Controllable effective resistance circuits.

Resistors are basic components of most electronic circuits. They are used for biasing, current limiting and a wide range of functions determined by the resistance of the resistor and the particular circuit in which they are used.

Filters are one type of circuit employing resistors that are used extensively in the field of electrical signal processing. Fundamentally, the filter has a capacitor and a resistor whose values define the frequency response of the filter. Thus, a different value for either the capacitor or the resistor will result in a change in the frequency response.

Typically for prior filters, the frequency response has been changed by substituting a capacitor of one value for a capacitor of another value or by changing a resistor of one value for a resistor of another value. Therefore, a given filter may not be easily used in two different applications requiring different frequency responses since it may be necessary to change either the capacitor or resistor, so that the filter may be usable in either application. Moreover, if

the filter is implemented as an integrated circuit, it would not be possible to change the capacitor or resistor.

One application of a filter involves its use in a circuit known as a phase-locked loop. One overall function of a phase-locked loop is to receive an input signal and then output a frequency signal in dependence on any difference in phase between the input signal and the output signal. The phase-locked loop includes a phase detector which receives the input signal and the output signal to generate a phase error signal, the filter that filters the phase error signal and a voltage controlled oscillator that responds to the filtered phase error signal to output and feed back to the phase detector the output frequency signal. The filter will have a frequency response that is predetermined based on the data rate of the input signal that the phase-locked loop is processing.

One problem with a prior phase-locked loop, particularly one being implemented as an integrated circuit, is its inflexibility in being able to be used with different input signals having different data rates. This stems in part from the use of the filter and its fixed frequency response based on the set capacitor and resistor values of the filter.

As the above examples' illustrate, the value of resistance of a resistor may have important effects on the operation of circuits employing the resistor.

According to one embodiment of the present invention, there is provided apparatus having a controllable effective resistance, the apparatus comprising a transistor means for providing a current at an input node responsive to an input voltage at the input node, coupled to a settable current source. The settable current source operates to control the effective value of the controllable effective resistance.

According to another embodiment, there is provided filtering apparatus comprising a filter which has a controllable filter response and includes a charge storage device, and an apparatus having a controllable effective resistance coupled to the charge storage device. The filter apparatus also includes a means for controlling the controllable effective resistance to set the filter response.

According to yet another embodiment there is provided apparatus providing an output frequency signal in response to an input signal having a nominal data rate, the apparatus comprising phase-locked loop means for generating the output signal in response to a difference in phase between the output frequency signal and the input signal, including a controllable filter having a frequency response in dependence on the nominal data rate of the input signal, and means for controlling the controllable filter to have the frequency response.

By way of example only, in the accompanying drawings:- <ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic diagram of an apparatus embodying the present invention;</li><li>Fig. 2 is a block diagram of a controlled filter embodying the present invention;</li><li>Fig. 3 is a block diagram of a phase-locked loop apparatus embodying the present invention; and</li><li>Fig. 4 is a more detailed schematic diagram of a preferred embodiment of an apparatus having a controllable effective resistance.</li></ul>

With reference to the Figures, a detailed description of the embodiments is provided. First, a basic controllable effective resistance embodying the invention is disclosed. Then, a filter apparatus employing the controllable effective resistance and a phase-locked loop apparatus employing the controllable effective resistance are described. Finally, a preferred embodiment of the controllable effective resistance for implementation as an integrated circuit component is described.

I. Overview

_Fig. 1 shows a schematic diagram of an apparatus 10 having a controllable effective resistance.

The apparatus 10 includes a transistor means 11 for providing a current I_OUT that is proportional to an input voltage V_IN supplied at the input node 12 for small input voltages. A settable current source 13 is coupled to the transistor means 11. The settable current source 13 supplies variable current in response to a reference signal generated by a control means 19. By varying the settable current source 13, the magnitude of the current I_OUT is varied, thereby providing a controllable effective voltage to current conversion. By coupling the current I_OUT on line 18 to the input node 12, the apparatus 10 behaves essentially as a resistor with controllable resistance.

The transistor means 11 in the embodiment of Fig. 1 includes a differential pair of transistors 14, 15. The base of transistor 14 is coupled to the input node 12 across resistor Rl. Also the base of transistor 14 is coupled to a reference voltage V_REF across resistor R2.

The base of transistor 15 is coupled to the reference voltage V_REF across resistor R3.

In the embodiment shown, the settable current source 13' includes a first variable current source 16 and a second variable current source 17. In the embodiment shown, the first variable current source 16 supplies a current 2I_C which is twice the current I_e of the second variable current source 17. The second variable current source 17 supplies a balanc_- ing current to the apparatus 10 so that it can behave as a classical resistor as discussed below, and can be varied or eliminated in some applications.

The emitters of the differential pair of transistors 14, 15 are coupled to the first variable current source 16. The collector of one transistor 14 of the differential pair of transistors 14, 15 is coupled to the second variable current source 17.

In operation, the controllable effective resistance 10 shown in Fig. 1 supplies a current on line 18 which is proportional to the voltage difference between the input voltage V_IN and the reference voltage V_REF for small values of V_IN.

If there is zero difference in voltage between V_IN and V_REF, the output current I_OUT on line 18 is zero. This is so because the second variable current source 17 supplies current I_e through the collector of the one transistor 14 which satisfies the balance condition of the differential pair 14, 15. So I_OUT must be zero. The other transistor 15 will conduct an equal current I_e through its collector to satisfy the second variable current source 16.

As the input voltage V_IN increases, the current conducted through the collector of the one transistor 14 likewise increases at the expense of the other transistor 15, since the sum of the currents through the differential pair of transistors 14, 15 is forced to be 2I_C by the first variable current source 16. An output current I_OUT is equal to the difference of the collector current of transistor 14 and the current I_e from the second variable current source 17.

For the opposite relationship, as the input voltage V_IN falls below the reference voltage V_REF, the current conducted by the other transistor 15 will be greater than that conducted by the one transistor 14. Thus an output current I_OUT will flow out of line 18 which subtracts current from the collector of the one transistor 14 in order to maintain the sum of the currents through the differential pair of transistors 14, 15 at the value 2I_c of the first variable current source 16.

The effective resistance of the apparatus 10 having a controllable effective resistance of Fig. 1 can be calculated in simplified form. By definition, the effective resistance R_EFF is determined by the following equation (1):<maths id="math0001" num=""><img file="EP0186401A2_D0001.tif" /></maths>

The output current IOUT on line 18 is derived by the following equation (2):<maths id="math0002" num=""><img file="EP0186401A2_D0002.tif" /></maths>where F is equal to a constant for small values of (V_IN - V_REF) determined by the characteristics of the differential pair of transistors 14, 15.

By combining equation (1) with equation (2), it is determined that the effective resistance R_EFF is defined by the following equation (3):<maths id="math0003" num=""><img file="EP0186401A2_D0003.tif" /></maths>

Accordingly, the apparatus 10 has a controllable effective resistance that is controllable in response to the settable current source 13. By varying the current supplied by the settable current source 13, the effective resistance R_EFF of the apparatus 10 is likewise controlled.

An additional feature of the present apparatus can be seen at line 18A in Fig. 1. By taking the output current ^I_OUT from the collector of the other transistor 15 of the differential pair of transistors, the apparatus 10 behaves as. a negative controllable effective resistance. By connecting the line 18A to the input node 12, a negative resistance is obtained which is controllable by selecting the value of the current source means 13. The output currents on line 18 and on line 18A normally will not be generated in a single apparatus 10.

The settable current source 13 receives a reference signal on a control line 21 from a control means 19 for controlling the settable current source 13. The control means 19 may include current mirrors or other current scaling apparatus which provide a reference signal on line 21 for controlling the settable current source 13. The operation of the control means 19 to generate the reference signal on control line 21 can be varied in accordance with the particular application of the apparatus 10. For instance, for apparatus 10 having a controllable effective resistance employed in a RC filter, the control means 19 may supply a reference signal on control line 21 which is proportional to the desired breakpoint frequency f_B of the RC filter. Other relationships between the control means 19 and the apparatus 10 will be determined by the particular application of the apparatus 10 having a controllable effective resistance.

II. A Filter Employing the Controllable Effective Resistance

Fig. 2 is a block diagram of a filter apparatus 20 employing the apparatus 10 having a controllable effective resistance. The filter apparatus 20 includes a charge storage means for storing charge, such as capacitor C, coupled across the input node 12 of the apparatus 10 to a ground terminal. As discussed above, the apparatus 10 provides an output current I_OUT which is proportional to the voltage (_VIN- V_REF). The value of the controllable effective resistance is determined by the reference signal on line 21 which controls the setting of the settable current source 13. The filter apparatus 20 provides a filter having a settable frequency response because of the manner in which the controllable effective resistance of apparatus 10 is employed. The following simplified equations prove the relationship.

The breakpoint frequency f of the filter apparatus 20 is given by the following equation (4):<maths id="math0004" num=""><img file="EP0186401A2_D0004.tif" /></maths>

Thus by changing R_EFF, the breakpoint frequency of the filter apparatus is changed. If you substitute the equation (3) for R_EFF in equation (4), the following equation (5) is obtained:<maths id="math0005" num=""><img file="EP0186401A2_D0005.tif" /></maths>

Thus by varying the current 2I_C, I_e supplied by the settable current source 13, the breakpoint frequency of f_a the filter apparatus 20 of Fig. 2 can be changed.

The control signal on line 21 is of a character for controlling the settable current source 13 of the controllable effective resistance 10. As mentioned above, the settable current source 13 can be manufactured in a variety of embodiments including digital or analog versions which are responsive to a control signal. For the RC filter apparatus 20, the reference signal is generated in response to the desired breakpoint frequency f_B, and the settable current source 13 is scaled to provide a current which results in the desired effective resistance R_EFF for the given capacitance of the charge storage device C.

The filter apparatus 20 in the preferred embodiment is formed on a semiconductor wafer in an integrated circuit. Because of the inability to vary charge storage devices in integrated circuits easily, the filter apparatus 20 employing the apparatus 10 having a controllable effective resistance has particular utility.

III. Phase Lock Loop Apparatus

Fig. 3 illustrates a block diagram of a phase-locked loop apparatus 30 employing the apparatus 10 having a controllable effective resistance. The phase-locked loop apparatus 30 includes a phase detector 31, a filter apparatus 20, and a voltage controlled oscillator 32. The phase detector 31 compares a difference in phase between an input signal V_IN having a given frequency, or nominal data rate, with the output frequency signal V_OUT from the voltage control oscillator 32 having a settable frequency. The output of the phase detector 31 is a signal proportional to the difference in phase between V_IN and V_OUT. The output of the phase detector 31 is filtered by the controllable filter apparatus 20 and supplied as input to the voltage controlled oscillator 32. The frequency of the voltage Controlled oscillator is shifted in response to the output from the controllable filter apparatus 20 in order to track the input signal ^V_IN'

The voltage controlled oscillator 32 operates at a center frequency in response to a reference signal I_REF on line 33. The filter apparatus 20 and the phase detector 31 are set to operate with a breakpoint frequency f_B that is proportional to the frequency of operation of the voltage controlled oscillator 32. Accordingly, the reference signal I_REF on line 33, the reference signal I_REF on line 21 for controlling the effective resistance of the controllable effective resistance 20, and the reference signal I_REF on line 50 for centering the phase detector 31 are generated in conjunction in control means 40 for generating the reference signal in dependence on the nominal data rate of V_IN to provide a phase-locked loop apparatus 30 which operates over a wide range of center frequencies.

IV. A Preferred Embodiment of the Controllable Effective Resistance

Fig. 4 illustrates a circuit diagram of a preferred embodiment for the present invention for implementation on an integrated circuit chip. The reference numbers from Fig. 1 are used in Fig. 4 where appropriate for consistency.

As in Fig. 1, the transistor means 11 is comprised of a differential pair of transistors 14, 15. Transistor Q7 corresponds to the one transistor 14 of Fig. 1 and transistor Q8 corresponds to the other transistor 15 of Fig. 1.

The settable current source 13 is comprised of a first variable current source 16 and a second variable current source 17. The first variable current source 16 supplies 2I_C current at the emitters of the differential pair of transistors Q7, Q8. The first variable current source 16 and second variable current source 17 are set to a desired current level by control 19. The control 19 is implemented with settable current mirrors or other control circuitry as suits the user.

The second variable current source 17 supplier current through the diode means Q10 to the collector of the one transistor Q7. The output current I_OUT is supplied over line 18 from the collector of transistor Q7 to the input node 12.

The apparatus 10 having a controllable effective resistance of the embodiment shown in Fig. 4 provides an effective resistance _REFF across a differential input voltage (^V_LEFT^- V_RIGHT).

A means 35 for providing overall temperature compensation is provided. Also, a means 36 for maintaining appropriate bias voltages at the bases of the differential pair of transistors Q7, Q8 is provided.

The means 35 for providing temperature compensation includes a first current path 37 and a second current path 38. The first current path 37 includes a current source 39 supplying current to the emitter of a transistor Q1. The collector of the transistor Q1 is connected to a node 40 which is connected to the base of the transistor Q8 of the transistor means 11. Another transistor Q3 is connected at its emitter to the node 40. The collector of the transistor Q3 is connected through resistor R2 to the supply voltage ^VCC^.

The second current path 38 likewise includes a current source 41 supplying current to the emitter of a transistor Q2. The collector of the transistor Q2 is connected to a' node 42 which is coupled to the base of the transistor Q7 of the transistor means 11. Another transistor Q4 is coupled at its emitter to the node 42. The collector of transistor _Q4 is coupled across resistor R3 to the reference voltage V_CC'

The emitters of the transistor Ql of the first current path 37 and Q2 of the second current path 38 are coupled together across resistor Rl.

The base of the transistor Q1 of the first current path 37 is coupled to the voltage V_LEFT at input node 12 across the emitter follower transistor Q12 over line 46 across diode means Q10. The base of transistor Q2 of the second currant path 38 is coupled to the voltage V_RIGHT across the emitter follower Q13 and diode means Q14 which matches diode means Q10.

The voltage node 42 follows the voltage VLEFT at node 12. Likewise, the voltage at node 40 follows the voltage V_RIGHT. This occurs because the differential voltage V_LEFT - VRIGHT will cause a current to flow across the resistor R1.

For example, if V_LEFT is at a higher voltage than V_RIGHT, then the current through transistor Q2 will be less than the current through transistor Q1. However, because the current sources 39, 41 draw equal current, a current must flow through R1 to the emitter of Q2 to balance the current. Because there is less current flowing through the collector of Q2 than flowing through the collector of Q1, the voltage drop from the base to emitter of transistor Q4 will be less than the voltage drop from the base to emitter of transistor Q3. Thus, the voltage at node 42 will be higher than the voltage at node 40. The same explanation applies for the opposite relationship.

Temperature compensation is provided by manufacturing transistor Q3 and Q4 to match transistors Q7 and Q8 of the transistor means 11 as closely as possible. Because a change in temperature will effect transistors Q3 and Q4 the same way as it will effect transistors Q7 and Q8, the voltages at nodes 40 and 42 applied to the bases of the differential pair of transistors Q7, Q8 will track the changes in the impedance of the base to emitter junction of the transistors Q7 and Q8 as the temperature changes.

In some environments, particularly for most integrated circuits, the range of bias voltages available is limited due to low supply voltages. The present apparatus overcomes that limitation by including the means 36 for providing bias voltages.

The means 36 for providing bias voltages for the transistor means 11 includes transistors Q5, Q6, Q9, Q10 and Qll. In order to maintain the transistor means 11 in its desired operating range without saturating the transistors in the ; means 35 for providing temperature compensation, the means 36 provides bias voltages to the transistor means 11 within the constraints of the integrated circuit environment.

The means 36 comprises a voltage path as follows. The voltage at node 45 at the collector of the one transistor Q7 and the voltage at node 44 at the collector of the other transistor Q8 are held roughly equivalent through the diode connected transistor Q10 and down through the emitter of transistor Q9, which is connected to the node 44. The node at the emitter of the transistor Q6 is 1V_BE below the voltage at the node 44. The voltage drop at the emitter of the transistor Q6 is made to track the voltage drops through the emitters of the transistors Q10 and Q9 by the controllable current source 90, which is controlled to deliver a current which is equal to about I_c. The voltage at the emitter of the transistor Q6 is supplied to the base of the pnp transistor Q5. The voltage at the emitter of the transistor Q5 is thus approximately equal to the voltage at the node 44. The diode connected transistor Qll maintains the voltage at the node 43 approximately. 1VBE above the voltage at the node 44 and at the node 45. This circuit path assures that the voltage at the nodes 40 and 42 at the bases of the one transistor Q7 and the other transistor Q8 of the differential pair of transistors are maintained within an acceptable range of the voltages of the collectors of the one transistor Q7 and the other transistor Q8.

The circuit shown in Fig. 4 illustrates a further feature of the preferred embodiment in that a means 46 for minimizing base current linkage through the apparatus 10 having a controllable effective resistance is provided. The means 46 is embodied by the emitter follower transistors Q12 and Q13 which provide additional gain at the input of the means 35 for providing temperature compensation. At very high values of effective resistance R_EFF for the controllable effective resistance 10, the current flowing through the base of the temperature compensation circuit transistors Ql and _Q2 becomes significant. Accordingly, the emitter follower transistors Q12 and Q13 are provided to minimize the base current linkage while providing sufficient drive for the apparatus 10 having a controllable effective resistance.

The variable current sources, including the first variable current source 16, the second variable current source 17 and the variable current source 90 in the circuit shown in Fig. 4 are preferably implemented by current mirrors that are adapted to provide a current that is a multiple of the reference signal I_REF. The current mirrors are scaled in the control 19 to provide the correct multiples of the reference current I_REp. When implemented in a phase-locked loop apparatus, the reference signal, IREF, which is used to center the VCO can be scaled in the control 19 to provide proper currents for the controllable effective resistance 10. As the center frequency of the VCO changes, the reference signal I_REF will change accordingly. As I_REF changes, the effective value of the controllable effective resistance 10 will also change so that the bandwidth of the filter 20 in the phase-locked loop apparatus 30 will automatically adjust to the center frequency of the VCO 32.

V. Conclusion

There is provided an apparatus 10 having a controllable effective resistance with a wide range of applications. The apparatus 10 can be used in the filter apparatus 20 : to provide a filter having a controllable frequency response over a wide range. Further there is provided a phase-locked loop apparatus which is operable over a wide range of frequency.

The controllable effective resistance is controllable by varying a settable current source 13. Thus, the controllable effective resistance can be controlled to dynamically change the effective resistance R_EFF of the apparatus in response to electronic signals.

The foregoing description has been presented for purposes of illustration . It is not intended to be exhaustive or to limit the invention to the precise form disclosed. And, obviously, many'modifications and variations are possible in light of the above teaching. The filter and integrated circuit embodiments of the controllable effective resistance were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.