Patent Application
20130113563
1. Field of the Invention
The present invention generally relates to amplifiers. More specifically, the present invention provides significant gain enhancement while minimizing noise introduction for differential amplifiers using cascode structures.
2. Background Art
Differential amplifiers are designed generally to amplify input signals while minimizing noise introduction and signal distortion. Conventional differential amplifiers are designed often using larger-sized transistors to boost gain and lower noise but at the expense of introducing higher levels of parasitic capacitance. Alternatively, conventional differential amplifiers can use smaller-sized transistors that provide less gain to maintain signal integrity. However, to provide required gain levels, conventional differential amplifiers that use smaller-sized transistors often require multiple gain stages and therefore additional circuit elements which increases design complexity and costs.
The cascode transistor pair 106 and 108 is coupled to the differential transistor pair 102 and 104 to increase the gain of the conventional differential amplifier 100. Transistor 106 is coupled to a biasing source (labeled for simplicity as “BIAS” in
The conventional differential amplifier 100 is coupled to a second stage amplifier (not shown in
As the size of a CMOS transistor is decreased, the intrinsic gain of the CMOS transistor—defined as the change in drain voltage for a change in gate voltage (or source voltage)—also decreases. The intrinsic gain can generally be considered to be the ratio of RDS to 1/gm of the CMOS transistor, where RDS is the drain-to-source resistance and gm is the transconductance of the CMOS transistor.
For a CMOS transistor fabricated in a 65 nm process, the intrinsic gain of the CMOS transistor is approximately 5 to 7. Using CMOS transistors of this scale in the conventional differential amplifier 100, the gain of the differential transistor pair 102, 104 is approximately 5 to 7, the gain of the cascode transistor pair 106, 108 is also approximately 5 to 7, and the overall gain of the conventional differential amplifier is approximately 25 to 49. While CMOS transistors at the 65 nm node generally provide greater bandwidth, the overall gain of the conventional differential amplifier 100 is generally too low for many applications when such transistors are used. Using larger-sized CMOS transistors to boost the overall gain of the conventional differential amplifier 100, however, introduces too much parasitic capacitance for many applications. The design of the conventional differential amplifier 100 may therefore be deficient in terms of overall gain when, for example, CMOS transistors fabricated in a 65 nm process, or even larger transistors, are employed.
Accordingly, what is needed is a differential amplifier design that can provide high levels of gain using low noise transistors without requiring multiple gain stages.
The accompanying drawings illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable one skilled in the pertinent art to make and use the invention.
Aspects of the present invention provide apparatuses and methods to provide significant gain enhancement for a cascode structure for a differential amplifier. The cascode structure of the differential amplifier can include first and second pairs of output transistors. The second pair of output transistors can be configured to approximately cancel modulation effects of the first pair of output transistors, thereby resulting in conditions for providing enhanced gain.
First outputs of the first pair of output transistors can provide a differential output of the differential amplifier. Second outputs of the first pair of transistors can be coupled to input transistors that receive a differential input signal. Inputs of the second pair of output transistors can be cross-coupled to the first outputs of the first pair of output transistors. First outputs of the second pair of output transistors can be coupled to the first outputs of the first pair of output transistors. The second pair of output transistors can be sized to be proportionally larger than the first pair of output transistors relative to the intrinsic gains of the first pair of output transistors. In doing so, second outputs of the second pair of output transistors can provide changes in output to approximately match changes in output of the second outputs of the first pair of output transistors when changes in the differential output occur. The modulation effects of the second outputs of the first pair of output transistors are thereby approximately canceled by the output provided by the second outputs of the second pair of output transistors. As a result, an approximately non-zero change in the differential output of the differential amplifier can be provided for approximately no change in the combined second outputs of the first and second pairs of output transistors.
The differential amplifier 200 can include a differential transistor pair 202-1 and 202-2 and cascode transistor pairs 206-1/206-2 and 208-1/208-2. Transistor 202-1 can be coupled to a first component of a differential input 210-1. The first component of differential input 210-1 can receive a first component of a differential input signal (labeled as “IN-i” in
As further shown in
The transistor 206-1 can be provided in a diode configuration. Specifically, a gate of transistor 206-1 can be coupled to its drain. The gate of transistor 206-1 can also be coupled to a drain of transistor 208-1 and can provide a first component of a differential output 214-1. The first component of the differential output 214-1 can provide a first component of a differential output signal (labeled as “OUT-n” in
As further shown in
The differential amplifier 200 can be configured to provide enhanced gain in comparison to the conventional differential amplifier 100 depicted in
The ability of the differential amplifier 200 to provide enhanced gain—using the cascode structure formed by the transistors 206-1, 206-2, 208-1 and 208-2—can be understood by example operation of the differential amplifier 200. Specifically, if a differential output voltage provided by the first differential output 214-1 changes by +1 volt (V), then the differential output voltage provided by the second differential output 214-2 corresponding will approximately change by −1 V. With this change at the differential outputs 214-1 and 214-2, the source of transistor 206-1 will be pulled to change by +1 V due to the configuration of the differential amplifier 200 (e.g., due to the diode follower configuration of the transistor 206-1). Further, the source of transistor 208-1 will be pulled to change by −1 V. The voltage change at the sources of the transistors 206-1 and 208-1 are therefore in tension and ideally can cancel each other out. When this occurs, the overall voltage change at the sources of the transistors 206-1 and 208-1 can be zero for a non-zero voltage change at the drains of the transistors 206-1 and 208-1. Recalling that the gain is defined to be the change in drain voltage for a change in source voltage, under such conditions the gain provided by the transistor pair 206-1 and 208-1 at the differential output 214-1 can be infinite.
In practice, the finite drain-to-source resistances of the transistors 206-1, 208-1 and 202-1 can challenge the ability of the transistor 208-1 to provide a change in voltage at its source that exactly counteracts the change in voltage at the source of the transistor 206-1. Therefore, the transistor 208-1 can be sized slightly larger than the transistors 206-1 and 202-1 to account for the finite drain-to-source resistances of these transistors by providing the transistor 208-1 with a larger gm as compared to the transistor 206-1. Specifically, the transistor 208-1 can be sized to be proportionally larger relative to the intrinsic gain of the transistor 206-1. In doing so, the transistor 208-1 can provide a voltage change at its source that more closely offsets or cancels the voltage change at the source at the transistor 206-1, thereby resulting in conditions providing enhanced gain. The transistors 206-1 and 202-1 can be sized to be approximately the same.
As a rule of thumb (for example, for transistors fabricated by a 65 nm process), transistor 208-1 can be sized approximately 20% larger than the transistor 206-1 to provide near ideal conditions such that enhanced gain conditions are achieved—i.e., such that approximately no overall voltage change occurs at the sources of the transistors 206-1 and 208-1 for a non-zero voltage change at the drains of the transistors 206-1 and 208-1. Infinite gain conditions can approximately occur when the following condition is met:
Rgm208-1≈Rgm206-1∥RDS206-1∥RDS208-1
where Rgm208-1 is the transresistance of transistor 208-1, Rgm206-1 is the transresistance of transistor 206-1, RDS206-1 is the drain-to-source resistance of transistor 206-1, and RDS208-1 is the drain-to-source resistance of transistor 208-1.
The diode follower configuration of the transistor 206-1, the cross-coupling of the transistor 208-1, and the increase of the sizing of the transistor 208-1 relative to the size of the transistor 206-1 (and therefore intrinsic gain of the transistor 206-1), enables the differential amplifier 200—and in particular the transistor 208-1—to cancel the modulation effects at the source of the transistor 206-1 due to the finite drain-to-source resistances of the constituent transistors. By canceling the modulation of the source of the transistor 206-1, the differential amplifier 200 can provide an approximately infinite voltage gain with the cascode transistors 206-1 and 208-1 (by having a differential voltage change of approximately zero at the sources of transistors 206-1 and 208-1 for a non-zero differential voltage change at the first differential output 214-1 as discussed above).
The operation and relative sizing of the transistors 206-2 and 208-2 can mimic the operation of the transistors 206-1 and 208-1 as described above with similar results and benefits. In short, the diode follower configuration of the transistor 206-2, the cross-coupling of the transistor 208-2, and the increase of the sizing of the transistor 208-2 relative to the size of the transistor 206-2 (and therefore intrinsic gain of the transistor 206-2), enables the transistor 208-2 to cancel the modulation effects at the source of the transistor 206-2 due to the finite drain-to-source resistance of the constituent transistors. As with the transistors 206-1 and 208-1, by canceling the modulation of the source of the transistor 206-2, the differential amplifier 200 can be able to provide an approximately infinite voltage gain with the cascode transistors 206-2 and 208-2 (by having a differential voltage change of approximately zero at the sources of transistors 206-2 and 208-2 for a non-zero differential voltage change at the second differential output 214-2 as discussed above).
The gain enhancement provided by the configuration of the differential amplifier 200 can achieve very high increases in gain over the gain provided by the conventional differential amplifier 100. The increased gain provided by the differential amplifier 200 can be achieved using larger-sized transistors to limit the amount of unwanted noise introduced by the differential amplifier 200. The very high gain levels provided by the differential amplifier 200 can vary with process and temperature.
The differential amplifier 200 can include additional biasing elements not shown to ensure proper operation of the transistors 202-1, 202-2, 206-1, 206-2, 208-1 and 208-2 (for example, to ensure operation in saturation mode) and which are not shown in
In the differential amplifier 300, the input differential transistors pair 202-1 and 202-2 of the differential amplifier 200 can be broken into two transistors—specifically, transistor pair 302-1 and 304-1 and transistor pair 302-2 and 304-2. The differential amplifier 300 can include cascode transistor pairs 306-1/308-1 and 306-2/308-2 coupled to the input transistor pairs 302-1/304-1 and 302-2/304-2, respectively, as shown in
Transistor pair 302-1/304-1 can be coupled to first differential input 310-1 while transistor pair 302-2/304-2 can be coupled to second differential input 310-2. As before, transistor 306-1 can be configured in a diode follower configuration and transistor 308-1 can be cross-coupled to second differential output 314-2. Transistor 306-2 can be similarly configured in a diode follower configuration and transistor 308-1 can be cross-coupled to first differential output 314-1.
In contrast to the differential amplifier 200, the transistor 302-1 can be coupled to transistor 306-1 and the transistor 304-1 can be coupled to transistor 308-1. Further, the transistor 302-2 can be coupled to transistor 306-2 and the transistor 304-2 can be coupled to transistor 308-2.
The ability of the differential amplifier 300 to provide enhanced gain—using cross-coupled transistors 308-1 and 308-2, diode follower configured transistors 306-1 and 306-2, and input differential transistor pairs 302-1/302-2 and 304-1/304-2—can be understood by example operation of the differential amplifier 300. Specifically, if a differential output voltage provided by the first differential output 314-1 changes by +1 V, then the differential output voltage provided by the second differential output 314-2 corresponding will approximately change by −1 V. With this change at the differential outputs 314-1 and 314-2, the source of transistor 306-1 will be pulled to change by +1 V due to the configuration of the differential amplifier 300 (e.g., due to the diode follower configuration of the transistor 306-1). Further, the source of transistor 308-1 will be pulled to change by −1 V. The different voltage change at the sources of the transistors 306-1 and 308-1 are therefore in tension and ideally can cancel each other out. Specifically, the net current change at the outputs of the sources of 306-1 and 308-1 can be approximately zero. When this occurs, the overall combined voltage change at the sources of the transistors 306-1 and 308-1 can be zero for a non-zero voltage change at the drains of the transistors 306-1 and 308-1, thereby enabling the differential amplifier 300 to ideally provide infinite gain.
As with the differential amplifier 200, in practice, the finite drain-to-source resistances of the component transistors can be accounted for to ensure the differential amplifier 300 provides significant gain enhancement. Specifically, the finite drain-to-source resistances of the transistors 302-1, 304-1, 306-1 and 308-1 can challenge the ability of the transistor 308-1 to provide an output source current that exactly counteracts the change in output source current of the transistor 306-1. Therefore, the transistor 308-1 can be sized slightly larger than the transistors 306-1, 302-1 and 304-1 to account for the finite drain-to-source resistance of these transistors by providing the transistor 308-1 with a larger gm as compared to the transistor 306-1. Specifically, the transistor 308-1 can be sized to be proportionally larger relative to the intrinsic gain of the transistor 306-1. In doing so, the transistor 308-1 can provide a voltage/current change at its source that more closely offsets or cancels the voltage/current change at the source at the transistor 306-1, thereby resulting in conditions providing enhanced gain. The transistors 302-1, 304-1, and 306-1 can be sized to be approximately the same.
The operation and relative sizing of the transistors 306-2 and 308-2 can mimic the operation of the transistors 306-1 and 308-1 as described above with similar results and benefits.
Infinite gain conditions can approximately occur when the following condition is met:
(1−KA)/(Rgm306-1∥RDS306-1)≈[RDS304-1(1−KB)−RDS308-1(1+KB)/RDS304-1∥RDS308-1]
KA≈(RDS302-1∥Rgm306-1∥RDS306-1)/(Rgm306-1∥RDS306-1)
KB≈(RDS304-1∥Rgm304-2∥RDS308-1)/RDS304-1∥RDS308-1)
where Rgm306-1 is the transresistance of transistor 306-1, RDS306-1 is the drain-to-source resistance of transistor 306-1, RDS304-1 is the drain-to-source resistance of transistor 304-1, RDS308-1 is the drain-to-source resistance of transistor 308-1, RDS302-1 is the drain-to-source resistance of transistor 302-1, and Rgm304-2 is the transresistance of transistor 304-2.
The diode configuration of the transistors 306-1 and 306-2, the cross-coupling of the transistors 308-1 and 308-2, and the increase of the sizing of the transistors 308-1 and 308-2 relative to the sizes of the other transistors (and therefore the intrinsic gain of the other transistors), enables the differential amplifier 300—and in particular the transistors 308-1 and 308-2—to cancel the modulation effects of the input transistors (302-1/302-2 and 304-1/304-2) and the diode follower configured transistors (306-1 and 306-2) due to their finite drain-to-source resistances. By canceling these modulation effects, the differential amplifier 300 can provide an approximately infinite voltage gain with the cascode transistor pairs 306-1/308-1 and 306-2/308-2 (by having a differential voltage/current change of approximately zero at the sources of transistors 306-1/308-1 and 306-2/308-2 for a non-zero differential voltage/current change at the first and second differential outputs 314-1 and 314-2 as discussed above).
The differential amplifier 400 can include differential transistor pair 402-1 and 402-2 and cascode transistor pairs 406-1/408-1 and 406-2/408-2. Transistor 402-1 can be coupled to a first component of a differential input 410-1. Transistor 402-2 can be coupled to a second component of a differential input 410-2. The differential transistor pair 402-1 and 402-2 can be coupled to a bias current source 412.
As further shown in
The differential amplifier 400 capacitively couples the first differential output 414-1 to the transistor 408-2 and capacitively couples the second differential output 414-2 to the transistor 408-1. Specifically, the gate of the transistor 408-1 can be coupled to the second differential output 414-2 through a capacitor 418-1. The gate of the transistor 408-2 can be coupled to the first differential output 414-1 through a capacitor 418-2. Further, the transistors 406-1 and 406-2 are no longer configured as diode followers. Instead, the gate of the transistor 406-1 can be coupled through a switch 416-1 to the gate of the transistor 408-1 and the gate of the transistor 406-2 can be coupled through a switch 416-2 to the gate of the transistor 408-2. The gates of the transistors 406-1 and 406-2 can also be coupled to biasing (labeled as “BIAS” in
The configuration of the differential amplifier 400 ensures the voltage applied to the gates of the transistors 406-1 and 406-2 can be a fixed reference voltage (by way of coupling to the biasing voltage). Further, the configuration of the differential amplifier 400 can force the same voltage to be applied to the gates of the transistors 406-1 and 408-1 along with forcing the same voltage to be applied to the gates of the transistors 406-2 and 408-2. In the differential amplifier 400, the transistors 406-1 and 406-2 are no longer configured as voltage followers tied to differential outputs 414-1 and 414-2, respectively.
As with the differential amplifiers 200 and 300 of the present invention, the differential amplifier 400 can be configured to provide a ideal voltage change of approximately zero at the sources of transistors 406-1 and 408-1 (and, correspondingly, at the sources of the transistors 406-2 and 408-2) for a change in voltage at the drains of the transistors 406-1 and 408-1 (and, correspondingly, at the drains of the transistors 406-2 and 408-2). In doing so, the differential amplifier 400 can provide significant gain enhancement over the cascoded arrangement of the conventional differential amplifier 100 of
The benefits of the differential amplifier 400 and the ability of the differential amplifier 400 to provide significant gain enhancement can be shown by way of example operation of the differential amplifier 400. Specifically, the differential amplifier 400 can begin in a reset mode. In reset mode, the first and second differential inputs 410-1 and 410-2 can be shorted. Additionally, the switches 416-1 and 416-2 can be closed. With the switch 416-1 closed, the voltage applied to the gates of the transistors 406-1 and 408-1 can be forced to be the same (e.g., forced to a reference voltage level supplied by the biasing voltage BIAS). Further, with the switch 416-2 closed, the voltage applied to the gates of the transistors 406-2 and 408-2 can be forced to be the same (e.g., forced to a reference voltage level supplied by the biasing voltage BIAS).
After reset, the switches 416-1 and 416-2 can be opened. Further, differential input signals can be applied to the differential inputs 410-1 and 410-2. When there is a change in the voltage provided by the differential outputs 414-1 and 414-2, the sources of the transistors 406-1 and 406-2 can correspondingly change but only by an amount that is proportional to their intrinsic gain. Specifically, for a +1 V change at the differential output 414-1, the change in voltage at the source of the transistor 406-1 can be proportional to the intrinsic gain of the transistor 406-1 and can be based on the reference voltage supply BIAS.
Likewise, for a corresponding −1 V change at the differential output 414-2, the change in voltage at the source of the transistor 406-2 can be proportional to the intrinsic gain of the transistor 406-2 and can be based on the reference voltage supply BIAS. Accordingly, the transistors 408-1 and 408-2—in order to cancel the modulation effects at the sources of the transistors 406-1 and 406-2 to achieve approximately zero voltage change at the sources of the transistors 406-1 and 406-2—can be sized larger than the other constituent transistors but smaller relative to their sizing in the prior described configurations and yet still achieve gain enhancement (i.e., near infinite gain under ideal conditions). This enables the differential amplifier 400 to use smaller sized transistors, thereby reducing the amount of noise introduced during amplification. Further, by not configuring the transistors 406-1 and 406-2 as diodes, further gain enhancement can be provided.
Infinite gain conditions can approximately occur when the following condition is met:
Rgm408-1≈RDS406-1∥RDS408-1*KCAP
KCAP≈C418-1/(C418-1+C408-1)
where Rgm408-1 is the transresistance of transistor 408-1, RDS406-1 is the drain-to-source resistance of transistor 406-1, RDS408-1 is the drain-to-source resistance of transistor 408-1, C418-1 is the capacitance of capacitor 418-1, and C408-1 is the effective input capacitance of transistor 408-1.
Similar to the introduction of additional input transistors in the differential amplifier 300, the differential amplifier 400 can be modified to include input transistors separately coupled to each cascode transistor 406-1, 406-2, 408-1 and 408-2.
The differential amplifiers of the present invention can include additional biasing elements not shown to ensure proper operation of the transistors (for example, to ensure operation in saturation mode) which have not been shown or described for simplicity. Further, the differential amplifiers of the present invention are not limited to implementation using NMOS transistors but can be implemented using PMOS technology as well as other transistor technologies including BJTs. Further, the gain enhancement structures of the present invention described above can be used for any process node with the benefits described herein becoming more advantageous with process node reduction.
At step 502, a differential input signal can be received. The differential input signal can include, for example, a first component of a differential input voltage and a second component of a differential input voltage. The differential input signal can be coupled to inputs of one or more input transistors. The input transistors can form a portion of an initial or input stage of a differential amplifier. As an example, a first component of a differential input signal can be coupled to a gate of one or more first input transistors and a second component of the differential input signal can be coupled to a gate of one or more second input transistors.
At step 504, a change in a differential output signal can be detected or sensed. The differential output signal can include, for example, a first component of a differential output voltage and a second component of a differential output voltage. The differential output signal can be provided by a cascode stage of a differential amplifier. For example, a first component of a differential output signal can be provided by an output of a first cascode transistor coupled to one or more first input transistors in cascode. A second component of the differential output signal can be provided by an output of a second cascode transistor coupled to one or more second input transistors in cascode.
The change (e.g., a voltage or current change) in the differential output signal can be sensed by the first and second output or cascode transistors. For example, a change in the first component of the differential output signal can be sensed by the first output transistor and a change in the second component of the differential output signal can be sensed by the second output transistor. The first and second output transistors can be configured, for example, as voltage followers or diode configured followers, to detect a change in the first and second components of the differential output signal, respectively. For example, a gate of the first output transistor can be coupled to a first output of the first transistor (e.g., a drain of the first transistor) that provides the first component of the differential output signal. A second output of the first transistor (e.g., a source of the first transistor) can be coupled to an output of one or more first input transistors (e.g., to drains of the one or more first input transistors).
Similarly, a gate of the second output transistor can be coupled to a first output of the second transistor (e.g., a drain of the second transistor) that provides the second component of the differential output signal. A second output of the second transistor (e.g., a source of the second transistor) can be coupled to an output of one or more second input transistors (e.g., to drains of the one or more second input transistors).
The change in the first and second components of the differential output signal detected by the first and second out put transistors, respectively, can result in a change in a voltage or current level at the second outputs of the first and second output transistors (e.g., the outputs coupled to the input transistors). Modulation effects can be induced at the second outputs of the first and second output transistors. The modulation effects can occur due to the sensed change in the differential output and due to the finite source-to-drain resistance of the constituent components of the differential amplifier.
At step 506, the change in output level (e.g., change in source voltage or current) at the second outputs of the first and second output transistors can be approximately matched by third and fourth output transistors. For example, the change in the output level of the second output of the first output transistor can be matched by an approximately equal and opposite change in output level provided by the third output transistor. The change in the output level of the second output of the second output transistor can be matched by an approximately equal and opposite change in output level provided by the fourth output transistor. This step can also be considered to include approximately canceling the induced modulation effects of the first and second output transistors using the third and fourth output transistors. The modulation effects of the first and second output transistors can be caused by the change in differential output.
The third and fourth output transistors can be cascode transistors arranged in cascode with the first and second out transistors, respectively. The third and fourth output transistors can be cross-coupled output transistors. That is, the third output transistor can be coupled to one or more first input transistors, the first component of the differential output signal, and the second component of the differential output signal. For example, a gate of the third output transistor can be coupled to the second component of the differential output signal. Likewise, the fourth output transistor can be coupled to one or more second input transistors, the second component of the differential output signal, and the first component of the differential output signal. For example, a gate of the fourth output transistor can be coupled to the first component of the differential output signal.
By using cross-coupled third and fourth output transistors, the change in the output levels of the second outputs of the first and second output transistors can be approximately matched and further approximately canceled out by the third and fourth output transistors, respectively. By approximately canceling out the change in the output levels of the second outputs of the first and second output transistors, the cascode transistors of the differential amplifier can provide a change in differential output with approximately no change in the output levels of the first and second output transistors coupled to the input transistors. As a result, the gain of the differential amplifier can be enhanced significantly (e.g., because a non-zero change in drain voltage can be provided with a zero change in source voltage).
The sizes of the third and fourth output transistors can be increased proportional to the intrinsic gains of the other transistors of the differential amplifier (e.g., the first and second output transistors and the one or more first and second input transistors—which can all be of approximately the same scaling) to better approximately match and cancel out the change in the output levels of the second outputs of the first and second output transistors to ensure significant gain enhancement.
As a variation, the first and second output transistors can alternatively be coupled to reference voltage or current sources. The third and fourth transistors can be capacitively coupled to the reference sources during a first stage of operation of the differential amplifier. During a second stage of operation, the third and fourth transistors can be de-coupled from the reference sources and can used stored charges from capacitors to approximately match and cancel out changes in output levels at the second outputs of the first and second output transistors.
The method 500 for providing enhanced gain according to an aspect of the present invention can be implemented using, for example, the differential amplifiers depicted in
While various aspects of the present invention have been described above, it should be understood that they have been presented by way of example and not limitation. It will be apparent to one skilled in the pertinent art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Therefore, the present invention should only be defined in accordance with the following claims and their equivalents.
