Buffer amplifier structures with enhanced linearity

[0002] 1. Field of the Invention

[0003] The present invention relates generally to buffer amplifiers.

[0004] 2. Description of the Related Art

[0005] The principal purpose of buffer amplifiers is to provide isolation while coupling signals between a preceding electronic circuit and a succeeding electronic circuit. Accordingly, a perfect buffer amplifier presents an infinite impedance to the preceding circuit and has zero output impedance and infinite bandwidth to facilitate driving the succeeding circuit. If the preceding circuit provides an input voltage V_in, the perfect buffer amplifier supplies the current required to drive the succeeding circuit and provides an output voltage V_outequal (although possibly offset) to the input voltage V_in.

[0006] Buffer amplifiers are essential elements in a great variety of signal-conditioning systems (e.g., analog-to-digital converters). Because real buffer amplifiers fail to provide the ideal parameters described above, their transfer functions generally exhibit nonlinearity so that they insert distortion into the output voltage V_outand thus degrade the performance of these systems.

[0007] Although this distortion can be reduced by increasing transistor bias currents to greatly exceed signal currents (thus reducing the relative effect of the signal current variations), this modification significantly degrades the buffer amplifier's efficiency. If the buffer amplifier's output load is capacitive, linearity will still degrade with increased signal frequency because of the increased current demand that is required to drive the load capacitance at higher frequencies.

[0008] The present invention is directed to buffer amplifiers that realize enhanced linearity.

[0009] These goals are realized with a replica current generator that supplements a buffer transistor and is configured to provide a replica current which substantially equals required load currents in the amplifier's output load. Because the current of the buffer transistor 24 now remains constant, its base-emitter voltage V_beremains constant and the amplifier linearly reproduces the input signal S_inacross the output load.

[0010] The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.

[0011] FIG. 1 is a schematic of a buffer amplifier embodiment of the present invention,

[0012] FIGS. 2A and 2B are schematics of another buffer amplifier embodiment and its responses to different phases of a differential input signal;

[0013] FIG. 3 is a schematic of a differential buffer amplifier embodiment of the invention;

[0014] FIG. 4 is a schematic of another buffer amplifier embodiment;

[0015] FIG. 5 is a block diagram of a pipelined analog-to-digital converter which includes a schematic of a switched-capacitor multiplying digital-to-analog converter; and

[0016] FIG. 6 is a schematic that illustrates an application of the differential buffer amplifier embodiment of FIG. 3 in the pipelined analog-to-digital converter of FIG. 5.

[0017] The present invention recognizes that the observed nonlinearity of buffer amplifiers is typically a result of current variation in transistors as they attempt to supply the current required by the buffer amplifier's output load. The invention further recognizes that these current variations induce changes in transistor parameters (e.g., base-emitter voltage V_be) that directly generate distortion in the buffer amplifier's output voltage.

[0018] In response to these recognitions, the invention injects the required current variations with a replica current generator so that other buffer currents remain substantially constant and, as a result, distortion is significantly reduced. The need for significantly increasing transistor bias currents to reduce distortion is thus avoided with consequent enhancement of buffer efficiency.

[0019] In particular, FIG. 1 illustrates a buffer amplifier 20 which drives an output load 22 in response to a differential input signal S_inat an input port 23. The amplifier includes a buffer transistor 24, a buffer current source 26 that provides a bias current I and a replica current generator 28.

[0020] The emitter of the buffer transistor 24 is available to provide a load current i_Lto the output load 22 when the buffer transistor's base receives an upper side 30 of the differential input signal S_in. The buffer current source 26 is coupled to the buffer transistor's emitter and the replica current generator 28 is coupled between a lower side 31 of the differential input signal S_inand the output load 22.

[0021] The replica current generator 28 is configured to provide a replica current 33 that is substantially equal to the required load current i_Land, accordingly, the current through the buffer transistor 24 remains substantially constant and equal to the bias current I of the buffer current source 26.

[0022] Because the current of the buffer transistor 24 is constant, its base-emitter voltage V_beis constant and, since the voltage across the output load 22 is the input voltage V_inof the upper side 30 (voltage between the upper side and circuit ground) plus V_be, the output voltage is now a faithful (although offset) copy of the input voltage V_in. The replica current generator 28 thus supplies the load current i_Lso that the buffer transistor 24 can accurately transfer the input voltage V_into the output load 22.

[0023] An embodiment of the replica current generator 28 is shown in the buffer amplifier 20 of FIGS. 2A and 2B which otherwise include elements of FIG. 1 with like elements indicated by like reference numbers. In this embodiment, the replica current generator 28 includes a replica load 32, a replica transistor 34 and first and second replica current generators 36 and 37.

[0024] The replica transistor's base receives the lower side 31 of the differential input signal S_in, its emitter is coupled to the replica load 32 and the first replica current source 36 and its collector is coupled to the second replica current source 37 and to the emitter of the buffer transistor 24. The first and second replica current sources each preferably provide the same bias current I that is provided by the buffer current source 26 and, in another important feature of the invention, the impedance of the replica load 32 substantially matches the impedance of the buffer load 22.

[0025] FIG. 2A illustrates a first phase of the input signal S_inin which the lower side 31 is at a low voltage −V_inrelative to a high voltage +V_inat the upper side. In contrast, FIG. 2B illustrates an opposite second phase of the input signal S_inin which the lower side 31 is at a high voltage +V_inrelative to a low voltage −V_inat the upper side. The buffer amplifier 20 must generate required load currents +i_L(in FIG. 2A) and −i_L(in FIG. 2B) in the buffer load 22 as the voltage of the upper side 32 successively rises above and drops below the lower side 31,

[0026] At the same times, the voltage of the lower side 31 drops and rises and induces (via the replica transistor 34) an upward replica current i_Rin the replica load 32 (in FIG. 2A) and a downward replica current i_R(in FIG. 2B). Because the replica current source 36 provides a bias current I to the replica transistor's emitter, the current through the replica transistor 34 thus alternates between I −i_Rand I +i_R. Because the second replica current source 37 provides a bias current I to the replica transistor's collector, a replica current i_Rflows to the circuit node 39 as the required load current i_Lflows away from this circuit node and a replica current i_Rflows from the circuit node 39 as the required load current i_Lflows towards this circuit node.

[0027] Finally, because the replica impedance Z_Rof the replica load 32 substantially equals the impedance Z_Lof the buffer load 22 and V_beof the replica transistor 34 substantially equals V_beof the buffer transistor 24, the replica currents i_Rat the circuit node 39 substantially match the required load currents i_Lat this node. The replica currents i_Rthus provide the required load currents i_Land current conservation at the circuit node 39 requires that the current through the buffer transistor 24 equals the constant bias current I of the buffer current source 26.

[0028] Because its current is constant, the base-emitter voltage V_beof the buffer transistor 24 is constant and distortion in the output signal across the buffer load 22 is significantly reduced. Although the buffer amplifier 20 includes three current sources, their currents I need only somewhat exceed the load current i_L. In contrast, power consumption is significantly greater in conventional buffers because they must substantially increase the current of the buffer current source (26 in FIGS. 2A and 2B) above the load current i_Lto achieve acceptable linearity.

[0029] FIG. 3 illustrates a differential buffer amplifier embodiment 60 which provides substantial immunity to the effects of a common-mode component in the differential input signal S_inat an input port 61.

[0030] This embodiment has a buffer portion 62 which includes first and second buffer transistors 63 and 64 and first and second buffer current sources 65 and 66 that are coupled to the emitters of respective buffer transistors. The emitters of the first and second buffer transistors are coupled to provide a load current i_Lthrough an output load 68 in response to a differential input signal S_inthat is received through the buffer transistors' bases from the input port 61.

[0031] The remainder of the buffer amplifier 60 comprises a replica current generator 70 which includes first and second replica transistors 73 and 74, first and second replica current sources 75 and 76 coupled to the emitters of respective replica transistors, third and fourth replica current sources 77 and 78 coupled to the collectors of respective replica transistors and a replica load 82.

[0032] The bases of the first and second replica transistors 73 and 74 are cross coupled (relative to the bases of the first and second buffer transistors 63 and 64) to the input port 61 and their emitters are coupled to drive a replica current i_Rthrough the replica load 82 in response to the differential input signal S_in.

[0033] The differential buffer amplifier 60 operates in a manner similar to that described above for the buffer amplifier 20 of FIGS. 2A and 2B except that it also provides significant rejection of a common-mode component of the input signal S_inbecause the common-mode component appears across both ends of the output load 68 and both ends of the replica load 82 and does not generate any substantial common-mode current through these loads.

[0034] Attention is now redirected to the buffer amplifier 20 of FIG. 2A in which it is noted that the currents I of the second replica current source 37 and the buffer current source 26 respectively flow towards and away from the same circuit node 39. The second replica current source 37 and the buffer current source 26, therefore, can be removed without substantially affecting the operation of the buffer amplifier.

[0035] Accordingly, FIG. 4 illustrates a buffer amplifier embodiment 90 which includes elements of the buffer amplifier 20 of FIG. 2A (with like elements indicated by like reference numbers) but which eliminates the second replica current source 37 and the buffer current source 26. This elimination alters the replica current generator 28 of FIG. 2A to the structure of the replica current generator 92 of FIG. 4.

[0036] In operation of the buffer amplifier 90, the input signal S_inat the lower side 31 generates replica currents ±i_Rin the replica load 32 so that currents through the replica transistor 34 are I ±i_R(as they were in the buffer amplifier 20 of FIGS. 2A and 2B). At the same time, the input signal S_inat the upper side 32 generates load currents ±i_Lin the replica load 32.

[0037] Because the replica impedance Z_Rof the replica load 32 substantially equals the impedance Z_Lof the buffer load 22, the replica currents i_Rat the circuit node 39 substantially provide the desired load currents i_Land current conservation at the circuit node 39 requires that the current through the buffer transistor 24 substantially equals the constant bias current I of the buffer current source 26.

[0038] It is further noted that the bases of the buffer transistor 24 and replica transistor 34 share the same common-mode input signal and that the collector of the replica transistor 34 is depressed by the base-emitter voltage V_beof the buffer transistor 24. Therefore, a level-shifting element E_l-sis preferably inserted ahead of the replica transistor 34 to similarly depress its base potential and insure that operation of the replica transistor remains in its active region (i.e., not in its saturation region).

[0039] In FIG. 4, this level-shifting element E_l-sis provided by a level-shifting diode 94 which receives a small trickle current I_Tfrom two trickle current sources 95 and 96 coupled between V_cand ground. Level shifting elements 97 are also shown in the control terminals of replica transistors in FIGS. 2A, 2B and 3. Rather than depress this control terminal in other buffer amplifier embodiments, it is noted that the level shifting can also be realized by elevating the control terminal of the buffer transistor (24 in FIG. 2A).

[0040] Buffer amplifiers of the invention are suitable for use in a variety of signal-conditioning systems such as the exemplary pipelined analog-to-digital converter (ADC) 100 of FIG. 5 which includes a buffer amplifier 102 that provides an analog input signal S_into N cascaded converter stages 104 that convert samples of the to input signal S_into a corresponding digital output signal S_out.

[0041] Each pipelined stage converts a respective analog signal to that stage's predetermined number of digital bits and passes an amplified residue signal S_resto a succeeding converter stage. As the succeeding converter stage converts its received residue signal S_resin a similar manner, the preceding stage is converting a succeeding analog input signal. All converter stages, therefore, are simultaneously converting succeeding analog input signals to their respective digital bits with final converted words issuing at the same rate as the sampling rate of input analog signals.

[0042] Broken expansion lines 106 in FIG. 1 indicate that an exemplary initial converter stage comprises an n-bit ADC 108 which provides at least a corresponding digital bit D₁and also comprises an n-bit digital-to-analog (DAC) 110 that converts these bits to an analog signal which is subtracted in a summer 112 from this stage's respective analog input to form an analog residue S_resthat is amplified in a respective amplifier 114 with a respective gain G and passed to a successive converter stage.

[0043] Generally, one or more redundant bits are generated and a control and correction logic 118 includes circuits (e.g., full adders) that use the bits of succeeding stages to correct preceding-stage errors that result from various degrading effects (e.g., offset and/or gain errors) and also includes circuits (e.g., shift registers) that time-align the corresponding digital bits

[0044] The m-bit DAC 110, the summer 112 and the amplifier 114 are generally referred to as a multiplying digital-to-analog converter (MDAC) which is collectively shown as the broken-line element 119 in FIG. 5. An exemplary arrow points from the MDAC 119 to indicate a particularly useful MDAC configuration 120 which is formed with a differential amplifier 122 that is coupled between a differential output port 124 and pairs of feedback capacitors C_fand sample capacitors C_s. Bottom plates of the feedback capacitors and sample capacitors are coupled together through a switch 126 and top plates of the feedback capacitors and the sample capacitors are respectively coupled to a differential input port 128 through switches 130 and multi-pole switches 132.

[0045] The switches 126, 130 and 132 are shown in a sample mode of the switched-capacitor MDAC 120 in which a sample of the input signal is stored as electrical charges on the feedback capacitors C_fand sample capacitors C_s. In an amplify mode, switch 126 is opened and switches 130 couple the top plates of the feedback capacitors C_fto the differential output port 124 (i.e., they are coupled across the differential amplifier 122).

[0046] In this amplify mode, the multi-pole switches 132 couple the bottom plates of the sample capacitors to −V_r, ground or +V_rin response to corresponding decisions by the n-bit ADC 108 wherein −V_rand +V_rare reference voltages that define the input limits of the ADC 108. This switch configuration is appropriate for a 1.5 bit ADC stage and is exemplary of other switch configurations that are appropriate for different stages.

[0047] It is apparent that the accuracy of the ADC 100 of FIG. 5 depends upon the ability of the buffer 102 to drive currents into the feedback capacitors C_fand sample capacitors C_sduring the sample mode of the MDAC 120. At the same time, the buffer 102 should faithfully reproduce the input signal S_inat the bottom plates of the feedback capacitors and sample capacitors.

[0048] It is noted that, during the sample mode, the switches 130 and feedback capacitors C_fare in parallel with corresponding ones of the switches 132 and the sample capacitors C_sto thereby form a series circuit consisting of two capacitors coupled between a central switch and two external switches. This capacitive load arrangement is shown as the buffer load 142 in the buffer amplifier embodiment 140 of FIG. 6 which is otherwise similar to the buffer amplifier embodiment 60 of FIG. 3 with like elements indicated by like reference numbers.

[0049] As stated above, it is important that the impedance of the replica load substantially matches the impedance of the buffer load and, accordingly, a similar capacitive load arrangement is inserted as the replica load 144. When the buffer amplifier embodiment 140 is used as the buffer 102 in FIG. 5, it will thus supply the currents required to drive the MDAC 120 and faithfully reproduce the input signal S_inat the top plates of the feedback capacitors C_fand sample capacitors C_r.

[0050] It was shown above with reference to the buffer amplifier 90 of FIG. 4, that the second replica current source (37 in FIG. 2A) and the buffer current source (26 in FIG. 2A) can be removed without substantially affecting the operation of the buffer amplifier. Accordingly, the second replica current sources and buffer current sources (77 and 65) of the buffer amplifier 60 of FIG. 3 are also eliminated in the buffer amplifier 140.

[0051] Simulations have been conducted with a conventional buffer and a buffer amplifier in accordance with the invention each driving a switched-capacitor MDAC. With a 2 volt peak-to-peak, 505 MHz input sine wave and a 100 Msamples/second sampling frequency, it was found that the output spectrum of the conventional buffer exhibited a spurious free dynamic range (SFDR) of approximately 54 dB and the output spectrum of the invention's buffer exhibited an SFDR of approximately 81.4 dB. Although bias currents (e.g., current of the buffer current source 26 in FIG. 2A) can be significantly increased in conventional buffers to realize SFDR performance that approaches that of the invention, this further increases their power consumption over that of the invention's buffer,

[0052] Structures of the invention have been described with reference to bipolar junction transistors but the teachings of the invention can be practiced with various transistor types. As is well known, transistors generally have a control terminal and first and second current terminals that are responsive to the control terminal.

[0053] Therefore, the teachings can be practiced, for example, with bipolar junction transistors in which the control terminal is a base and the first and second current terminals are emitters and collectors and with metal-oxide semiconductor (MOS) transistors in which the control terminal is a gate and the first and second current terminals are sources and drains. The equivalent use of MOS transistors, for example, is indicated in FIG. 6 by the MOS transistor 146 which is substituted for the bipolar junction transistor 73 as indicated by substitution arrow 147.

[0054] Although buffer amplifiers of the invention have been illustrated, for simplicity of description, as responding to differential input signals, other buffer embodiments may respond to single-ended input signals. For example, replacement arrow 150 in FIG. 1 indicates that the differential input signal at the differential input port 23 can be replaced with a single-ended input signal, a single-ended input port 151 and an inverter 152. In the description of FIG. 1, the differential input signal was described as having upper and lower sides 30 and 31. With this single-ended replacement, the single-ended input signal provides the upper side 30 and the inverter 152 responds to the upper side and provides the lower side 31.

[0055] The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.