Output stage with low output impedance and operating from a low power supply

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the design of electronic circuits, and more specifically to a low-output-impedance circuit operating from a low supply voltage.

[0003] 2. Related Art

[0004] An output stage generally refers to a portion of a circuit which provides the output signals of interest on corresponding output terminals. The terminals may connect the output stage with the input stage of a subsequent circuit (or to a load in general). The output stage is characterized by an output impedance, which is in theory defined as the impedance seen looking into the terminals.

[0005] It is generally desirable that the output impedance be low since the signal strength (voltage, current) would then be available for the actual load that the electrical circuit may be designed to be connected to. Low output impedance can lead to several benefits such as reduced power consumption, potential simplification of circuit design, etc., as is also well known in the relevant arts.

[0006] Output stages generally operate from a power supply. The power supply provides the energy (in the form of voltage/current) to generate the output signals. For example, a supply voltage, often termed as Vdd, is provided in operation of several circuits.

[0007] It may be desirable to provide output stages with low output impedance which operate from low power supplies in several environments. For example, there is an increasing demand to implement circuits operating with lower supply voltages (e.g., from 3.0 to 1.8 Voltage), and it may thus be desirable to ensure that the output stage also can operate at corresponding low supply voltage.

[0008] Various aspects of the present invention address one or more of the above-noted requirements, as described below in further detail.

[0016] In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

1. Overview

[0017] An output stage provided according to an aspect of the present invention includes an amplifier and two transistors coupled in a manner such that the output stage provides a low output impedance and operates from a low power supply. In an embodiment, the output stage contains an operational amplifier driving a gate terminal of a first MOS transistor, ensuring that there is no loading of the operational amplifier. The output of the output stage is provided at the drain terminal of a second MOS transistor, and the source and bulk (substrate) terminals of the second MOS transistor are substantially at the same potential. This enables the use of a low power supply and further, the output voltage of the output stage to be substantially independent of the power supply. A feedback path is provided from the output (drain) terminal of the second MOS transistor to the drain terminal of the first MOS transistor. Such an active feedback connection ensures that stability of the circuit is maintained.

[0018] According to another aspect of the present invention, a capacitor is provided between the output terminal of the output stage and the input terminal of the last transistor. As a result, at higher frequencies, the feedback factor is reduced, thereby increasing gain in the last transistor. This, to a certain extent, increases the local loopgain, and consequently compensates increase in output impedance at higher frequencies.

[0019] Several aspects of the invention are described below with reference to examples for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One skilled in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details, or with other methods, etc. In other instances, well known structures or operations are not shown in detail to avoid obscuring the features of the invention

2. Example Environment

[0020] FIG. 1 is a block diagram of an example environment in which various aspects of the present invention can be implemented. FIG. 1 is shown containing voltage generator 110 and analog-to-digital converter (ADC) 120. The blocks may be used as a part of a data acquisition system or instrumentation or control system. Such systems typically have other elements (blocks, not shown) to process the digital data received on path 125 and to control various functions such as sampling times and sampling rates, etc.

[0021] ADC 120 converts analog signals received on path 126, to equivalent digital representations according to the reference voltage received on path 112. The digital representations (digital codes) are made available on path 125 to an external system (such as a microprocessor, not shown). ADC 120 may be implemented as any of several different types of ADCs, such as successive approximation (SAR) ADC, integrating type ADC, pipelined ADC etc.

[0022] Voltage generator 110 provides a stable reference voltage output to ADC 120 on path 112. Power and ground connections to the various blocks shown are represented by 140 and 150 respectively.

[0023] The accuracy with which ADC 120 converts an input analog signal to an equivalent digital code depends, among other factors, on the accuracy of the reference voltage on path 112. One of the parameters that determines accuracy of the reference voltage is load regulation, which is a measure of the change in the output reference voltage due to a change in the current drawn from voltage generator 110.

[0024] This parameter depends on the output impedance of voltage generator 110. The lower the output impedance, the less change in output voltage due to variations in load current (and hence greater accuracy in reference voltage), and vice versa. Accordingly, it may be desirable to design voltage generator 110 with a low output impedance.

[0025] Further, it may be desirable, for reasons such as lower power consumption, that all the blocks in a system referred to above operate from a low power supply voltage. As such, it is also desirable that voltage generator 110 be able to operate from a low power supply voltage, while still being able to provide a reasonably wide range of reference voltages.

[0026] Various aspects of the present invention meet one or more of such objectives. The features of the present invention can be appreciated in comparison to a prior approach using which output stages are implemented. Accordingly, the next section contains the description of a prior approach to such a circuit.

3. Prior Output Stage

[0027] FIG. 2 is a schematic representation of a prior output stage that provides a low output impedance. The output stage is shown containing operational amplifier (OPAMP) 260, N-channel MOSFET (NMOS) 210, current source 250 and resistors 270 and 280. Each component is described below in further detail.

[0028] OPAMP 260 is used in a closed-loop configuration and produces an amplified voltage (on path 263) of an input voltage received on path 265. The gain of OPAMP 260 is determined by resistors 270 and 280. The output terminal of OPAMP 260 is connected to the gate terminal of NMOS210.

[0029] NMOS 210 is used in a source follower configuration and operates to reduce the output impedance compared to if the output were given directly from path 263. The output (VREF) is taken from the source terminal (220) of NMOS 210. The gate terminal of NMOS 210 is connected to the output of OPAMP 260.

[0030] Current source 250 provides a constant current on path 215, and helps in maintaining a stable voltage output (VREF) at point 220. The source follower configuration of NMOS 210 provides a low output impedance, equal to 1/g_m, wherein g_m is equal to the transconductance of NMOS 210.

[0031] As NMOS 210 is used in a feedback configuration with OPAMP 260, the output impedance is reduced by a factor equal to the open-loop gain of OPAMP 260, as is well known in the relevant arts.

[0032] That is, the output impedance of output stage shown in FIG. 1 is given by:
Z_o=1/(g_m*A_o)  Equation 1

[0033] wherein:

[0034] Z_o is the output impedance of the output stage,

[0035] g_m is the transconductance of NMOS 210

[0036] and A_o is the open-loop gain of OPAMP 260.

[0037] One disadvantage with the output stage shown in FIG. 2 is that it requires a comparatively high value of supply voltage for operation. Specifically, the minimum value of supply voltage AV_dd is governed by the following mathematical relation:
AV_dd>VREF+V_TN+3V_ON  Equation 2

[0038] wherein:

[0039] AV_dd is the minimum required value of supply voltage 290

[0040] VREF is the value of the reference voltage output

[0041] V_TN is the threshold voltage of the NMOS transistor

[0042] V_ON (the minimum voltage needed to keep the transistors in saturation) represents a voltage difference between the operating voltage across the gate and source terminals (Vgs) of transistor 210 and V_TN i.e (Vgs−V_TN). V_ON also represents the difference (Vgs−V_TN) of each of two cascoded transistors (not shown) contained in OPAMP 260

[0043] > represents the greater-than logical relationship.

[0044] The substrate (bulk) terminal of NMOS 210 is tied to ground potential in one embodiment. From FIG. 2, it can be seen that the source terminal of NMOS 210 is at a potential higher than ground by a value VREF. Due to this reason, the threshold voltage V_TN could be high (e.g., of the order of 1 volt). Further, V_ON is approximately equal to 200 milli volts for a required VREF of 1 volt.

[0045] Therefore, to obtain a reference voltage (VREF) of 1 Volt, the minimum value of power supply voltage AV_dd is given by:
AV_dd>2.6V  Equation 3

[0046] (for the circuit of FIG. 3, where OPAMP 260 contains a cascode stage at its output.)

[0047] Providing an output voltage using a source follower stage as shown in FIG. 3, has the disadvantage that the range of output voltages is constrained by the bulk-bias (V_TN of equation 2), as illustrated above using example values in equation 3. Although, FIG. 1 is shown using NMOS transistors, a source follower circuit using PMOS transistors may have similar limitations.

[0048] As noted earlier, it is desirable to operate integrated circuits from lower power supply voltages. Various aspects of the present invention provide a low-impedance output stage which can operate from lower supply voltages as described below in further detail.

4. Low Impedance Output Stage Operating from Low Power Supply

[0049] FIG. 3 is a schematic representation of an output stage according to an aspect of the present invention. The output stage is shown containing operational amplifier (OPAMP) 360, P-channel MOSFETs (PMOS) 310 and 330, current sources 340 and 350, and resistors 370 and 380. The output terminal of the output stage is on path 385 and provides a voltage (VREF).

[0050] OPAMP 360 is used in a closed-loop configuration and produces an amplified voltage (on path 363) of the input voltage received at its non-inverting terminal on path 365. The output terminal of OPAMP 360 is connected to the gate(input) terminal (333) of PMOS 330.

[0051] PMOS 330 has its gate terminal (333) connected to the output of OPAMP 360, source(output) terminal (331) connected to the gate (input) terminal (313) of PMOS 310 and drain terminal (332) connected to the output terminal 385. Current source 340 provides biasing (operating point) for PMOS 330.

[0052] PMOS 310 has its source terminal (311) connected to power supply (AVdd) 390, and drain terminal (312) connected to the output terminal 385. Current source 350 provides biasing (operating point) for PMOS 310.

[0053] Resistor 380 is connected between the output terminal 385 and the non-inverting input terminal of OPAMP 360. Resistor 370 is connected between the non-inverting input terminal of OPAMP 360 and ground. The two resistors cause OPAMP 360 to operate in the closed-loop configuration. Feedback is provided from output terminal 385 to drain terminal (332) of PMOS 330.

[0054] The topology described above operates as an output stage with low impedance, as described in sections below (described with respect to FIGS. 4 and 5).

[0055] Continuing with the description of FIG. 3, capacitor 325 is connected across the output 385 and gate terminal (313) of PMOS 310. As described below, the use of capacitor 325 reduces the feedback factor at higher frequencies, thus reducing the output impedance at higher frequencies. This offsets, to a certain extent, the gain reduction at higher frequencies in OPAMP 360, and consequently compensates for increase in output impedance at higher frequencies.

[0056] From FIG. 3, it may be appreciated that since the output of OPAMP 360 is connected to the gate terminal 333 of PMOS 330, OPAMP 360 is not loaded. The feedback provided by PMOS 330 to the gain stage formed by PMOS 310 prevents any circuit stability problems.

[0057] In comparison to the output stage of FIG. 2, it may be appreciated that since the output of FIG. 3 is provided between the drain terminal of PMOS 310 and ground, there is no dependence of the output voltage on bulk-bias effects (explained earlier). Thus, the output voltage may be chosen substantially independent of the supply voltage AVdd.

[0058] The circuit of FIG. 3 uses two PMOS transistors (330 and 310). It should be understood, that a circuit similar to that of FIG. 3, but using NMOS transistors (with appropriate changes in connections) can also operate as an output stage providing a low output impedance (as will be apparent to one skilled in the relevant arts by reading the disclosure herein).

[0059] The operation of the circuit of FIG. 3 using a low power supply is described below.

5. Lower Power Supply Voltages

[0060] The power supply voltage 390 (AVdd) for a required value of output (VREF) is given by:
AV_dd>VREF+V_TP+V_ON1+V_ON2  Equation 4a

[0061] wherein:

[0062] AV_dd is the minimum required value of supply voltage

[0063] VREF is the value of the reference voltage output

[0064] V_TP is the threshold voltage of the PMOS transistors 310

[0065] V_ON1 represents a voltage difference between the operating voltage across the gate and source terminals (Vgs) of PMOS transistor 330 and V_TP (in this case it is the minimum voltage to keep PMOS 330 in saturation).

[0066] V_ON2 represents a voltage difference between the operating voltage across the gate and source terminals (Vgs) of PMOS transistor 310 and V_TP.

[0067] As is well known, MOSFETs are characterized by a parameter commonly referred to as threshold voltage(V_T), which is the minimum value of the voltage between the MOSFET (s gate and source terminals at which a drain current begins to flow, and the MOSFET may be used in normal operation. The threshold voltage for a PMOS transistor is lower than the corresponding threshold voltage for an NMOS transistor due to the fabrication process.

[0068] The threshold voltage V_T is not a constant, but a function of the voltage (V_SB) between the source and substrate (bulk) terminals of a MOSFET. V_T is minimum when V_SB is zero, and increases for non-zero values of V_SB. When used in integrated circuits, the substrate terminals of PMOS transistors is normally connected to the power supply, while the substrate terminals of NMOS transistors is normally connected to the ground terminal.

[0069] The substrate (bulk) terminal (not shown in figure) of PMOS 310 is connected to power supply (AV_dd)390. The source terminals 311 of PMOS 310 is also connected to power supply 390. Consequently, the potential difference (V_SB) between source and bulk terminals of PMOS 310 is always 0 volts irrespective of output voltage at terminal 385. Therefore, the value of V_TP in equation 4A is always the minimum specified for PMOS transistor 310, and further does not vary with the value of VREF desired. Similarly bulk of PMOS 330 may be connected to source terminal 331 to achieve a lower value of threshold voltage in the bias provided to PMOS 310.

[0070] In contrast, the output stage shown in FIG. 2 uses an NMOS transistor which has a higher value of threshold voltage V_T due to additional implants during the fabrication process. Further, due to the source follower configuration (which is necessary to achieve a low output impedance), source terminal of NMOS transistor 210 is at a potential different from ground (to which the substrate terminal is connected). Due to the non-zero value of the potential difference(V_SB) between source and bulk terminals of NMOS 210, the value of threshold voltage V_T is higher than its minimum.

[0071] Therefore, it may be appreciated that the output stage of FIG. 3 has a lower power supply voltage requirement for the same value of output voltage VREF, as may be seen by comparing equations 3 and 4b and further illustrated below .

[0072] Substituting for typical values for the terms in equation 4a gives,
AV_dd>VREF+0.9V (A value of 0.5V for V_TP and 0.2V for each of V_ON1 and V_ON2 have been used)  Equation 4b

[0073] Thus for a VREF of 1 Volt, AV_dd>1.9V, which is less than the requirement indicated by equation 3.

[0074] The substrate terminal (not shown in FIG. 3) of PMOS 330 is connected to its source terminal 331. Such a connection results in a minimum value of threshold voltage V_TP for PMOS 330 and further helps in minimizing power supply voltage (AV_dd) requirement.

[0075] It may be seen that the constraint on a minimum value of power supply AV_dd due to the bias to PMOS 310 (formed by PMOS 330 and current source 340) results in a value that is lower than 1.9V as determined from the equation below:
AV_dd>2V_ON(360)+V_TP(330)+V_ON(330)+V_TP(310)+V_ON(310)  Equation 4c

[0076] Wherein 2V_ON(360) represents a factor due to the output transistorstage (not shown in FIG. 3) of OPAMP 360

[0077] V_TP(330) is the threshold voltage of PMOS 330.

[0078] V_ON(330) is a factor due to saturation voltage requirement of PMOS 330.

[0079] V_TP(310) is the threshold voltage of PMOS 310.

[0080] V_ON(310) is a factor due to saturation voltage requirement of PMOS 310.

[0081] It may be verified that the AVdd requirement as given in equation 4c is less than 1.9 Volts.

[0082] Thus, it may be appreciated that the circuit of FIG. 3 may operate from a low value of power supply voltage.

[0083] Further, as the potential difference(V_SB) between source and bulk terminals of PMOS 310 of FIG. 3 is at a fixed minimum value (zero volts) irrespective of the output voltage VREF, the output voltage VREF can be chosen substantially independent of the supply voltage AV_dd.

[0084] The feedback used in the output stage shown in FIG. 3, results in a low output impedance as is shown below.

6. Low Output Impedance

[0085] FIG. 4 shows a schematic representation of the portion of output stage (of FIG. 3) containing PMOS transistors 310 and 330 and biasing current sources 340 and 350 . For the sake of ease of description, OPAMP 360 is considered to be ideal and only the portion of the output stage containing elements 310, 330, 340 and 350 is used for computing the output impedance. In FIG. 4, PMOS 310 and 330 have been replaced by equivalent models in order to derive the parameters of interest.

[0086] Element 420 (representing the transconductance of PMOS 330) represents the source-to-drain current source according to the small signal model for PMOS 330. Resistor 460(R_ds2) represents the equivalent source-to-drain resistance according to the small signal model for PMOS 330.

[0087] Element 430 (representing the transconductance of PMOS 310) represents the source-to-drain current source according to the small signal model for PMOS 310. Resistor 480 (R_ds1) represents the equivalent source-to-drain resistance according to the small signal model for PMOS 310.

[0088] Resistor 450 is the equivalent resistance presented by current source 340. Resistor 470 is the equivalent resistance presented by current source 350.

[0089] As is well known, for the purpose of computation of output impedance in a circuit, all independent voltage sources are shorted (made 0 volts). Therefore, in FIG. 4, power supply terminal 390 is shown connected to ground, and gate(input) terminal 333 of PMOS 330 is connected to ground, in order to compute the output impedance. Further a current source 410, supplying a current I_in is connected to output terminal 385.

[0090] The description is now continued with respect to equations demonstrating that the model of FIG. 4 provides low output impedance. Various symbols and parameters used in deriving the output impedance are first given below:

[0091] g_m1 represents the transconductance of PMOS 310.

[0092] g_m2 represents the transconductance of PMOS 330.

[0093] R_ds2 represents the equivalent source-to-drain resistance according to the small signal model for PMOS 330.

[0094] g_ds2=1/R_ds2

[0095] R_ds1 represents the equivalent source-to-drain resistance according to the small signal model for PMOS 310.

[0096] g_ds1=1/R_ds1

[0097] R_dsb1 represents the resistance of current source 350(of FIG. 3) and is represented by element 470(of FIG. 4).

[0098] g_dsb2=1/R_dsb2

[0099] R_dsb2 represents the resistance of current source 340(of FIG. 3) and is represented by element 450(of FIG. 4).

[0100] g_dsb2=1/R_dsb2

[0101] V₀(terminal 385) represents the voltage at the output terminal of the output stage.

[0102] V_x(terminal 313) represents the voltage at the gate terminal 313 of PMOS 310.

[0103] I_in represents the current from a current source 410 supplying a constant current connected on path 385. (This is used to mathematically derive the output impedance of the output stage).

[0104] Z_OL represents the open-loop output impedance of the portion of the output stage (of FIG. 3) that is shown in FIG. 4 .

[0105] Z_CL represents the (closed-loop) output impedance of the overall output stage (that is, of the output stage of FIG. 3).

[0106] A₀ represents the open-loop gain of OPAMP 360.

[0107] A_PMOS represents the gain due to PMOS transistors 330 and 310.

[0108] V_in represents the voltage of a voltage source (510 in FIG. 5) connected at terminal 333. (This is used to mathematically derive the gain due to PMOS transistors 330 and 310).

[0109] Open-loop output impedance of the portion of the output stage shown in FIG. 4 is given by:
Z_OL=V₀/I_in  Equation 5

[0110] Applying Kirchoff(s Current law (KCL) at terminal 385 gives:
V₀g_ds1+V₀g_dsb1+V_xg_m1+(V₀−V_X)g_ds2−V_xg_m2+1_in=0  Equation 6

[0111] Applying KCL at terminal 313 gives:
V₁g_ds2+V_xg_m2−(V₀−V_x)g_ds2=0  Equation 7

[0112] Rearranging the terms in equation 7 gives:

[0113] $\begin{array}{cc}{V}_0=\frac{V_x\left(g_{m2}+g_{\mathrm{ds}2}+g_{\mathrm{dsb}2}\right)}{g_{\mathrm{ds}2}}& \mathrm{Equation}8\end{array}$

[0114] Substitution of term V_x (obtained from equation 8) in equation 6 gives:

[0115] $\begin{array}{cc}\frac{I_{\mathrm{in}}}{V_0}=\left(g_{\mathrm{ds}1}+g_{\mathrm{dsb}1}+g_{\mathrm{ds}2}\right)+g_{m1}\frac{1-\left(g_{m2}/g_{m1}\right)-\left(g_{\mathrm{ds}2}/g_{m1}\right)}{1+\left(g_{m2}/g_{\mathrm{dsb}1}\right)}& \mathrm{Equation}9\end{array}$

[0116] Terms containing G_ds1 and G_ds2 can be neglected as they are very small.

[0117] Therefore, Z_OL may be approximated by the relation below.

[0118] $\begin{array}{cc}{Z}_{\mathrm{OL}}\approx \frac{1}{g_{m1}}\frac{1+\left(g_{m2}/g_{\mathrm{dsb}1}\right)}{1-\left(g_{m2}/g_{m1}\right)}& \mathrm{Equation}10\end{array}$

[0119] wherein the term

[0120] $\frac{1+\left(g_{m2}/g_{\mathrm{dsb}1}\right)}{1-\left(g_{m2}/g_{m1}\right)}$

[0121] can be considered a scaling factor which is represented for convenience as P.

[0122] Z_OL thus computed is in turn used for computing the closed loop output impedance of the output stage. The closed-loop output impedance Z_CL also depends on the open loop gain of the output stage. Accordingly, the description is continued with respect to the computation of the open loop gain.

[0123] FIG. 5 is a schematic diagram illustrating the manner in which gain (from terminal 363 to 385 of FIG. 3) due to PMOS 330/310 of the circuit of FIG. 3 is computed. The gain thus computed is then used to compute the open loop gain of the output stage. The diagram is described in comparison to the circuit of FIG. 4 for conciseness. The circuit of FIG. 5 differs from the circuit of FIG. 4 in that voltage source 510 supplying V_in volts is connected between gate terminal of PMOS 330 and ground, and current source 410 is removed.

[0124] The open-loop output impedance (Z_OL) of the output stage is represented by equation 10.

[0125] The closed-loop output impedance of the output stage (that is, of the circuit of FIG. 3) further depends on the total open-loop gain in the output stage (gain in OPAMP+gain due to PMOS transistors 330 and 310).

[0126] To derive the gain A_PMOS due to PMOS transistors 330 and 310, a voltage V_in is applied between terminal 333 and ground, and an expression for the gain A_PMOS (which is equal to V₀/V_IN) is computed as shown below:

[0127] Applying KCL at terminal 333:
(V_x−V₀)g_ds2+V_xg_dsb2−(V_in−V_X)g_m2=0  Equation 11

[0128] Applying KCL at terminal 385:
V₀g_dsd1+V₀g_ds1+V_Xg_m1+(V₀−V_X)g_ds2+(V_in−V_X)g_m2=0  Equation 12

[0129] Rearranging the terms in equation 11, gives

[0130] $\begin{array}{cc}{V}_x=\frac{V_0{g}_2+V{g}_1}{g_1+g_2+g_1}& \mathrm{Equation}13\end{array}$

[0131] Substitution of term V_x (obtained from equation 13) in equation 12 gives:

[0132] $\begin{array}{cc}\frac{V_0}{V_{\mathrm{in}}}=\frac{g}{\left[\frac{g+g}{g}\times \frac{g+g+g}{2g-g}+1\right]\times g}& \mathrm{Equation}14\end{array}$

[0133] Wherein, V₀/V_in of equation 14 represents the gain A_PMOS due to PMOS 330 and 310.

[0134] If the terms g_m1 is much greater than g_m2 and if the g_ds terms are much less than the g_m terms, then it may be shown that gain due to PMOS transistors 330 and 310 is substantially the same as factor P of equation 10.
Therefore A_PMOS=P (approximately)  Equation 15a

[0135] Due to feedback used:
Z_CL=Z_OL/(G_OLT)  Equation 15

[0136] wherein G_OLT=A₀*A_PMOS

[0137] Substituting the values for Z_OL (from equation 10) and G_OLT
Z_CL=((1/g_m1)*P)/(A₀*P)  Equation 16

[0138] wherein approximation of equation 15a has been used.

[0139] Therefore the (closed-loop) impedance of the output stage may be approximated by
Z_CL=1/(g_m1*A₀)  Equation 17

[0140] As terms g_m1 and A₀ typically have very large values, it may be seen that the impedance of the output stage is very low.

[0141] It must be understood that the above computation of Z_CL has been approximated to show that the closed loop output impedance is at least as low as that of the source follower structure of FIG. 2. The actual closed-loop output impedance Z_CL of the circuit of FIG. 3 may be shown to be even lower than that of the source follower of FIG. 2.

[0142] Thus, the output stage of FIG. 3 operates from a low power supply and has a low output impedance.

[0143] The output stage of FIG. 3 may be used as an output stage of voltage generator 110 (shown in FIG. 1), where a stable DC voltage is a requirement. Frequently, there are requirements in electronic circuits where a low-output impedance output stage has to drive a predominantly capacitive load. In such environments, the output signal of interest is typically an AC voltage.

[0144] From equation 15, it may be seen that the output impedance Z_CL of the output stage is inversely proportional to the open-loop gain A₀ of OPAMP 360. This gain A₀ is not a constant, but varies inversely as frequency of the input signal (operating frequency) applied to OPAMP 360 (on path 365). Therefore it can be seen that output impedance Z_CL of the output stage increases with increasing operating frequency. Thus, Z_CL behaves similar to an inductor.

[0145] When it is a requirement that the output stage of FIG. 3 be connected to a predominantly capacitive load (such as in a switched capacitor load in an ADC), there is a possibility of resonance occurring in the output signal of the output stage. It is therefore desirable that the output impedance Z_CL of the output stage not increase in value significantly as frequency increases.

[0146] An aspect of the present invention addresses such a requirement, as is described below.

7. Feedback Capacitor

[0147] As noted above,

[0148] Z_CL=Z_OL/(G_OLT)

[0149] wherein G_OLT=A₀*A_PMOS

[0150] A_PMOS represents the gain contribution due to PMOS 330 and PMOS 310. As the operating frequency increases A₀ decreases (due to capacitance inherent in various stages in the OPAMP, as is well known in the relevant arts), thereby causing an increase in Z_CL.

[0151] If Z_OL is designed to decrease with increasing frequency by ensuring that the feedback from terminal 385 to 313 increases with increasing frequencies, the local loop gain (from terminal 363 to 385) increases, thus resulting in lower overall impedance Z_CL. This is achieved according to an aspect of the present invention by connecting capacitor 325 between the output terminal 385 of the output stage and the gate terminal 313 of PMOS 310.

[0152] As is well known, a capacitor presents a smaller impedance at higher frequencies. As operating frequency increases, capacitor 325 effectively presents a lower impedance and consequently the feedback factor reduces.

[0153] This causes more output to be fedback to the gate of PMOS 310, consequently reducing Z_OL, which offsets, to a degree, a decrease in A₀. Therefore, any increase in the value of the term Z_OL is minimized, thereby minimizing any change in the output impedance Z_CL of the output stage, and thereby minimizing any possibility of resonance occurring in the output signal of the output stage

[0154] Thus, the connection of capacitor 325 from the output terminal of the output stage to the input terminal of PMOS310 results in an output stage whose output impedance remains low even at higher frequencies.

[0155] The description is continued with respect to an example device in which various aspects of the present invention are implemented.

8. Device

[0156] FIG. 6 is a block diagram of receiver system 600 illustrating an example system in which various aspects of the present invention may be implemented. For illustration, it is assumed that receiver system 600 is implemented within a Wireless Receiver. However, receiver system 600 can be implemented in other devices (wireless as well as wire-based communications) as well.

[0157] Receiver system 600 is shown containing low noise amplifiers (LNA) 610, mixer 620, filter circuit 660, analog to digital converter (ADC) 670, and processor 680. Each block/stage is described in further detail below.

[0158] LNA 610 receives signals on path 601 and amplifies the received signals to generate a corresponding amplified signal on path 612. For example, in wireless systems, the signals that are transmitted from satellites, etc. may be received by an antenna (not shown) and the received signals are provided on path 601. The received signals may be weak in strength and thus amplified by LNA 610 for further processing. LNA 610 may be implemented in a known way.

[0159] Mixer 620 may be used to down-convert the received amplified signal on path 612 into an intermediate signal with the frequency band of interest centered at a lower frequency than the carrier frequency of the received signal. In an embodiment, a signal with the frequency band of interest centered at 2.4 GHZ (carrier frequency) is converted to a signal with the frequency band of interest centered at zero frequency.

[0160] Mixer 620 may receive the amplified signal on path 612 and a signal of fixed frequency on path 622 as inputs, and provides the intermediate signal on path 626. The signal of fixed frequency on path 622 may be generated by a phase locked loop (not shown) in a known way.

[0161] Filter circuit 660 may correspond to a low pass filter, which allows the desired low frequencies and rejects all other unwanted high frequencies present in the signal received on line 626. The filtered signal, which contains the frequency band of interest, is provided on path 667.

[0162] ADC 670 converts (samples) the filtered signal received on path 667 to a corresponding digital value, which represents the signal of interest in received signal 601. Processor 680 processes the received digital values to provide various user applications and may be implemented as multiple processing units, each potentially operating independently. ADC 670 may correspond to the ADCs described in sections above (and implemented according to various aspects of the present invention).

9. Conclusion

[0163] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.