Low voltage variable gain amplifier with constant input impedance and adjustable one-pole filtering characteristic

[0002] The invention relates to the field of variable gain amplifiers, and in particular to variable gain amplifiers with constant input impedance and adjustable one-pole filtering characteristics.

[0003] Variable gain amplifiers (VGA) are known in the art and are utilized in a number of applications. Traditionally, a VGA comprises an operational amplifier (op-amp) having adjustable resistances in either the feedback path (from op-amp output to op-amp input), the signal input path, or both. By adjusting the resistance in either of these paths, the feedback factor of the amplifier is adjusted, thereby providing the variable gain.

[0004] One disadvantage of the traditional implementation of a VGA is that, as the gain of the VGA is adjusted by changing the feedback factor, the closed-loop bandwidth of the VGA changes. This is due to the fact that the closed-loop bandwidth of the VGA is approximately the open-loop bandwidth, which is fixed, of the op-amp (op-amp transfer function) multiplied by the feedback factor. As the gain increases, the feedback factor decreases and, therefore, the closed-loop bandwidth decreases. Therefore, for higher gains, the bandwidth of the VGA is more limited, limiting the range of input signals applied to the VGA.

[0005] U.S. Pat. No. 5,285,169, entitled Monolithic Integrated Differential Amplifier with Digital Gain Setting, seeks to correct this disadvantage by providing an operational amplifier in which the gain-bandwidth product (open-loop bandwidth) of the op-amp is adjustable. The gain-bandwidth product is made adjustable by adjusting the channel width Wt and source current “it” of transistors t1 and t2 in the differential stage “tv” of the op-amp. These variables are adjusted by activating a parallel stage “to”, comprising transistors t7 and t16.

[0006] While the '169 patent addresses the drawback of a fixed gain-bandwidth product, there are a number of disadvantages to the VGA of the '169 patent, as well as the traditional VGA. The adjustment of the resistance in the input path causes the input impedance to change. Particularly when a VGA is utilized to implement an analog front-end system, this creates difficulties in matching the driver of the input to the input impedance.

[0007] In addition, at times it is desirable provide a one-pole filtering characteristic with the VGA. Often this is done to reduce noise in the input signal. This is typically accomplished by providing capacitance in the feedback path of the op-amp. However, when traditional VGAs that adjust the resistance in the feedback path to adjust the gain are utilized in conjunction therewith, the time constant, and implicitly, the pole location of the VGA is changed as the gain is changed.

[0008] Therefore, what is needed is a variable gain amplifier which has constant input impedance and one-pole filtering characteristics which are adjustable so as to remain constant for a change in gain.

[0009] The present invention provides for a variable gain amplifier with constant input impedance and an adjustable one-pole filtering characteristic. Preferably, a first set of n+1 parallel resistors is connected to input V_inand a second set of n+1 parallel resistors is connected to input V_ip. Except for one resistor in each set, each of the resistors of the two sets is connected to its corresponding summing junction (op-amp input), or to a corresponding resistor of the other set via a switch. Switching the resistors to their corresponding summing junction or to the corresponding resistor of the other set provides for the variable gain function, where the gain is proportional to the number of resistors connected to the summing junction. However, regardless of the number of resistors switched to the summing junctions, the input impedance of the VGA remains a constant impedance of R/(n+1), where R is the resistance value of the resistors. In addition to the input resistor networks, the VGA of the present invention comprises a fixed feedback resistance, along with, preferably, a parallel capacitor array in the feedback paths of the differential op-amp to provide one-pole filtering of the input signal. Changing the value of the capacitance, by switching capacitors in the array in and out, changes the one-pole filtering characteristics without affecting the VGA gain. To maintain the filtering characteristics constant for a change in the gain, the op-amp comprises an internal compensation network for adjusting the open-loop bandwidth of the op-amp.

[0010] FIG. 1aillustrates a differential variable gain amplifier of the present invention.

[0011] FIG. 1billustrates a single-ended variable gain amplifier of the present invention.

[0012] FIG. 2 illustrates a small signal model of the operational amplifier of the present invention including a compensation network.

[0013] FIG. 1aillustrates the variable gain amplifier 100 of the present invention which comprises two sets of resistor networks 104aand 104b, each with resistors having a resistance R and connected to the positive input V_ipand negative input V_in, respectively; op-amp 114, variable capacitor arrays 110aand 110band feedback resistors 112aand 112b, having a resistance R_f. Amplifier 100 is a double-input and double-output differential amplifier with outputs V_onand V_op. Feedback resistor 112aand capacitor array 110aare connected in parallel between output V_onand non-inverting input (summing junction) 116 of op-amp 114 (feedback path). Likewise, feedback resistor 112band capacitor array 110bare connected in parallel in the feedback path corresponding to output V_opand inverting input (summing junction) 118. Each set of resistors has n+1 resistors, and in each set of resistors, except for one resistor R₁a, R₁b, each of the resistors R₂a-R_(n+1)a and R₂b-R_(n+1)b are switched to the corresponding summing junction (which is virtual ground) or switched to the corresponding resistor connected to the other polarity input via n switches sw₁a-sw_na and sw₁a-sw_nb.

[0014] Switching of resistors R₁a-R_(n+1)a and R₁b-R_(n+1)b provides for the variable gain function of VGA 100. When m resistors of each set are switched to the corresponding summing junction, the overall DC gain is m*R_f/R. Preferably, the gain change is made linear in decibels with 6 dB steps by making the number of resistors in a set (n+1) a power of two. Hence, for example, if n=31 and 31 resistors are switched to the summing junction, there is a gain of 32*R_f/R, a gain of 16*R_f/R for 15 resistors connected to the summing junction, and so on.

[0015] For the preferred implementation of a differential variable gain amplifier, the inputs V_ipand V_inare substantially equal in magnitude, but opposite in polarity. Due to this, coupled with the fact that the resistance values of the input resistors are all equal, from a differential standpoint, when a resistor is connected to the corresponding resistor in the other set, node 120 between the resistors is a virtual ground. Therefore, as the summing junctions (inputs) are also virtual ground, no matter whether the resistors are switched to a summing junction, or to a corresponding resistor, the input impedance to VGA 100 for either input V_ipor V_inis constant with a value of R/n+1. This allows for matching the driver of the input to the input impedance of VGA 100.

[0016] Another embodiment of the present invention is a single-ended configuration of the VGA 100. This embodiment is illustrated in FIG. 1b, where like numerals represent like elements of FIG. 1a. Single-ended VGA operates similar to differential VGA 100, however, for single-ended VGA 122, instead of being a virtual ground, node 120 is connected to ground along with non-inverting input 116 of op-amp 114. In addition, the feedback is provided between the op-amp output and the inverting input, so as to provide negative feedback.

[0017] For either a differential or single-ended VGA, in the preferred embodiment of the present invention, one-pole filtering characteristics are provided by VGA 100 in order to reduce noise in the input. This is accomplished by variable capacitor arrays 110aand 110bin the feedback paths of op-amp 114. While the one-pole filtering characteristics are able to be provided by a single capacitor in the feedback path, it is preferable to utilize a capacitor array so that the filtering characteristics can be adjusted by switching capacitors into and out of the feedback path, rather than feedback resistors. This provides distinct advantages for certain uses of VGA 100. For instance, when VGA 100 is fabricated monolithically, often times the fabricated component values are not precise and, by adjusting the total capacitance in the feedback array, the filtering characteristics can be adjusted to the nominal design characteristics. In addition, it may be desirable to continually compensate for variations in the component values during operation of VGA due to variations in temperature or supply voltage, so as to maintain the filter characteristics near the nominal design value.

[0018] The one-pole transfer function of VGA 100 is determined by the time constant R_fC_a(where C_ais the total capacitance in the feedback path). By providing a fixed feedback resistance R_fin the feedback paths, the time constant, and implicitly, the pole location is adjustable independently from the switching of the resistors to effect a gain change of VGA 100.

[0019] However, a change in the gain affects the closed-loop bandwidth, as the closed-loop bandwidth is approximately the product of the open-loop bandwidth and the feedback factor. Therefore, in order to maintain the closed-loop bandwidth constant, the open-loop bandwidth needs to be adjusted when there is a change in the gain.

[0020] In order to adjust the open-loop bandwidth of op-amp 114, a preferred embodiment of the present invention comprises a compensation network internal to op-amp 114. FIG. 2 illustrates a small-signal model of op-amp 114, including compensation networks 210aand 210b. Compensation network 210aadjusts the open-loop bandwidth for the non-inverting input 116 and 210badjusts for the inverting input 118. Preferably, VGA 100 is utilized in low voltage supply applications, therefore, the op-amp is assumed to be a two-stage topology. For low voltage supply applications, a two-stage topology is preferred, as a single stage topology normally does not provide enough gain with restricted headroom due to a low power supply. However, the present invention should not be seen as limited to a two-stage topology.

[0021] As illustrated, op-amp 114 includes two equal sets of k parallel capacitor-resistor branches around second stage gm2. Each branch has a capacitor 212, with capacitance Cm, in series with a resistor 211. As illustrated, resistor 211 is preferably a triode-mode Metal Oxide Semiconductor (MOS) device having a corresponding gate. The open-loop bandwidth of op-amp 114 is incrementally changed by switching a MOS device in a branch on or off. Therefore, the overall adjustment to the open-loop bandwidth is performed by switching a number of the MOS devices on or off, depending upon the overall adjustment needed, by applying the power supply voltage to the appropriate gates (switch on) or by applying ground to the appropriate gates (switch off), i.e. the signals bw0-bwk are either ground or power supply. If the gates of n MOS devices 211 are tied to the power supply (i.e. switched on), then the open-loop bandwidth of the amplifier 114 is approximately gm1/(kC_m), where gm1 is the transconductance of the first stage. Therefore, by adjusting the total number of MOS devices turned on, the open-loop bandwidth is adjusted and the change in the closed-loop bandwidth due to the change in the gain is compensated for. For example, as the closed-loop bandwidth (which is approximately the open-loop bandwidth multiplied by the feedback factor) decreases with the gain of the VGA increasing (because the feedback factor is decreasing), the open-loop bandwidth is increased by turning off MOS devices.

[0022] While not a part of the present invention, it should be understood that, typically, a control circuit outputs digital signals to appropriately set the various parameters of VGA 100. For instance, the control circuit outputs a digital word of n bits, each of which are applied to a switch of the resistor networks 104aand 104bto adjust the gain of VGA 100. Preferably, at the same time, the control circuit outputs a digital word of k signals which is input to op-amp 114 (as illustrated by signal BW in FIG. 1a) where each of the k signals are applied to a gate of a MOS device in the compensation network. Also, the control circuit outputs a digital word which controls the number of capacitors switched into the capacitor arrays 110aand 110bso as to set the filter characteristics of VGA 100.

[0023] Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.