SENSE AMPLIFIERS INCLUDING BIAS CIRCUITS
Sense amplifiers including bias circuits axe described. Examples include bias circuits having an adjustable width transistor. A loop gain of the bias circuit may be determined in part by the adjustable width of the transistor. Examples of sense amplifiers including amplifier stages configured to bias an input/output node to a reference voltage.
This application is a continuation of U.S. patent application Ser. No. 13/191,263, filed Jul. 26, 2011. This application is incorporated herein by reference, in its entirety, for any purpose.
TECHNICAL FIELDEmbodiments of the invention relate generally to integrated circuits, and more particularly, in one or more of the illustrated embodiments, to integrated circuit sense amplifiers.
BACKGROUNDCurrent mode sense amplifiers (CSAs) have been used in integrated circuits to sense and amplify differential input currents. For example, in applications in semiconductor memory, the CSAs are often used to sense and amplify input currents resulting from reading memory cell data and being provided over relatively long signal lines. As a result, the input currents may be very weak and low in magnitude. In applications such as these, control of the CSA's loop gain may be important because it affects the operating characteristics of the CSA. That is, where the loop gain of a CSA is approximately equal to 1, the dominant mode of operation for the CSA is sensing differential input currents, in contrast, as the loop gain of a CSA increases to be greater than 1, the dominant mode of operation for the CSA transitions from current sensing to behaving as a latch circuit. Thus, controlling loop gain may be desirable in order to control the behavior of the CSA.
Certain details are set forth below to provide a sufficient understanding of embodiments of the invention. However, it will be clear to one skilled in the art that embodiments of the invention may be practiced without these particular details. Moreover, the particular embodiments of the present invention described herein are provided by way of example and should not be used to limit the scope of the invention to these particular embodiments, in other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail in order to avoid unnecessarily obscuring the invention.
Examples of sense amplifiers have been described in co-pending application U.S. Ser. No. 12/694,136, filed Jan. 26, 2010 entitled “Sense amplifier having loop gain control.”
The loop gain for the bias circuit 210 (e.g., gm230·R240), where gm230 is the transconductance of transistor 230, may be given as:
As a result, the loop gain for the bias circuit 210 may be constant (e.g., gm230·R240=1) for
(W/L)232=4·(W/L)230
With transistor dimensions of the transistors 230, 232 scaled accordingly, for example, ratio for transistor 230 is four times the (W/L) ratio for transistor 232, the bias circuit 210 has a loop gain that is 1, and is substantially constant despite variations in process, voltage, and/or temperature. For example, where the resistance of the resistance 240 is lower due to variation in the fabrication process, the gm increases due to an increased current Ibmr resulting from the lower resistance, and likewise, the Vbias also related to Ibmr changes as well. As a result, the product of gin and R, that is, the loop gain for the bias circuit 210, is kept substantially constant despite the variation in R.
An example amplifier stage 250 is also illustrated in
The amplifier stage 250 includes transistors 260, 262, such as n-FET transistors, having gates cross coupled to the drains of the other transistor. Transistors 270, 272 are coupled to sources of the transistors 260, 262, respectively, and have sources coupled to a reference voltage, such as ground. Gates of the transistors 270, 272 are coupled to receive a Vbias voltage from the bias circuit 202. Resistances 280, 282 having magnitudes R are coupled to drains of the transistors 260. 282, respectively, and receive Vcc. The amplifier stage 250 senses and amplifies current at input-output (10) nodes IO, IOb, 290, 292. As known, the loop gain for the amplifier stage 250 for matched transistors 260, 262 and matched resistances 280, 282 is:
gm260·R280=gm262·R282
Operation of the amplifier stage .250 will be described with the Vbias provided by the bias circuit 210 of
With the transistors and resistances matched and Vbias provided to the amplifier stage 250 by a matched bias circuit 210, the loop gain of the amplifier stage 250 will be generally independent of process, voltage, and/or temperature variations. For example, again considering the case where the resistance of the resistance 240 is lower due to variation in the fabrication process, the resistance of the resistances 280, 282 would also be lower since they were subjected to the same variation in the fabrication process. As previously discussed, the Vbias provided by the bias circuit 210 changes as a result of the change in Ibmr due to the lower resistance of the resistance 240. Turning to the amplifier stage, the Vbias voltage from the bias circuit 210 sets the transistors 270, 272 (which are matched to transistor 230 of the bias circuit 210) to the same bias condition of transistor 230. This in turn adjusts the bias current of the amplifier stage 250 in the same manner Hum of the bias circuit 210 was altered by the lower resistance of the resistance 240. As a. result, the gm of the amplifier stage 250 may increase (as it did for the bias circuit 210) due to the increased bias current to compensate for the decreased R of resistances 280, 282 and the loop gain of the amplifier stage 250 may remain substantially constant despite the variation in R. Generally the Vbias voltage can be used to change the bias current of the amplifier stage, which in turn changes the gm of the amplifier stage. As a result, the loop gain (e.g., gm·R) may be controlled by adjusting the Vbias voltage, for example, to be substantially constant. In this manner, the bias circuit may be configured to provide a bias voltage having a magnitude that is configured to maintain a substantially constant loop gain.
In some embodiments, the transistors 260, 262, 270, 272 and resistances 280, 282 are not matched to transistors 230, 232 and resistance 240, as previously described. For example, transistors 260, 262 and transistors 270, 272 may be designed (e.g., scaled) to provide transconductances k times the transconductance of transistor 230 of the bias circuit 210, and the magnitude of the resistances 280, 282 are (l/k) times the magnitude of resistance 240 of the bias circuit 210. Although the transistors are not matched, the amplifier stage 250 has a loop gain substantially equal to the loop gain of the bias circuit, that is, 1. In sonic embodiments, the transistors 260, 262, 270, 272 and resistances 280, 282 are designed to provide a amplifier stage having a loop gain other than 1. For example, where the loop gain of the amplifier stage 250 is greater than 1, the amplifier stage exhibits a latch circuit behavior.
In the embodiment shown in
Sense amplifiers in accordance with embodiments of the present invention may allow the input/output of an amplifier stage to be precharged to a reference voltage, such as a supply voltage, which may be preferable over an intermediate analog voltage.
In an analogous manner as described above, a current IBMR may accordingly be provided, in the transistors 405 and 410 and mirrored to the transistors 406 and 411. The bias circuit may be configured to provide a substantially constant loop gain. A bias voltage biasp may be provided to the amplifier stage 404 from the gates of the p-FET transistors 405, 406. This may be in contrast to the circuit described above with reference to
In this manner, a bias voltage biasp may be provided, to the amplifier stage 404. The amplifier stage 404 may include p-FET transistors 425, 426. The bias transistors 425 and 426 have their gates coupled together and are coupled to as reference voltage, e.g. VCC. In some embodiments, one p-FET transistor may be used in place of the two bias transistors 425, 426. Resistances 430, 411 are coupled to the drains of bias transistors 425, 426, respectively. Cross-coupled n-FET transistors 435, 436 are coupled to the resistances 430, 431. The gate of the transistor 436 is coupled to the resistance 430, while the gate of the transistor 435 is coupled to the resistance 431. The drain of the transistor 435 is coupled to the resistance 430. The drain of the transistor 436 is coupled to the resistance 431. The source of the transistor 435 is coupled to an input/output of the sense amplifier 404 (node 450). The source of the transistor 436 is coupled to another input/output of the sense amplifier 404 (node 451). As mentioned above, the input/outputs may be coupled to relatively long lines Gio/Giob 452, 453. In some embodiments, the nodes 450, 451 are coupled to a reference voltage, e.g. ground. In some embodiments, the nodes 450, 451 are coupled to the reference voltage through n-FFT transistors 460, 461 are coupled to the nodes 450, 451. The amplifier stage 404 may also be configured to provide a substantially constant loop gain. As mentioned above, the bias circuit may be configured to provide a bias voltage having a magnitude that is configured to maintain a substantially constant loop gain.
Generally, the resistances 430 and 431 may be equal to the resistance 412. Transistor dimensions of the transistors 435 and 436 may be equal to transistor dimensions of the transistor 411. For example, the widths of transistors 435 and 436 may be equal to the width of the transistor 411. The widths of bias transistors 425 and 426 may be equal to the widths of the transistors 405 and 406. Recall that the width of the transistor 410 may be adjustable. The width of the transistor 410 may be an integer multiple, K, of the width of the transistor 411. The current IBMR is generated such that g of transistor 411 multiplied by RBMR may be given as
where K is the width ratio of transistor 410 to that of 411. In this manner, the loop gain of the bias circuit (e.g. gm411RBMR product) may depend on K, and not on process, voltage, or temperature variation. Where K=4, gm411RBMR=1. Accordingly, K=4 may be used in some embodiments. In other embodiments K=1, 2, 3, 5, 6, or 7 may be used. Other values of K. may also be used. The biasp voltage is provided to the amplifier stage 404 such that a current based on IBMR is also provided in the legs of the amplifier stage 404 and the loop gain of the amplifier stage (e.g. gm of the transistor 435 or 436 multiplied by the resistance 430 or 431) is also
Provided the voltages at nodes 450, 451 are sufficiently low, the current in the legs of the amplifier stage 404 should also be IBMR due to current mirroring. The gm of 435 or 436 multiplied by the resistance of 430 or 431 may accordingly also be substantially constant, 1 in some embodiments. In some embodiments, different multiples may be used. For example, the transistor 425 may have a width 2 times that of the transistor 411, while the resistances 430, 431 may be ½ RBMR. Then, the g, of 435 or 436 (e.g. 2/RBMR) multiplied, by the resistance of 430 or 431 (e.g. ½RBMR) may still be substantially constant. Other multiples may be used, generally where the width of transistors 425 and 426 are N times a width of the transistor 411, the resistances 430 and 431 are 1/N the resistance of the resistance 412 (RBMR).
The bias transistors 425 and 426 may also have their drains connected together. Connecting the drains, such as by including the short 470 may ensure that the load provided to the transistors 435, 436 is simply the resistance of the resistor 430, 431, respectively. If the short 470 is not included, the load resistance would include the resistance of the bias transistors 425, 426, which may be too large for desirable operation. Accordingly, the drains of the bias transistors 425, 426 may be coupled together. In some embodiments, a single transistor may be used in place of the bias transistors 425. 426. The single transistor may be twice as wide as one of the bias transistors 425, 426 in some embodiments. While a bard-wired short 470 is shown in
The bias circuit 502 includes a resistance 512 RBMR. The transistor 510 may have an adjustable width, and may be K times a width of the transistor 505. The transistors 506 and 511 provide a bias, biasn, at the gates of the transistors 506 and 511 to the amplifier stage 504. n-FET bias transistors 525, 526, which may have a same width as the transistors 506, 511, receive the biasn signal and provide currents in the respective legs of the amplifier stage 504. As was described above, drains of the bias transistors 525, 526 may be shorted together using a short 570 to reduce a resistance seen by the cross-coupled transistors 535, 536. In some embodiments, a single wider transistor may be used in place of the two bias transistors 525, 526. The resistances 530, 531 may each equal the resistance RBMR in some embodiments. As was described above, a gmRBMR product may be substantially constant over process, voltage, and temperature variation,
The input/outputs 550, 551 may accordingly be precharged to a reference voltage, Vcc. The input/outputs 550, 551 may be coupled to relatively long conductive lines 552, 553. It may be advantageous to charge the relatively long conductive lines 552, 553 to a reference voltage, e.g. VCC rather than an intermediate analog, voltage.
Any structure or method may be used to provide the adjustable width transistors described above with reference to
As generally described above, embodiments of sense amplifiers described herein may have substantially constant gmR products, which may be unity in some embodiments. In practice, however, the gmR product associated with a sense amplifier may vary as a reference voltage, e.g. VCC varies, due to circuit non-idealities such as, but not limited to, channel length modulation or short channel effect. For a given K value, the gmR product may increase with increasing VCC. In some embodiments, one K value may be used at a lower VCC value and a different K value used at a higher VCC value. That is, when Vcc is at or below a threshold, a lower K may be used than when VCC is above a threshold. In one example, the threshold may be IV.
The dummy signal driver 703 is coupled to the transistors 715, 716. The dummy signal driver 703 may provide a differential current to the transistors 715, 716. In this manner, a differential voltage may be generated at nodes 730, 731. The differential voltage may be provided, to the comparator 705 that may provide an output signal indicative of the differential voltage to the logic 708. Based on the output signal, the logic 708 may provide a control signal to the bias circuit 710 to change a width of an adjustable transistor in the bias circuit 710. The control signal may be provided to change the width of the adjustable transistor to reduce the voltage difference generated at the nodes 730 and 731 responsive to the input current difference. The control signal provided by the logic 708 may include, for example, a control signal to open a particular number of switches to connect a number of transistors in parallel to provide an effective transistor of a particular width. In some examples, ideally, no voltage difference may ultimately be generated responsive to an input differential current, corresponding to an effective 0 input resistance to the dummy amplifier stage 702. The bias voltage, e.g. biasn, may be provided to other amplifier stages for use by sense amplifiers, e.g. the amplifier stages 404 or 504 of
The row and column addresses are provided by the address latch 810 to a row address decoder 822 and a column address decoder 828, respectively. The column address decoder 828 selects bit lines extending through the array 802 corresponding to respective column addresses. The row address decoder 822 is connected to word line driver 824 that activates respective rows of memory cells in the array 802 corresponding to received row addresses. The selected data line (e.g. a bit line or bit lines) corresponding to a received column address are coupled to a read/write circuitry 830 to provide read data to a data output buffer 834 via an input-output data bus 840. Write data are applied to the memory array 802 through a data input buffer 844 and the memory array read/write circuitry 830. The read/write circuitry 830 includes at least one sense amplifier 832 according to an embodiment of the invention. Read data and write data provided to the read/write circuitry 830 is transmitted over input-output lines and are amplified by the sense amplifier 832 to be provided to the output buffer 834 and before being written to the memory array 802. The sense amplifier 832 may be implemented by any of the embodiments of sense amplifiers described herein.
The portion of the memory device shown in
From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.
Claims
1. A circuit comprising:
- an amplifier stage configured to provide a differential voltage;
- a bias circuit configured to provide a bias voltage to the amplifier stage, wherein the bias circuit is configured to provide a constant loop gain and the bias circuit includes a transistor having an adjustable dimension; and
- logic configured to adjust the adjustable dimension of the transistor based on the differential voltage.
2. The circuit of claim 1, wherein the amplifier stage is configured to sense input current and to amplify the input current based on the sensed input current to provide the differential voltage.
3. The circuit of claim 1, wherein the amplifier stage is configured to control a loop gain based on the bias voltage.
4. The circuit of claim 1, wherein the bias circuit further comprises a resistance coupled between the transistor and a reference voltage, where the resistance matches a resistance of the amplifier.
5. An apparatus, comprising
- a bias circuit configured to provide a bias voltage having a magnitude configured to maintain a constant loop gain, wherein the bias circuit comprises a beta multiplier reference circuit; and
- an amplifier stage coupled to the bias circuit to receive the bias voltage and configured to bias an input-output node to a reference voltage, the amplifier stage having a loop gain controlled at least in part by the bias voltage.
6. The apparatus of claim 5, wherein the beta multiplier reference circuit is configured to control the loop gain of the bias circuit.
7. The apparatus of claim 5, wherein the beta multiplier reference circuit comprises a plurality of transistors selectively coupled in parallel based on comparison of two input signals to the bias circuit.
8. The apparatus of claim 7, further comprising logic configured to provide a control signal having a value that reduces a voltage difference between the two input signals.
9. An apparatus, comprising
- a bias circuit configured to provide a bias voltage having a magnitude configured to maintain a constant loop gain, wherein the bias circuit comprises: a first pair of transistors including an adjustable transistor having gates coupled together, a transistor of the first pair of transistors other than the adjustable transistor having a gate coupled to its drain; a second pair of transistors having gates coupled together, a first transistor of the second pair of transistors having a gate coupled to its drain and a bias voltage output node from which the bias voltage is provided; and a resistance coupled to a second transistor of the first pair of transistors coupled to its drain; and
- an amplifier stage coupled to the bias circuit to receive the bias voltage and configured to bias an input-output node to a reference voltage, the amplifier stage having a loop gain controlled at least in part by the bias voltage.
10. The apparatus of claim 9, wherein the transistor of the first pair of transistors is a p-type transistor and the adjustable transistor is an n-type transistor, and wherein a first transistor of the second pair of transistors is a p-type transistor and a second transistor of the second pair of transistors is an n-type transistor.
11. The apparatus of claim 9, wherein the adjustable transistor includes an adjustable dimension that is greater than a corresponding dimension of a transistor of the second pair of transistors.
12. The apparatus of claim 9, wherein a gate of the adjustable transistors is coupled to the bias voltage output node.
13. An apparatus, comprising
- a bias circuit configured to provide a bias voltage having a magnitude configured to maintain a constant loop gain; and
- an amplifier stage coupled to the bias circuit to receive the bias voltage and configured to bias an input-output node to a reference voltage, the amplifier stage having a loop gain controlled at least in part by the bias voltage, wherein the amplifier stage comprises: a pair of cross-coupled transistors; a bias transistor coupled to the pair of cross-coupled transistors and further configured to receive the bias voltage from the bias circuit; and a pair of resistances, each of the pair of resistances coupled between a respective one of the pair of cross-coupled transistors and the bias transistor, the loop gain of the current amplifier stage matching a loop gain of the bias circuit.
14. The apparatus of claim 13, wherein the bias transistor is a first bias transistor coupled to a first resistance of the pair or resistances, the amplifier stage further comprising a second bias transistor coupled to a second resistance of the pair or resistances, the second bias transistor configured to receive the bias voltage.
15. The apparatus of claim 14, wherein a node between the first bias transistor and the first resistance is coupled to a node between the second bias transistor and the second resistance.
16. The apparatus of claim 13, wherein the amplifier stage further comprises a precharge transistor configured to couple a precharge voltage to the input-output node.
17. An apparatus, comprising
- a bias circuit configured to provide a bias voltage having a magnitude configured to maintain a constant loop gain; and
- an amplifier stage coupled to the bias circuit to receive the bias voltage and configured to bias an input-output node to a reference voltage, the amplifier stage having a loop gain controlled at least in part by the bias voltage, wherein a loop gain of the amplifier stage is the same as the loop gain of the bias circuit.
18. The apparatus of claim 17, wherein the bias circuit comprises logic configured to control the loop gain of the bias circuit.
19. The apparatus of claim 18, wherein the bias circuit further comprises a dummy amplifier stage configured to match operation of the amplifier stage, the bias circuit further comprising a dummy signal driver configured to provide a differential current to the dummy amplifier stage.
20. The apparatus of claim 19, further comprising a comparator configured to provide a comparison of two input signals provided from the dummy signal driver, wherein the logic is configured to control the loop gain of the bias circuit based on the comparison of the two input signals.
21. The apparatus of claim 20, wherein the logic is configured to reduce a voltage difference between the two input signals.
Type: Application
Filed: Aug 18, 2014
Publication Date: Dec 4, 2014
Inventor: SEONG-HOON LEE (Boise, ID)
Application Number: 14/462,340
International Classification: H03F 3/45 (20060101); G11C 7/06 (20060101);