WO2007019509A2

WO2007019509A2 - Feedback network for high gain amplification with enhanced bandwidth and gain control

Info

Publication number: WO2007019509A2
Application number: PCT/US2006/030899
Authority: WO
Inventors: Robert D. Washburn; Robert F. Mcclanahan
Original assignee: Thunder Creative Technologies, Inc.
Priority date: 2005-08-05
Filing date: 2006-08-07
Publication date: 2007-02-15
Also published as: WO2007019509A3

Abstract

The system is an amplifier feedback circuit that permits a significant increase in useable bandwidth with comparable gain compared to amplifiers of the present art incorporating conventional feedback circuit topologies. The increased bandwidth is accomplished by significantly increasing the frequency of the pole in the transfer characteristic that is formed by the feedback resistor and the parasitic amplifier input capacitance. Embodiments shown include application to a transimpedance amplifier such as used with photo detector diodes in fiber optic receiver circuits. A basic voltage input, voltage output, operational amplifier circuit embodiment is also shown. Means for controlling the gain of the amplifier with a variable resistor that may be implemented with a digital potentiometer is shown.

Description

FEEDBACK NETWORK FOR HIGH GAIN AMPLIFICATION WITH ENHANCED BANDWIDTH AND GAIN CONTROL

FIELD

[0001] The network relates to the field of high gain, wide bandwidth amplifiers.

BACKGROUND ART

[0002] Since their inception, there has been a steady improvement in the performance capability of virtually every type of integrated circuit. Finer control of internal features as well as improved manufacturing techniques and process controls has resulted in high density or small circuits with much higher operating frequencies. This is true as applied to analog amplifiers including operational amplifiers. Over the period referenced above, the state-of-the-art gain- bandwidth product of the operational amplifiers has increased from approximately 1 MHz to approximately 10 GHz. This trend is expected to continue for at least the immediate future.

[0003] Unfortunately, the practical benefits of this capability are often unrealized due to application requirements or circuit limitations. This is certainly true with regard to high gain, wide bandwidth operational amplifier circuits. The basic problem can be illustrated using the simplest form of circuit, a transimpedance amplifier as illustrated in Figure 1. Figure 1 shows the transimpedance amplifier in a common application, converting a photodiode current into an output voltage.

[0004] Referring to Figure 1, reverse biased photo detector diode DlOO couples the positive terminal of bias voltage source +VD_D at node NlOO to the negative input of operational amplifier AlOO at node NlOl. The negative terminal of source +VDD and the positive input of operational amplifier AlOO are connected to ground. Feedback resistor RlOO couples the output of operational amplifier AlOO at node N102 to the negative input of operational AlOO at node NlOl. The transimpedance of the amplifier circuit of Figure 1 is set by the Value of resistor RlOO.

[0005] Gain is a dimensionless quantity representing the ratio of powers, voltages, or currents at 2 nodes in a circuit or a ratio of one the quantity at one node to a defined standard value for the quantity. Transimpedance is a ratio between a voltage (output) and a current (input) and has the basic unit of ohm. Particularly with transimpedance amplifiers, the transimpedance functions in a like manner as voltage gain in a voltage input- voltage output amplifier. The transimpedance is set by the feedback resistor value which functions as the proportionality constant between the input and output. For convenience, the term gain will be used as a substitute for transimpedance in the remainder of this application.

[0006] Transimpedance amplifiers frequently have high gain. An example is the Analog Devices AD8015, which is described by the manufacturer as "...a wide bandwidth, single supply transimpedance amplifier optimized for use in a fiber optic receiver circuit". The AD8015 has an internal lOK-ohm feedback resistor. Of course the high resistance value makes the feedback resistor a relatively large thermal noise source. The AD8015 die has a typical input capacitance of 0.2pF and a SOIC package capacitance of approximately 0.4pF. Allowing approximately 0.5pF for the circuit board pad and input node trace capacitance implies a total input capacitance of approximately IpF. The resulting RC time constant is 10ns with an associated transfer function pole at 15.9MHz. Both significantly limit practical performance and bandwidth.

[0007] The above described problem and resulting performance limitations are well known to manufacturers of analog integrated circuit amplifiers. The manufacturers have employed various circuit techniques that at least partially mitigate these limitations. This can be seen from the specifications of the AD8015, which has a bandwidth of 240MHz, and rise and fall times of 1.5ns. Both performance characteristics are substantially better than the pure limitations imposed by the RC time constant, yet remain limited. In the AD8015, the mitigation of the performance limitations is accomplished by the use of an integrator feedback circuit, which is discussed on in the device specification.

SUMMARY

[0008] The system is a feedback network structure, used in one embodiment with high gain, wide bandwidth amplifiers. It allows realization of substantially more of the potential amplifier bandwidth capability made possible by modern semiconductor fabrication techniques and equipment than has been achieved using conventional gain setting, feedback circuits.

[0009] The system overcomes a limitation imposed by the presence of a pole in the transfer function of an amplifier, formed by the feedback resistor and the parasitic capacitance of the amplifier input. In conventional designs, high gain requires a large value feedback resistor, use of which results in a low frequency of the pole, and severely limited useable bandwidth. Expressed another way, the RC time constant formed by the feedback resistor and the parasitic capacitance significantly limit the large signal transient response time of the amplifier. The system provides for use of lower value resistors, substantially increasing the pole frequency and allowing significantly greater realizable bandwidth.

[0010] In one or more embodiments, the system includes a Tee resistor network to perform feedback and gain setting functions for an amplifier.

[0011] In one or more embodiments, the system includes a Tee resistor network to perform feedback and gain setting functions for an operational amplifier.

[0012] In one or more embodiments, the system includes a Tee network to perform feedback and gain setting functions for a voltage amplifier circuit.

[0013] In one or more embodiments, the system includes a Tee resistor network to perform feedback and gain setting functions for a transimpedance amplifier.

[0014] In one or more embodiments, the system includes a Tee resistor network to perform feedback and gain setting functions for an amplifier, wherein the resistor-capacitor time constant formed by the equivalent resistance of the Tee resistor network and the parasitic capacitance coupling the amplifier input to ground is substantially less than the resistor-capacitor time constant formed by the resistance of the feedback resistor of the present art and the parasitic capacitance coupling the amplifier input to ground, wherein the amplifiers and the gains of both amplifier circuits are substantially the same.

[0015] In one or more embodiments, the system includes multiple Tee resistor networks to perform feedback and gain setting functions, wherein the Tee resistor networks are connected in cascade, wherein series connected resistors in the cascade connection are or are not combined into a single resistor of equivalent value.

[0016] In one or more embodiments, the system includes one or more Tee resistor networks to perform feedback and gain setting for an amplifier circuit, wherein the resistance of one or more shunt legs of the Tee resistor networks is a variable resistance and wherein the variation of the variable resistance varies the gain of the amplifier circuit.

[0017] In one or more embodiments, the system includes one or more Tee resistor networks to perform feedback and gain setting for an amplifier circuit, wherein the resistance of one or more shunt legs of the Tee resistor networks is a variable resistance and wherein the variable resistance is set in whole or in part by a digitally controlled variable resistor such as a digital potentiometer.

[0018] In one or more embodiments, the system includes a Tee resistor network wherein the shunt resistor is coupled to a DC offset voltage source.

[0019] In one or more embodiments, the system includes a compensated Tee resistor- capacitor network to perform feedback and gain setting functions for an amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS [0020] Figure 1 is a circuit diagram of a transimpedance amplifier of the present art.

[0021] Figure 2 is a circuit diagram of a fixed gain embodiment of the system.

[0022] Figure 3 is a circuit diagram of a variable gain embodiment of the system.

[0023] Figure 4 is a circuit diagram of a variable gain embodiment of the system with feedback circuit comprised of multiple Tee networks.

DETAILED DESCRIPTION

[0024] The system is directed to the feedback network used in high gain, wide band amplifiers. In the following description, numerous specific details are set forth to provide a more thorough description of embodiments of the system. It is apparent, however, to one skilled in the art, that the system may be practiced without these specific details. In other instances, well known features have not been described in detail so as not to obscure the system.

[0025] The system enables electronic circuit designers to realize wider useable bandwidth and faster large signal response in high gain, feedback amplifiers than is achieved using classic (prior art) feedback network configurations. This is accomplished by replacing the classic feedback network configuration with a Tee structure that is implemented with relatively low value resistors while achieving equal or higher gain than realized with the classic feedback network configurations and their higher value resistors.

[0026] In the various embodiments of the system, circuits are typically shown as purely resistive networks. This is done for purposes of example only and is not intended to be a limitation of the system. This does not imply that parasitic reactive elements and networks are not present, but that the significance of the reactive elements is dependent on the detailed implementation for a specific application and set of performance specifications.

[0027] Among the figures illustrating various embodiments and features of the system, like circuit elements retain previously defined identifiers.

[0028] Amplifier Tee Resistor Feedback Networks for High Gain and Wide Bandwidth

[0029] The system and resultant benefits will be illustrated using a transimpedance amplifier configuration with the same input source, overall gain (transimpedance), and output signal level as shown in Figure 1 and discussed as part of the present background art. The illustration represents one embodiment of the system for a transimpedance amplifier and is illustrated in Figure 2.

[0030] The values for resistors R200 and R201 maybe chosen or estimated based upon required amplifier performance characteristics and then adjusted as part of a design optimization. In one embodiment, desired performance is achieved when the values of resistors R200 and R202 are approximately equal. The value of resistor R200 may not be set arbitrarily small as the overall amplifier gain would not be realizable with a resistor R202 of approximately equal value. A larger value for resistor R202 could provide the necessary gain while reducing the basic benefits of the system. The loss of benefit occurs because the larger resistance value for resistor R202 and the parasitic capacitance associated with the physical resistors and mounting pads form a transfer function pole at a lower frequency and a larger RC time constant than for the case where resistors R200 and R202 are of similar value. However, this typically is not a problem due to the low RC time constant on the T-node (Node 200) due to the relatively low resistance value for resistor R201. A larger resistance value for resistor R202 may also increase thermal noise.

[0031] The value of resistor R201 should be chosen to be significantly lower than the value of resistor R200, yet not so small that the parasitic inductance of resistor R201 becomes significant in determining the impedance of the shunt resistor branch of the Tee network. All other elements of the amplifier circuit being equal, the lower the value of resistor R201, the higher the gain of the amplifier circuit. Although small values for resistor R201 are not precluded, it is generally preferred to keep the value of resistor R201 at or above 10-ohms for non-integrated implementations to avoid effects of parasitic inductance of resistor R201 at high frequencies.

[0032] Selecting values for resistors R200 and R201 to be 300-ohms and 10-ohms respectively, for example, the value of resistor R202 calculates to be approximately 334-ohms. Assuming the same lpf amplifier input parasitic capacitance previously discussed, the RC time constant has been reduced from 10ns to 0.3ns and the associated pole in the transfer function has been moved from a frequency of approximately 15.9MHz to a frequency of approximately 530.5MHz where it poses far less potential to limit amplifier performance capability.

[0033] Although illustrated in terms of a transimpedance amplifier configuration, the system is readily applied by one skilled in the art to most forms of amplifiers using feedback, including operational amplifier and non-operational amplifier amplifiers, voltage and current amplifiers, and amplifiers implemented with a wide variety of semiconductor and non-semiconductor gain elements. Instrumentation amplifiers are a particularly attractive application for the system since they have traditionally had only low frequency capability.

[0034] Amplifier Tee Resistor Feedback Networks with Variable Gain Control

[0035] Whether an embodiment of the system uses a single Tee resistor feedback network as previously described, or uses multiple Tee resistor networks in cascade as described in the following section, the mathematical expression for overall gain of the amplifier circuit includes elements that are or include the ratio of a series to shunt resistor value. Making either resistor a variable resistor such as a potentiometer can then make the overall gain variable and controllable. The shunt resistor(s) is typically chosen since the controller mechanism is much easier to implement than one for a series resistor that requires a differential control signal. Figures 3 and 4 illustrated typical implementations of a variable gain capability.

[0036] Particularly useful embodiments include a digital potentiometer, which is readily available with up to 1024 selectable values. Use of a digital potentiometer can allow the system processor to set the gain as part of an overall configuration of an entire system. The digital potentiometer, or any other form of variable resistor, can be used as a single component implementation for resistors R201 of Figure 3 as well as R401 and R403 of Figure 4. It is also extremely useful to use one or more fixed resistors in conjunction with the variable resistor to form a series, parallel, or series-parallel network configuration to replace the single variable resistor. The replacement network not only provides for precise limiting the range of gain variation, it also provides for finer resolution for gain settings within the limited range.

[0037] Cascaded Amplifier Tee Resistor Feedback Networks

[0038] As discussed previously, embodiments of the system can be realized using a wide variety of resistor values that depend on the overall gain to be realized and the chosen value of the feedback resistor coupled to the amplifier input (R200). The lowest values for series resistors (R200 and R202) occur when the resistors are approximately equal in value. Realization of some gain levels may mean that resistor values remain unacceptably high.

[0039] Realization of high gain with even lower value resistors than previously discussed can be achieved by the use of embodiments of the system with multiple Tee resistor networks connected in cascade. Figure 4 illustrates a cascade of 2 Tee resistor networks with resistor R402 representing the series combination of the output resistor of the first Tee resistor network and the input resistor of the second Tee resistor network. It should be noted that there is a design tradeoff between the higher resistor value that results from combination into a single resistor R402 as opposed to 2 lower value resistors R402A and R402B (not shown) in series with resulting increased parasitic capacitance and inductance. Figure 4 shows the shunt resistors as variable resistors, which provides embodiments with adjustable gain as discussed in the previous section. Cascaded embodiments can be implemented with either fixed or variable shunt resistors as appropriate for a particular application.

[0040] The benefit of a cascade configuration can be readily seen by comparing the configuration shown in Figure 2 with a cascade configuration using a 30-ohm input resistor, 10- ohm shunt resistors, and resistors of approximately 30-ohms for the remaining series resistors. For purposes of illustration, the shunt resistor value of 10-ohms has been retained. The circuit of Figure 4 will be used for element identification purposes when reference is made to the cascade configuration. Figure 4 nominally illustrates a true voltage gain amplifier. The circuits of Figures 3 and 4 can be analyzed as the previous transimpedance amplifiers if input signal source V300 and resistor R300 are replaced by a current source of value V300/R300 where the resistance value of resistor R300 is large.

[0041] Referring to Figure 2 as described above, a lOOuA input current and a 1 volt output signal requires a gain (transimpedance) is 10000 and the values of resistors R200, R201, and R202 to provide the gain are approximately 300-ohms, 10-ohms, and 334-ohms respectively. Assuming the same overall input/output/gain for the cascade configuration, with R400 chosen to be 30-ohms or 10% of the value for resistor R200 in the configuration of Figure 2. The magnitude of the voltage at node N400 is 3mv and the current through resistor R401 is 30OuA. The current through resistor R402 is thus 40OuA, and the magnitude of the voltage at node N401 is 15mv. It should be noted that this is the equivalent output voltage for the circuit of Figure 2 with the values of resistors R200, R201, and R202 being 30-ohms, 10-ohms, and 30-ohms respectively. The gain of the configuration is 150. [0042] Continuing with the circuit of Figure 4, the currents through resistors R403 and R404 are 1.5mA and 1.9mA respectively. The magnitude of the voltage at node Nl 02 is 72mV and the overall gain is 720. It is again clear that either some of the resistor values are too low to achieve the overall gain or at least one additional Tee resistor network needs to be added in cascade.

[0043] Assuming an additional Tee resistor network is added and the value of resistor R404 remains unchanged (resistor R404 would couple node N401 to node N402, resistor R405 (10- ohms) would couple node N402 to ground and resistor R406 (30-ohms) would couple node N402 to node Nl 02. The currents through resistors R405 and R406 are 7.2mA and 9.ImA respectively. The magnitude of the voltage at node N102 is 0.345 and the overall gain is 3450.

[0044] Again, either resistor values could be adjusted or another Tee resistor network section could be added to the cascaded. Since this overall gain is within a factor of less than 3 of the required value and with the values of 7 resistors in the feedback network are available for adjustment, another stage would not normally be added except for illustrative purposes.

[0045] Again assuming an additional Tee resistor network is added and the value of resistor R406 remains unchanged. Node Nl 02 becomes node N403 with new resistors R407 and R408 coupling node N403 to ground and to node N 102 respectively. The magnitude of the new output voltage at node Nl 02 would be 1.653V and the overall gain of 16530. Since the output voltage and gain exceed the overall performance requirements, the likely choice would be to make a minor adjustment in the value of shunt resistor R407. Any or all of the shunt resistor values could be adjusted to provide the reduced gain. This may be useful in high frequency applications, where the likely choice would be to adjust shunt resistor values rather than add an additional section with the additional parasitic elements to the Tee resistor network cascade.

[0046] If configured as above for an overall gain of 16530, the feedback current would be 43.6mA. This may be considered high for a low noise, low distortion, operational amplifier based system, a factor that will significantly influence the number of cascade stages and resistance values used.

[0047] Amplifier Tee Resistor Feedback Networks with DC Offset Voltage Control [0048] In a variety of amplifier configurations, it may be beneficial to provide the capability to supply an external DC offset voltage to the feedback network of the configurations. Examples of the configurations can include circuits using operational amplifiers with a high DC offset voltage specification or circuits wherein the operational amplifier positive input is not connected to ground. The capability is realizable for various embodiments of the system by coupling one or more shunt resistor branches within the Tee resistor feedback network to a DC voltage source of the appropriate DC offset voltage value instead of ground. In so doing, it is necessary to insure that either the DC offset voltage sources adequately approximate the "zero" source impedance characteristic of an ideal voltage source or the impedance characteristics of the DC offset sources are incorporated within the desired impedance of the shunt resistor branches.

[0049] Amplifier Compensated Tee Resistor-Capacitor Feedback Networks

[0050] A well-known technique associated with resistor networks is frequency compensation. This technique replaces some or all of the individual resistors in a network with resistor-capacitor combinations. Various circuit topologies can be employed depending on the nature and function of the overall network. Compensation is particularly useful with large resistor values (and resulting large R-C time constants), or at very high frequencies such as defined in relation to the upper frequency capabilities of associated amplifiers.

[0051] Although the low value resistors implicitly part of the system reduce the need for frequency compensation, there will be an upper frequency limit on amplifier performance imposed by material and the state-of-the-art technology. As a result, frequency compensation for the system may prove useful. There is nothing associated with the system that precludes application of numerous frequency compensation techniques that would be readily applied by one skilled in the art.

[0052] Thus, a feedback network for high gain amplification with enhanced bandwidth and gain control has been described.

Claims

CLAIMSWe claim:

1. A circuit comprising: an input; an amplifier coupled to the input; a tee network coupled to the amplifier to perform feedback and gain setting functions for the amplifier.

2. The circuit of claim 1 wherein the tee network comprises a tee resistor network.

3. The circuit of claim 2 wherein the tee resistor network comprises a first resistor coupled to an input of the amplifier and a node; a second resistor coupled to the node and to the output of the amplifier; a third resistor coupled to the node and to ground.

4. The circuit of claim 3 wherein the first resistor and the second resistor have approximately equal resistance.

5. The circuit of claim 4 wherein the third resistor has a resistance lower than the first resistor.

6. The circuit of claim 3 wherein the third resistor is a variable resistor.

7. A circuit comprising: an input; an amplifier coupled to the input; a first tee network coupled to the amplifier to perform feedback and gain setting functions for the amplifier; a second tee network coupled to the amplifier to perform feedback and gain setting functions for the amplifier.

8. The circuit of claim 7 wherein the first and second tee networks comprises tee resistor networks.

9. The circuit of claim 8 wherein the first tee resistor network comprises a first resistor coupled to an input of the amplifier and a first node; a second resistor coupled to the first node and to a second node; a third resistor coupled to the first node and to ground.

10. The circuit of claim 9 wherein the second tee resistor network comprises a fourth resistor coupled to the second node and to the output of the amplifier; a fifth resistor coupled to the second node and to ground.

11. The circuit of claim 10 wherein the third resistor and the fifth resistor are variable resistors.