Impedance trim network for use in integrated circuit applications
CROSS REFERENCE TO A RELATED APPLICATION 
     The subject matter of the subject application is related to the subject matter of an application entitled &#34;A TRIM NETWORK FOR MONOLITHIC CIRCUITS AND METHOD OF TRIMMING&#34;, Ser. No. 719,549 by John J. Price, assigned to Motorola, Inc. 
     BACKGROUND OF THE INVENTION 
     1. Field of Art 
     This invention relates generally to adjustable impedance networks and more particularly, to an impedance trim circuit suitable for use in monolithic circuit applications for adjusting operating characteristic error signals of the circuit. 
     2. Description of the Prior Art 
     The need for a monolithic adjustable trim circuit for use in several integrated circuit applications has long been felt. Many integrated circuit structures have errors associated therewith caused by processing tolerance problems. For instance, most all monolithic differential amplifiers have input voltage offset errors caused primarily by mismatch in the differential transistors which comprise the amplifier. In the past, to eliminate these offset errors, discrete trimming potentiometers have been employed. Several disadvantages are associated with such discrete potentiometers. First, the physical size of such potentiometers may limit their use in some application. For each differential amplifier that is used in a system a respective potentiometer would be required that must be manually adjusted to trim the amplifier. As each monolithic differential amplifier employed in a system may require a trimming potentiometer the cost of such systems linearly increases accordingly. Thus, there is a need for an integrated adjustable impedance network which may be utilized in monolithic differential amplifier circuits to eliminate the voltage offset errors associated therewith. Additionally, there is a need for a relatively simple adjustable impedance network structure which may be employed in several integrated circuit applications. 
     At least two prior art teachings have recognized the above needs. For instance, in the application by John J. Price, supra, there is disclosed a trim circuit and method of trimming the same for eliminating voltage errors otherwise associated with a monolithic digital to analog circuit. U.S. Pat. No. 4,016,483, Rudin, teaches a trim scheme including impedance elements which requires the use of electrical energy for trimming the impedance value of the disclosed impedance network. 
     From above it is quite apparent that there is a further need for a simple and inexpensive adjustable integrated impedance network that is suitable for use in several different integrated circuit applications. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an adjustable impedance network suitable to be utilized in several integrated circuit applications. 
     It is another object of the invention to provide an adjustable impedance network for use in a monolithic differential amplifier circuit to eliminate input voltage offset errors. 
     A further object of the invention is to provide a trimable impedance network suitable to be utilized in a variety of integrated circuit applications which is capable of being automatically trimmed during the fabrication processes for such integrated circuits. 
     Still another object of the invention is to provide a monolithic current reciprocator circuit including an adjustable impedance network. 
     In accordance with the foregoing and other objects there is provided an adjustable impedance network suitable for use in integrated circuit applications. The impedance network comprises: a plurality of impedance elements serially connected between first and second terminals, a plurality of alterable metal links each electrically shorting a respective one of the plurality of impedance elements, an additional pair of serially connected links electrically shorting a selected number of the impedance elements to a common terminal of the network, the impedance value between the common terminal and either one of the first or second terminals being adjusted by selectively open circuiting predetermined ones of the plurality of metal links and the appropriate one of the pair of additional links. 
     In one feature of the invention the aforedescribed impedance network is utilized between the collector outputs of a differential amplifier and a source of operating potential for eliminating input voltage offset errors. 
     In another feature of the invention the aforedescribed impedance network is utilized in a current reciprocator circuit for causing the magnitude of the output current to remain constant although changing in polarity depending upon the polarity of an applied input signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in schematic form a differential amplifier circuit including the laser trim network of the present invention; and 
     FIG. 2 illustrates in schematic form a current reciprocator circuit including the trim circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Referring to FIG. 1 there is shown differential amplifier 10 which is suitable to be manufactured in monolithic integrated circuit form. As is understood, differential amplifier 10 may be utilized as an input stage of an operational amplifier or a comparator circuit. Generally, differential amplifier 10 includes transistors 12 and 14 differentially connected with respective emitter electrodes commonly connected to current source 16. Current source 16 which is returned to a reference potential at terminal 20 produces a current that is evenly divided through the transistors with amplifier 10 being in a balanced operating state. Normally, load resistors 22 and 24 would couple respective collector electrodes of transistors 12 and 14 to a source of operating potential V CC . Hence, as is known, in response to a differential input voltage ±(V 1  -V 2 ) being applied across the base electrodes of transistors 12 and 14 a differential output voltage is derived at the collectors of transistors 12 and 14, between output terminals 26 and 28. 
     Ideally collector load resistors 22 and 24 as well as transistors 12 and 14 are perfectly matched such that the balanced condition may be maintained at the quiescence. Thus, input offset error signals are cancelled. However because of processing tolerances in the fabrication of the resistors and especially due to the mismatch of transistors 12 and 14 an input offset voltage is generated. Input offset voltage is defined as the input voltage required to provide a zero output voltage between terminals 26 and 28. The input offset voltage can be eliminated by adjusting the value of load resistors 22 or 24 depending upon the polarity of the error. 
     Trim network 30 of the invention may be utilized in differential amplifier 10 to trim the value of the appropriate load resistor, either resistor 22 or 24. Trim network 30 is shown as being connected between internal nodes 32 and 34 of differential amplifier 10 and power supply terminal 36. Trim network 30 comprises a plurality of binary weighted serially connected resistors 38, 40, 42 and 44 which are electrically shorted by thermally machinable metallic links L 1  and L 2 . Links L 1  and L 2  are commonly connected to power supply terminal 36. In addition each of the forementioned resistors are electrically shorted by parallel links L 3  -L 6  respectively. Fine trim resistors 46 and 48 are provided which are connected respectively between resistors 38 and 44 to nodes 32 and 34. Shorting links L 7  and L 8  are connected in parallel across resistors 46 and 48, respectively. In the preferred embodiment resistors 38-44 are binarily scaled having values R, 2R, 4R, 8R respectively. Resistors 46 and 48 typically have a value R. Resistors 38-48 may be of any suitable form, e.g., thin film resistors known in the art and are formed in a known manner on the insulating substrate comprising monolithic amplifier 10. 
     With trim resistors 38-48 being electrically shorted by respective links L 1  -L 8 , differential amplifier 10, comprising an active circuit portion that includes transistors 12 and 14, is in a balanced configuration. Using the same technique of measuring and test as disclosed in Assignee&#39;s pending patent application, referenced above, and illustrated in the article entitled &#34;A passive Laser Technique to Improve the Linearity of a Ten Bit D/A Converter&#34;, published by the inventor in the IEEE Journal of Solid State Circuits, Vol. SC-11, No. 6, (Dec., 1976), a computer can be programmed with a look-up table to determine which of the links L 3  -L 6  should be cut to reduce the offset voltage error by increasing the resistance of either resistor 22 or 24. After determination of which links are to be cut, the computer can instruct a laser apparatus described in the above application and article, or any other high energy source, to open circuit the links by thermal machining. It should be understood that test and trim is done at wafer level in the preferred embodiment. In sequence, link L 1  would first be cut to add resistance to resistor 22 or link L 2  to add resistance to resistor 24. Next links L 3  -L 6  would be cut as determined. After this initial trim step the offset error may again be measured and checked for required tolerance. If the offset error is still not in tolerance an additional incremental trim can be made in either polarity by cutting links L 7  or L 8  accordingly. 
     The advantage of trim network 30 is that an &#34;on-chip&#34; network is provided for trimming of offset voltage errors. The testing and trimming can be accomplished automatically and quickly. The unique and novel trim network of the invention requires very few components to reduce or eliminate offset errors which are common to monolithic differential amplifiers. Using trim network 30 input offset voltages can typically be trimmed to less than 0.3 millivolts. 
     Referring now to FIG. 2 there is illustrated a current reciprocator circuit 50 comprising an active circuit portion having first and second internal nodes 52 and 54 and trim network 30 connected therebetween. Current reciprocator circuit 50 either sinks or sources the current I at output terminal 56 depending upon the polarity of the differential input voltage ±(V 1  -V 2 ) applied at input terminals 58 and 60. Ideally it is desired that the magnitude of the current I remains constant even though its polarity is reversed. However, due again to processing tolerances, the value of I may vary between polarity states which is undesirous. Trim network 30 may be utilized to reduce or substantially eliminate errors in the value of output current I. 
     The active circuit portion of current reciprocator 50 includes differentially connected input transistors 62 and 64 each having respective base electrodes coupled to input terminals 58 and 60 of reciprocator circuit 50. The emitter electrodes of transistors 62 and 64 are connected to current source 66 which in turn is coupled to a reference potential at terminal 68. The collector electrodes of transistors 62 and 64 are coupled to power supply terminal 70 through a current mirror circuit comprising transistors 72, 74, 76 and resistors 78 and 80. Transistors 82 and 84 are provided to reduce error in output current I which would be caused by transient conditions if these transistors were otherwise not in the circuit. Transistors 82 and 84 are differentially coupled to resistor 86 through trim network 30. The base electrodes of transistors 82 and 84 are coupled to the base electrodes of transistors 62 and 64 respectively with the collector electrodes thereof being cross coupled to the collectors of transistors 62 and 64. Current through transistors 82 and 84 is supplied by current source 66, through resistor 86. 
     Without transistors 82, 84, trim network 30 and resistor 86 current reciprocator 50 suffers from several deleterious operating conditions as will be explained. Assuming that the aforementioned components are not part of reciprocator 50 the circuit would operate in the following manner. If the voltage V 1  is greater than V 2  transistor 62 will be conducting and transistor 64 will be in a non-conducting state. The value of the collector current flowing through transistor 72, produced by transistor 62 being rendered conductive, is mirrored by transistors 74 and 76, as understood, to produce a collector current through transistor 76 substantially equal in value to that of transistor 72. Hence, current is supplied or sourced out of terminal 56 which has a value substantially equal to I 1 . In the next mode or state, with the voltage V 2  being greater than V 1 , transistor 62 is caused to be non-conductive and transistor 64 conductive. In this mode the current mirror circuit is rendered non-conductive and transistor 64 sinks current from a load through terminal 56. Ideally, the current that is supplied by the load is equal to the current I 1 . Thus, depending on the values of the input voltage, the reciprocator circuit would either source or sink current to or from a load which is coupled to terminal 56. 
     The above circuit suffers from two common problems: (1) poor matching of the magnitude of the two polarities of the output current, and (2) slow settling time in the output current after a change in the mode of operation of the circuit. Both of these problems arise because of the current mirror circuit. When V 2  becomes greater than V 1 , transistors 72, 74 and 76 will continue to conduct until the base of transistor 76 rises to the voltage V CC . Until these transistors are shut off there is an error in the output current. Thus, there is required some settling time until the value of current sourced into the reciprocator circuit equals the current supplied by the circuit after transition between operating modes. Mismatch between transistors 72 and 74 causes error between current into and out of the reciprocator circuit when the current mirror is active. 
     The addition of trim network 30 to reciprocator circuit 50 eliminates current mismatch problems, mentioned previously, at lower currents where matching is most important in many applications and which is typically the hardest to achieve. The addition of transistors 82 and 84 cause current to always be flowing in the current mirror circuit even during transistion so that settling time is reduced. 
     In the preferred embodiment resistor 86 is selected so that the current conducted through either transistor 82 or 84 (depending on the value of input voltage V 1  and V 2 ) is a predetermined fraction of I 1 , e.g., one-tenth of the value of I 1 . When transistors 62 and 82 are conducting, the current I flowing out of terminal 56 becomes less than I 1  by twice the value of the current flowing through transistor 82 which is stolen first from transistor 62 and then from transistor 76. When transistors 64 and 84 conduct the current flowing into terminal 56 is less than I 1  by twice the value of the current conducted through transistor 84. Therefore the magnitude of the output current I remains a constant value but is of opposite polarity depending upon the mode of operation of reciprocator circuit 50. 
     By cutting appropriate links, L 1  -L 8 , using the same method discussed supra, resistance can be added to resistor 86 in the current path of either transistor 82 or 84. If, for example, more resistance is added to the path through transistor 82 the current conducted therethrough will be less than before trim. In response, transistor 62 will conduct more current and current I will increase. This will compensate for mismatches in the current mirror circuit which caused transistor 76 to conduct less current than it should. Conversely, when resistance is added in the path of transistor 84 the magnitude of the current flowing into reciprocator circuit 50 is increased. Thus, trim network 30 provides a method whereby mismatches between transistors 72 and 74 can be compensated therefor. 
     What has been described above is a unique and novel laser trim network suitable to be utilized in monolithic integrated circuit applications to correct circuit operating errors caused by processing tolerances. In accordance with the invention there has been illustrated a differential amplifier and a novel reciprocator circuit employing the trim network.