US3582689A

US3582689A - Current conveyor with virtual input reference potential

Info

Publication number: US3582689A
Application number: US769798A
Authority: US
Inventors: Kenneth C Smith; Adel S Sedra
Original assignee: Canadian Patents and Development Ltd
Current assignee: TORONTO INNOVATIONS FOUNDATION UNIVERSITY OF TORONTO 203 COLLEGE ST TORONTO ONTARIO CANADA A Co OF UNIVERSITY OF TORONTO, University of
Priority date: 1968-10-23
Filing date: 1968-10-23
Publication date: 1971-06-01
Anticipated expiration: 1988-06-01

Abstract

A basic circuit comprising a first transistor having an input terminal for introduction of a current, an output connected to a diode-connected second transistor, and a control electrode connected to the control electrode of a diode-connected third transistor whose output is adapted for connection to a reference potential source and whose input is connected to the output of a fourth transistor, with the control electrodes of the second and fourth transistors interconnected and the output of the second and the input of the fourth transistors connected via equal resistors to a power supply source such that the current flowing in the first and second transistors is equally reproduced in the third and fourth transistors and a fifth transistor connected to extract a current equal to or related to these currents and to feed it to an output terminal.

Description

United States Patent CURRENT CONVEYOR WITH VIRTUAL INPUT REFERENCE POTENTIAL 2 Claims, 7 Drawing Figs.

U.S. CI 307/297, 307/200, 325/38 1 330/24 Int. Cl H03k 1/12, H03f 3/04 Field of Search 307/203,

Primary Examiner-Stanley D. Miller, Jr. Attorney-James R. Hughes ABSTRACT: A basic circuit comprising a first transistor having an input terminal for introduction of a current, an output connected to a diode-connected second transistor, and a control electrode connected to the control electrode of a diodeconnected third transistor whose output is adapted for connection to a reference potential source and whose input is connected to the output of a fourth transistor, with the control electrodes of the second and fourth transistors interconnected and the output of the second and the input of the fourth transistors connected via equal resistors to a power supply source such that the current flowing in the first and second transistors is equally reproduced in the third and fourth transistors and a fifth transistor connected to extract a current equal to or related to these currents and to feed it to an output tenninal.

CURRENT CONVEYOR WITH VIRTUAL INPUT REFERENCE POTENTIAL This invention relates to a current conveyor circuit with virtual input reference potential and more particularly to a method and apparatus for accepting a current at one terminal and conveying it or a current related to it in a predetermined manner to an output terminal.

The invention described herein introduces a new electrical circuit principle or concept which will be referred to as Current Conveying" and the circuit for doing this is called a Current Conveyor." As the name implies the circuit accepts a current at one .tenninal and conveys it to another. A special input terminal setsa reference potential to which all other input terminals appear to be connected irrespective of the value of the input current. This gives the input the property of virtual zero input impedance and permits the forcing of any desirable input current which the circuit in turn conveys and reproduces at a high output impedance in the manner of an ideal current source. An additional property of the reference terminal is that its current is determineddirectly by the current through the input tenninal irrespective of the value of the reference voltage. In this way, the reference tenninal exhibits a virtual open circuit incremental characteristic. Morever the current in the reference terminal is in the same direction as the current in the input terminal and does not correspond to the polarity of the reference terminal voltage.

A more precise definition of circuit terminal properties can be obtained in terms of hybrid-h two-port parametric notation. The matrix relating the input terminal'to the output terminal with the reference terminal held fixed is 0 H X! [1 The matrix relating the input terminal to the. reference terminal with the output terminal held constant in potential is The circuit described, because of its unique properties including that of an impedance transformer for currents and which therefore has some relation to the now conventional cathode and emitterfollower configurations, is foreseen to have many and varied interesting applications some of which will be described below.

It is an object of the invention to provide a circuit method and apparatus for conveying a current accepted at one input terminal at a low including zero impedance level to an output terminal at a different impedance level.

It is another object of the invention to provide a current source thatapproaches the idealized state.

It is another object of the invention to providea basic circuit building block that can be readily adapted, complemented, added to, -or multiplied to obtaindevices that can be used as, for example, a wide range current source, a digital to analog converter, a wideband current-measuring probe, .a negative impedance converter, etc.

It is another-object of the invention to provide a-circui t'that is reasonably well suited to integrated circuit produotion and thathas very good temperature stability characteristics.

These and otherobjects of the invention are achieved by a basic circuit comprising a first transistor having an input terminal for introduction of a current, and'output-connected toa diode-connected second transistor, and a control electrode connected to thecontrol electrode of a diode-connectedthird transistor whose output is adapted for connection *to a reference potential source source .and whose input is connected to the output of a fourth transistor, said second and fourth transistors having their control electrodes interconnected and the output of the second and the input of the fourthconnected via 'equal resistors to a powersupp'ly-source such that thecurrent flowing in the first and'secondtransistors is substantially equally reproduced in the third and fourth transistorsand means for extracting a current equal to or'related to these currents and feeding it to an output terminal. In actual practice the first and third transistors would be a matched pair, the second and fourth would be a matched pair, and the two resistors would also be matched. The current extractor means would preferably be a fifth transistor matched to the second and fourth having its control electrode connected to the control electrodes of the second and fourth transistors and its input connected via a matched resistor to the power supply such that its output will engender an output current that is a replica of the currents flowing through the secohd and fourth transistors.

In drawings which illustrate embodiments of the invention,

FIG. 1 is a block representation of the current conveyor,

FIG. 2 is a circuit implementation of the basic current conveyor,

FIG. 3 is a more complex form of the circuit of FIG. 2 which provides additional output terminals,

FIG. 4 shows in block form an adaptation and application of the device as a wide range current source,

FIG. 5 shows an adaptation of the device as a digital to analog converter,

FIG. 6 is a circuit representation of the device of FIG. 5, and

FIG. 7 shows an adaptation of the device as a wideband level shifter.

Referring to FIG. 1 a block representation of the current conveyor is shown generally as 10. Terminals x and y are the input terminals and z is the output current source terminal. In its simplest form, the circuit requires only a single supply voltage (-V For some applications the y terminal is connected to a reference voltage V,. An equal potential V, (except for a negligible offset) will appear on the input terminal x. Having virtually established the potential of the input terminal x, it becomes easy to force the required amount of current I through 1:. An almost equal amount of current I will be supplied through the output terminal 1 at a high impedance level providing an almost idealized current source. It should be noted that the virtual potential appearing on terminal x will not be affected by the value of the current I. Hence, the device shows, in this sense, a zero input impedance and represents a virtual short circuit input characteristic. Also, as will beseen later, the circuit operation is independent, within limits, of the value of the supply voltage (V FIG. 2 is the circuit implementation to provide the characteristics of the block of FIG. 1. A first transistor'Ql has its input connected to the .1: terminal and its output connected to a diode-connected second transistor Q2'whose output is connected via a resistor R1 toa power supply source (V,, The control electrode or base of O1 is interconnected to the base of a diode-connected third transistor Q3 whose output is connected to terminal y. The input of O3 is connected to the output of a fourth transistor Q4 whose input is connected via resistor R2 to the power supply (V and whose control electrode is connected to the control electrode of Q2. It is preferable that Q1-and-Q3 be a matched pair, Q2 and Q4 a matched pair, and -R1 and R2 be matched resistors. It would be possible to replace Q2 and Q3 with diodes but this would make matching a muchmore 'difficult problem and it is standard practice to use diode-connected transistors as diodes especially in integrated circuitapplications. A fifth transistor Q5 is used to extract a cur-rent equal to that in Q2 and Q4 and has its input connected via a third resistor R3, preferably matched to the others, to the power supply (-V,,,;') and its control electrode connected to the control electrodes of Q2 and Q4; The output of this transistor '05 actually provides an output current (-I) to terminal 1 that is equal to or closely related to input current I.

In operation, if a current 'I is forced through the emitter of transistor Q1, this current is sent to the diode-connected transistor Q2 and hence establishes the potential of its base. Assuming that Q2 and Q4 are a matched pair of high current gain'(h -or [3) transistors and that -R1=R2, then=Q4 will carry an equal current I (irrespective of the value of the negative supply voltage V Q4 extracts this current I from the diode-connected transistor Q3 which, if matched to Q1 will have a base-emitter drop equal to that of 01. In this way, the emitter of 01 will be held at a virtual potential equal to V, irrespective of the value ofthe current I. As the potential of terminal x may thus be established at will, the current I can be easily forced through Q1 from standard sources. Q provides the necessary output current at terminal 2 which is an inflowing current I similar to that flowing into terminals x and y. A

current conveyor complementary to the one shown in FIG. 2 with all current directions and voltage polarities reversed is also feasible.

FIG. 3 shows a circuit similar to that of FIG. 2 but has additions to provide for a complementary output current I on terminal z and a current l at a higher voltage level on terminal In this circuit transistor Q7 extracts a current I and supplies to the dual" arrangement of transistors Q8 and Q9 and resistors R5 and R6 which are connected to a positive supply voltage. Again the circuit operation is independent of supply voltage and an output current at a higher voltage level and reversed to that of FIG. 2 is obtained at terminal An output positive (complementary) current I is obtained at terminal 12 via transistor Q6 connected to input terminal x Terminal z, still provides the same output current as the circuit of FIG. 2. Following the same procedure, more output terminals can be provided. It may be noted that by using the complementary form of each transistor and reversed voltage polarities, a complementary circuit with reversed current directions can be provided and this may be useful for some applications.

The design depends upon matched pairs (and sometimes triples or quadruples) of transistors and resistors. The exact values of resistors used are not of primary importance. To provide for exact tracking of the various junction drops, the matched transistors should preferably be kept in the same environment. All the transistors should have a high current gain and in some applications involving AC signals should have low junction capacitances. Although in the prototype tested matched pairs of transistors in the same package were used, better results will definitely be obtained, if the circuit is integrated. The circuit is felt to be suitable for production with integrated circuit techniques clue to the fact that it uses complementary transistors (PNP and NPN) hybrid circuit techniques are more suitable. However, improved economical techniques for producing high h PNP and NPN transistors integrated on the same chip appear to be available shortly. The choice of resistor values should be made in connection with choosing the supply voltage values, on the basis of ensuring linear operation of all the transistors throughout the required current range, and at the same time providing sufficient voltage to mask second order variations in transistor junction drops. Reasonable choices are 1K for all resistors and +15 and I5 volts for the supply voltages. This conveniently provides currents from zero to over 10 ma.

FIG. 4 shows in block form an application of the basic concept as an accurate widerange current source. Terminal y is at ground level and thus terminal x will be held at virtual ground. The current I will be directly proportional to the control voltage V, which provides current I via resistor R, i.e., I=V /R FIG. 5 and FIG. 6 show a digital to analog converter or what is in essence a digitally controlled current source. This application makes use of the potential tracking of the input terminals. The current conveyor 10 shown is the complement of the device of FIGS. 1 and 2 and has input terminal y connected to reference supply +V, Terminal x at the virtual potential of +V,,.,, is connected to ground through a parallel series of binary weighted resistors 2R, 2 R, ..,2""R. These resistors are switched in and out by switches a 0,, "1/1,, which are controlled by an n bit digital signal with the decimal equivalent of P cuitry in block 11 is required to stabilize V against load variations. The switches comprise a series of transistors O9 to Q15 with attendant resistors R9 to R15. These latter should be individually trimmed to compensate for any offset in the conveyor and for the saturation drops of the transistor switches.

FIG. 7 shows the application of the device as a wideband level shifter. This circuit accepts input signals (within certain limits) and DC levels (also within limits) and produces the sum of both. Terminal 1: which is at virtual ground is the summing point for a biasing current V /R the DC level V and the input signal V,. The biasing current allows the circuit to handle bipolar inputs. By taking the output V across R23 (where R23=R22=R2l=R20) the biasing is independent of the precise value of supply voltage +V The output of the circuit V,,,,,=V ,+V,,,. The full circuit implementation can be readily put into effect and is not described further here. It should be noted that the level shifter will require an impedance transfonning stage of unit voltage gain between the output terminal and the load to operate satisfactorily. Amplifiers of this type are available.

The Current Conveyor described can also be readily adapted to a negative impedance converter whose transfer characteristic is linear over a wide range and moreover is directly coupled with no offset, such that its graphical transfer characteristic passes through the origin. The fact that both current conveyor input terminals x and y (of FIGS. 1 and 2) draw currents in the same direction allows the use of the conveyor in this fashion. This is demonstrated by the hybrid-h matrix relating input to reference, namely,

. For example, if in the circuit of FIG. 2, the output terminal is grounded and if V, is a negative voltage changing towards zero, then the same voltage appears at terminal x. Thus if terminal x is connected via a resistor R (not shown) to ground, then as V,. increases the current I through x and hence through y decreases giving V,/I=R or the negative impedance characteristic.

The circuits described above show the flexibility and wide adaptation and application of the basic current conveyor circuit. Other uses and modifications will suggest themselves to those skilled in this field.

We claim:

1. A current conveyor circuit for accepting a current at an input at low or zero impedance and providing an output current of the same or a related value at high impedance comprising:

a. a first transistor having an input for introduction ofa current,

a diode connected second transistor whose input is connected to the output of the first transistor and whose output is connected via a first resistor to a voltage supply,

0. a diode connected third transistor whose output is con- 'nected to a reference voltage terminal,

. a fourth transistor whose control electrode is connected to the control electrode of said second transistor and whose input is connected via a second resistor, equal in value to the first resistor, to the voltage supply, and whose output is connected to the input of said third transistor,

. the control electrode of the third transistor being connected to the control electrode of the first transistor, and a fifth transistor whose input is connected via a third resistor equal in value to the first and second resistors to the voltage supply, whose control electrode is connected to the control electrodes of the second and fourth transistors, and which supplies an output current to an output terminal equal in value to the current flowing in the second and fourth transistors.

2. A current conveyor as in claim 5 wherein the first and third transistors are a matched pair of high current gain transistors and the second and fourth transistors are a matched pair of high current gain transistors, and the first and second resistors are a matched pair of resistors.