Abstract:
A circuit with low noise and reduced offset that feeds an input of an opamp with a programmable feedback resistor that provides variable gain settings. Input biasing currents are varied using control bits that are also used to adjust the gain. When the input signal is small (gain at higher setting), a minimum bias current is provided to source the input voltage swing. This scheme reduces the noise and offset generated by the lower transconductance of a biasing transistor while maintaining a constant SNR and fixed offset even in the presence of relatively small input swings. Also, when the input signal is large (gain at lower setting), a maximum bias current can be provided to accommodate the relatively large input swing level. Although the overall noise and offset current are increased for large input swings, the overall SNR and offset is maintained for relatively lower input swings.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a variable gain voltage amplifier, and more particularly, to a current conveyor type voltage amplifier circuit that employs a gain dependent biasing scheme to reduce offset and noise to maintain the signal integrity when amplifying a varying input signal. 
     BACKGROUND OF THE INVENTION 
     In the past, different types of circuits have been employed to improve the performance of variable gain amplifiers that employ automatic gain control (AGC) circuits. AGC amplifiers are widely used in communication channels to automatically adjust gain in response to a varying peak to peak swing of an input signal. A typical AGC circuit detects an outputted signal&#39;s peak-to-peak swing, and then adjusts the gain by varying the feedback impedance network which is connected across an opamp to maintain a relatively constant level for the swing of the outputted signal. For example, when an inputted signal swings (rises) to a relatively high level, the AGC circuit will adjust the feedback impedance of the amplifier so that its gain is reduced. Similarly, when the inputted signal&#39;s swing is at a relatively low level, the AGC circuit adjusts the feedback impedance to provide more amplifier gain. 
     However, an unfortunate side effect of increasing amplifier gain in current conveyor type voltage amplifiers for a faint/weak input signal with a relatively small swing is an increase in noise and the offset voltage due to an increase in the feedback impedance that multiplies noise and offset contributions from the current conveyor&#39;s biasing (current sources) circuits. Thus, for relatively faint/weak signals, the operation of an AGC amplifier can degrade the signal to noise ratio (SNR). Accordingly, it would be advantageous to provide a variable gain amplification circuit that retains the benefits of an AGC circuit without degrading the SNR for weak/faint input signals with a relatively small swing. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for amplifying a signal. A variable feedback resistance is coupled between the input and output terminals of an opamp. The input terminal of the opamp is connected to the output of the current conveyor circuit (the high impedance node). The input signal is converted into input current by a fixed resistor which connects to the input of the current conveyor circuit (low impedance node). The signal current then is conveyed (directed) to the input of the opamp and, through the feedback resistor to produce output voltage. Due to the high output impedance of the current conveyor circuit, the feedback factor of the amplifier remains close to unity and thus, operating at a loop gain which is close to the open-loop performance. This also results in reduction of power consumption by the opamp since the feedback factor remains close to unity across the AGC range. The current conveyor circuit consists of sink and source currents that are provided partly to bias the input signal swing current. In particular, the source current is scaled such that for a given input swing current, sufficient biasing current is available to maintain signal integrity. The sink current is scaled according to the source current to maintain the biasing conditions. The (sink and source) current sources produce noise and offset contributions that are proportional to the Gm (transconductance) of the transistors. To minimize the offset and noise for a given drain-to-source voltage the transistors are biased such that minimum Gm is achieved while the transistors remain in saturation region. The sink and source current sources are then scaled by the gain control setting of the amplifier to provide sufficient current for the required input current swing. In other words, when the input swing current is at minimum (maximum feedback resistance), the current sources provide minimum bias currents such that maximum SNR is achieved. On the other hand, when the input swing current is at maximum (minimum feedback resistance), the current sources provide large bias currents such that the overall signal-to-noise ratio is relatively constant. Thus, the SNR remains constant across the range of the ADC. 
     For one embodiment, the invention is directed to at least one switch for disabling at least a portion of the source current source if the high swing of the signal is small. The operation of the switch causes noise produced by at least a portion of the source current source to be reduced. Also, at least another switch is directed to disabling at least a portion of the sink current source if the low swing of the signal is small. The operation of this switch causes noise produced by at least a portion of the sink current source to be reduced. 
     For yet another embodiment, the invention is directed to at least one of a plurality of PMOS transistors that is employed by the source current source to provide the selected amount of current. Also, at least one of a plurality of NMOS transistors is employed by the sink current source to provide the selected amount of current. 
     For yet still another embodiment of the invention, the controller includes an automatic gain control component. Also, the controller can be used to determine the size of a peak-to-peak swing in the signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a signal amplification circuit with automatic gain control; 
     FIG. 2 shows a graph for the open loop and closed loop performance of the circuit illustrated in FIG. 1; 
     FIG. 3 shows another signal amplification circuit with automatic gain control that is further configured to have a loop gain substantially similar to open loop performance of the amplifier; 
     FIG. 4 illustrates a graph for the open loop gain and the loop gain performance of the circuit shown in FIG. 3; and 
     FIG. 5 shows another signal amplification circuit with automatic gain control that is further configured to operate in a manner substantially similar to the circuit illustrated in FIG.  3  and further provide reduced noise and offset when a received signal is weak, in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanied drawings, which form a part hereof, and which is shown by way of illustration, specific exemplary embodiments of which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” The term “connected” means a direct electrical connection between the items connected, without any intermediate devices. The term “coupled” means either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, or data signal. Referring to the drawings, like numbers indicate like parts throughout the views. 
     The present invention can be employed with any amplification circuit where it is desirous to provide reduced power consumption when amplifying a signal of varying strength with some form of variable gain, such as provided by an AGC circuit. The invention enables an amplifier to operate with loop gain configuration close to open loop performance. Also, the invention enables relatively stable noise and offset contributions for an amplification circuit even when the size of the peak-to-peak swing for an input signal varies. Typical applications might include wireless transceivers for mobile nodes such as mobile telephones, pagers, walkie-talkies, and the like. Similarly, wired transceivers employed with network interface cards might include applications that use the present invention. 
     FIG. 1 illustrates a signal amplification circuit  100  with automatic gain control (AGC). Amplifier  102  is shown configured for closed loop operation and provides variable gain by way of adjustments to the impedance of feedback resistor Rf, which is coupled between the input and output terminals of amplifier  102 . An end of AGC component  104  is coupled to feedback resistor Rf to enable the AGC to vary the gain of circuit  100  in response to the size of the peak-to-peak swing of the signal outputted at terminal Vout. 
     The feedback factor (voltage at the input of the opamp) at Node A is represented by the equation Rin/(Rin+Rf). For a given Rf setting, the feedback factor of the amplifier varies across the AGC gain range or in other words, the loop gain and loop gain-bandwidth varies across the range of AGC. For example, assuming a unity gain configuration (Rin=Rf), the feedback factor equals ½ which, corresponds to a loop gain of 6 dB or less than the open-loop gain. This reduction in loop gain also reduces the gain-bandwidth frequency by ½. Thus, to operate the opamp at a desired loop gain and frequency, the open-loop gain and bandwidth of the amplifier must be twice as large as the desired gain and bandwidth. 
     FIG. 2 shows graph  200 , which illustrates a gain versus frequency relationship for a signal outputted by an amplification circuit such as the circuit shown in FIG.  1 . As shown in FIG. 2, assuming Rin equals Rf, the amplification circuit operates at frequencies that are half as large under an open loop configuration and provide 50% (6 decibels) less gain than the frequencies and gain enabled in a closed loop configuration for this circuit. 
     FIG. 3 illustrates signal amplification circuit  300  with AGC that is further configured to have an open loop gain similar to the open loop performance of the amplifier. 
     The configuration of circuit  300  is as follows. An input terminal Vin is coupled to an end of an input impedance Rin where a signal is inputted to the circuit. The other end of input impedance Rin is coupled to a drain terminal of PMOS transistor  302  and a source terminal of PMOS transistor  304 . A source terminal of PMOS transistor  302  is coupled to a positive voltage supply Vdd. Also, a drain terminal of PMOS transistor  304 , a drain terminal of NMOS transistor  306 , the input to amplifier  310  and an end of feedback resistor Rf are coupled together at Node B. A source terminal of NMOS transistor  306  is coupled to a grounded terminal. The other end of feedback resistor Rf is coupled to the output of amplifier  310  and the output terminal Vout where the outputted signal is provided for use with other circuits. Additionally, although not shown, in some embodiments, the source terminal of NMOS transistor  306  may be coupled to a negative voltage supply. 
     In FIG. 3, AGC circuit  308  is coupled to feedback resistor Rf AGC circuit  308  can vary the feedback resistor&#39;s impedance (amplification circuit  300  gain) in proportion to the swing/strength of the signal received at input terminal Vin. In this way, the signal outputted at the output terminal Vout remains relatively constant as the strength of the signal varies. 
     AGC component  308  controls the operation of PMOS transistor  302  and NMOS transistor  306  so that relatively large currents are conveyed with the inputted signal to the input of amplifier  310  when the signal swings towards a positive or negative peak, respectively. For example, when the inputted signal is positive, PMOS transistor  302  conducts a current from the positive voltage supply to the input of amplifier  310  (Node B). A biasing voltage VB 1  for PMOS transistor  302  is controlled by AGC component  308  in response to the swings of the inputted signal. Similarly, when the inputted signal swings low, NMOS transistor  306  is turned on by AGC component  308  with biasing voltage VB 2  so that a current will flow to the input of amplifier  310 . 
     Biasing PMOS transistor  302  provides the source current in the current conveyor circuit, which provides the required current when the input swing level is negative relative to the voltage at the biasing transistor&#39;s drain. Since the input impedance of the current conveyor (drain of biasing transistor  302 ) is proportional to the 1/Gm of PMOS transistor  304 , the swing at the source of PMOS transistor  304  (or drain of biasing PMOS transistor  302 ) is relatively small compared to the input swing (virtual ground). The biasing NMOS transistor  306  provides the sink current source of the current conveyor circuit, which sinks the current associated with PMOS transistor  302  to maintain proper biasing conditions at Node B. 
     It is understood that the biasing PMOS transistor  302  and biasing NMOS transistor  306  are fixed transistors, which are scaled to provide the required biasing current when the input swing current (Vin/Rin) is at a maximum. In other words, the PMOS and NMOS biasing transistors are scaled for worst-case input current levels. The output of the current conveyor circuit at Node B, is a high impedance node given by the cascading of transistors  302  and  304  in parallel with biasing transistor  306 . The feedback factor associated with this circuit topology is Ro/(Rf+Ro) where Ro is the output impedance of the current conveyor. The feedback factor can be designed where Ro&gt;&gt;Rf such the feedback factor is close to unity across the range of Rf. In this way, a loop-gain and loop-gain-bandwidth can be maintained close to open-loop performance. 
     Also, in FIG. 3, it is understood that the amount of current that is supplied by the current conveyor sources is fixed, e.g., 100 microamps might be supplied by the conduction of either PMOS transistor  302  or NMOS transistor  306 . This fixed amount of current is typically selected to enable optimal operation of the circuit when the signal inputted at terminal Vin is relatively large. However, when the received signal is relatively small/weak, this same amount of current can lead to a degradation in the signal to noise ratio (SNR) for the circuit. In particular, the SNR for inputted signals with relatively small peak to peak swings may be adversely affected by the amount of noise in the relatively large currents provided by the current conveyor sources, i.e., PMOS transistor  302  and NMOS transistor  306 . In some cases, the currents supplied for a large signal swing may be less than optimum for an inputted signal with a relatively small signal swing. Additionally, since so much more of the actual signal outputted by amplifier  310  can be made available for use with other circuits at terminal Vout, circuit  300  can consume substantially less power than other amplification circuits operating in the closed loop mode, such as shown in FIG.  1 . 
     FIG. 4 illustrates graph  400  for the open loop gain and the loop gain performance of the circuit shown in FIG.  3 . 
     FIG. 5 illustrates another amplification circuit  500  that can operate in a closed loop configuration at frequencies and gain that are substantially similar to the operation of an amplifier in an open loop configuration while consuming less power and adding less noise to an inputted signal with a varying swing. The circuit shown in FIG. 5 is configured and operates in substantially the same manner as the circuit illustrated in FIG.  3  and discussed above. However, the circuit shown in FIG. 5 is configured to provide relatively less current and less noise from a current conveyor source when the swing of an inputted signal is relatively low and relatively more current when the inputted signal&#39;s swing is relatively large. 
     As shown in FIG. 5, a signal is received at input terminal Vin, which is coupled to an end of input impedance Rin. The other end of input impedance Rin is coupled to a source terminal of PMOS transistor  402  and the drains of PMOS transistors  408 A,  408 B and  408 C. The drain terminal of PMOS transistor  402  is coupled to the drains of NMOS transistors  410 A,  410 B and  410 C, and an end of feedback resistor Rf at Node C. The input to amplifier  404  is also coupled to Node C and the output of the amplifier is coupled to both the other end of feedback resistor RF and the output terminal Vout. 
     Automatic Gain Control (AGC) component  406  is coupled to feedback resistor Rf and controls the impedance (amplifier gain) provided by the resistor in accordance with the size of the swing of the inputted signal. Also, AGC component  406  controls the operation of three enable switches  414 A,  414 B, and  414 C, and three disable switches  412 A,  412 B and  412 C for PMOS transistors  408 A,  408 B and  408 C, respectively, in response to the size of the high swing in the inputted signal. Similarly, AGC component  406  controls the operation of three enable switches  416 A,  416 B, and  416 C, and three disable switches  418 A,  418 B and  418 C for PMOS transistors  410 A,  410 B and  410 C, respectively, in response to the size of the low swing in the inputted signal. 
     When an enable switch is closed for an associated MOS transistor, the corresponding disable switch is opened and the transistor begins to conduct and convey a current along with the inputted signal to Node C at the input to amplifier  404 . For example, when enable switch  414 A is closed and disable switch  412 A is opened, PMOS transistor  408 A conducts and a current is supplied to the input terminal of amplifier  404  while the inputted signal is swinging high. Similarly, closing enable switch  416 A and opening disable switch  418 A causes NMOS transistor  410 A to conduct and provide a current to the input terminal of amplifier  404  when the inputted signal is swinging low. 
     Depending on the size of swing of the inputted signal, AGC component  406  will enable or disable more or less MOS transistors in the current conveyor sources. In particular, the AGC component&#39;s coordinated operation of PMOS transistors  408 A,  408 B and  408 C form a current conveyor source that provides a variable current in response to the size of a high swing for an inputted signal. Thus, when the signal has a relatively large high swing, all three of these PMOS transistors can be enabled to conduct, i.e., enable switches  414 A,  414 B, and  414 C close and disable switches  412 A,  412 B, and  412 C open. Also, when the inputted signal has a relatively small high swing, only one of the three PMOS transistors would be enabled to conduct, e.g., enable switch  414 A would close, enable switch  414 B and  414 C would open, disable switch  412 C would open and disable switches  412 B and  412 C would close. 
     Additionally, the closing of the disable switches for either PMOS transistors  408 A,  408 B,  408 C or NMOS transistors  410 A,  410 B and  410 C stops their conduction and causes a reduction in the amount of noise that these transistors add to the operation of circuit  500 . When AGC component  406  is not enabling one of the MOS transistors to provide a portion of the current supplied by a current conveyor source, its corresponding disable switch is closed to reduce noise and improve the overall SNR for circuit  500 . This feature is particularly helpful in reducing SNR when circuit  500  is employed with an inputted signal that has a relatively small swing. Although not shown, disable and enable switches may be formed with different types of electronic devices, including, but not limited to transistors, analog switches, and the like. 
     Furthermore, the amount of current supplied by each MOS transistor in the source and sink current conveyor sources for circuit  500  may be substantially the same or varied. For example, PMOS transistor  408 A and NMOS transistor  410 A could provide 50%, PMOS transistor  408 B and NMOS transistors  410 B might provide 30%, and PMOS transistor  408 C and NMOS transistor  410 C could provide the remaining 20% of the total amount of current provided by their respective source and sink current conveyor sources. Moreover, although the embodiment shown in FIG. 5 illustrates source and sink current conveyor sources with three PMOS or NMOS transistors, respectively, it is understood that the operation of the invention with two or substantially more PMOS and NMOS transistors would also provide the significant benefits discussed herein. Also, it is understood that the operation of the PMOS and NMOS transistors in the current conveyor sources does not have to be symmetrical. For example, if it is determined that the size of the high swing for an inputted signal is substantially larger than the size of the low swing, substantially more PMOS transistors could be enabled for providing current than NMOS transistors in the current conveyor sources. 
     Additionally, overall power consumption by circuit  500  can be further reduced because less current is provided for an inputted signal with a relatively small swing than the amount of current provided for an inputted signal with a relatively large swing. In other words, the bigger the signal, the more MOS transistors that are turned on and the more current that is provided; and where the signal is smaller, fewer MOS transistors are turned on to provide current and power is saved. 
     It should be appreciated that circuit  500  shown in FIG. 5 enables an amplifier to control the amount of current supplied by current conveyor sources in proportion to the size of the peak-to-peak swing in the received signal without decreasing the SNR for signals with a relatively small swing. This circuit also enables almost open loop performance while operating in a closed loop mode in substantially the same manner as discussed for the circuit shown in FIG.  3 . Additionally, since the current provided by the current conveyor sources is reduced for a relatively small inputted signal, PMOS transistor  402  can maintain a relatively stable biasing for efficient operation of circuit  500  even though the swing of the inputted signal can vary substantially. 
     As mentioned above, circuit  500  conserves power by adjusting the amount of current provided by current conveyor sources in proportion to the size of the swing of the received signal, disables any non-conducting MOS transistors in the current conveyor sources to further reduce noise, provides a relatively stable bias current for varying sizes of peak-to-peak swing in the signal and enables an amplifier to provide substantially open loop performance when arranged in a closed loop configuration. It is further understood that although the embodiments were discussed in terms of the general operation of an automatic gain control component, the invention may be employed with any amplification circuit that enables variable gain of an amplification circuit for an type of inputted signal. 
     The above specification, examples and data provide a complete description of the manufacture and use of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.