the magnitude of the differential input voltage Vout-VoutF, the differential-to-single converter 80 thus provides a single voltage FB corresponding to the magnitude of the different output voltage Vout-FoutF of the bias voltage generator 54.
The compensating circuit 50 or a compensating circuit according to some other embodiment may be used with an embodiment of an analog delay line 100 as shown in
The manner in which the analog delay line 100 operates may be explained with reference to the chart shown in
As can be seen from
Insofar as the Bias voltage applied to the gate of the current sink transistor 130 in each of the delay stages 112 may be set to cause the delay stage to have a substantially unity gain at a differential output voltage amplitude corresponding to the SwingRef (
Although the present invention has been described with reference to the disclosed embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the invention. Such modifications are well within the skill of those ordinarily skilled in the art. Accordingly, the invention is not limited except as by the appended claims.
This application is a divisional of U.S. patent application Ser. No. 12/854,749, filed Aug. 11, 2010, issued as U.S. Pat. No. 8,283,950 on Oct. 9, 2012. This application and patent are incorporated herein by reference, in their entirety, for any purpose.
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Number | Date | Country | |
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Number | Date | Country | |
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Parent | 12854749 | Aug 2010 | US |
Child | 13605739 | US |