US3798468A

US3798468A - Analog voltage switch

Info

Publication number: US3798468A
Application number: US00308926A
Authority: US
Inventors: K Elshuber
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1972-01-18
Filing date: 1972-11-22
Publication date: 1974-03-19
Anticipated expiration: 1991-03-19
Also published as: NL7300129A; ES410711A1; FR2168390B1; SE388987B; JPS49106273A; GB1409918A; DE2202250B1; DE2202250C2; IT976913B; DK132780C; DK132780B; BE793546A; FR2168390A1; AT327292B; ATA38173A

Abstract

This disclosure concerns an analog voltage switch in which the output thereof reproduces as exactly as possible the analog voltage supplied to the input. The analog voltage switch includes a plurality of channels, each of which is provided with an analog voltage input and a control input for enabling the channel to be selected. The selected channel as determined by the control input receives an analog voltage input which appears at the output thereof. The analog voltage switch employs primarily semiconductor components and is especially applicable to use in an integrated form in that current amplifications of complementary semiconductor components and forward voltages of diodes employed in the analog voltage switch are of substantially the same magnitude and constant current sources included therein supply substantially the same current. By virtue of this arrangement, the temperature-dependent deviations of the semiconductor components employed in the analog voltage switch effectively cancel out each other such that the circuit comprising the analog voltage switch is substantially independent of temperature.

Description

United States Patent Elshuber Mar. 19, 1974 ANALOG VOLTAGE SWITCH [75] Inventor: Karl Elshuber, Schnotting nr.

Gemeinde Kirchdorf, Germany [73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

22 Filed: Nov. 22, 1972 211 App]. No.: 308,926

[30] Foreign Application Priority Data Jan. 18. 1972 Germany 2202250 [52] U.S. Cl 307/242, 307/254, 307/255 [51] Int. Cl. H03k 17/00 [58 Field of Search 307/255, 254, 241, 242, 307/243, 239

[56] References Cited UNITED STATES PATENTS 3.521450 8/1970 Muenter 307/255 3.526.785 9/l970 Birl 307/243 Primary ExaminerRudolph V. Rolinec Assistant Examiner-B. P. Davis Attorney, Agent, or Firm-James 0. Dixon; Hal Levine; Jim Comfort [57] ABSTRACT This disclosure concerns an analog voltage switch in which the output thereof reproduces as exactly as possible the analog voltage supplied to the input. The analog voltage switch includes a plurality of channels, each of which is provided with an analog voltage input and a control input for enabling the channel to be selected. The selected channel as determined by the control input receives an analog voltage input which appears at the output thereof. The analog voltage switch employs primarily semiconductor components and is especially applicable to use in an integrated form in that current amplifications of complementary semiconductor components and forward voltages of diodes employed in the analog voltage switch are of substantially the same magnitude and constant current sources included therein supply substantially the same current. By virtue of this arrangement, the temperature-dependent deviations of the semiconductor components employed in the analog voltage switch effectively cancel out each other such that the circuit comprising the analog voltage switch is substantially independent of temperature.

2 Claims, 3 Drawing Figures 7/3 r2; r33 r43 r/2 r2 r22 75/ r32 71/ r42 1 1 r4 Z l 2/2 3 E I i 52/ 22 I 32 2 4/ 54g Y J x Y Y J Y J CHANNELI CHANNELZ CHANNEL3 CHANNEL4 ANALOG VOLTAGE SWITCH This invention relates to an analog voltage switch comprising a plurality of channels which each have an analog voltage input and 'a control input for selecting the channel, and an analog voltage output at which the analog voltage fed to the analog voltage input of the selected channel appears.

A requirement of such analog voltage switches is that the analog voltage appearing at the analog voltage output reproduces as exactly as possible the analog voltage supplied to the selected channel. This requirement is, however, particularly difficult to meet when the'analog voltage switch is constructed with semiconductor circuit elements, especially in integrated form. In this case, errors arise in the transmission of the analog voltage from the selected input to theoutput, due in particular to the temperature dependence of the semiconductor circuit elements.

Furthermore, in analog voltage switches a large input voltage range is to be covered with small input power and the output impedance should be low.

The problem underlying the invention is the provision of an analog voltage switch with small input current, low temperature drift and low output impedance which is particularly suitable for construttion in the form of an integrated circuit.

According to the invention, this is achieved in that each channel contains a transistor whose emittercollector path is connected on the one hand to ground and on the other, via a constant current source to the first pole of a voltage source whose second pole isconnected to ground, that the junction between transistor and constant current source of each channel is connected via a decoupling diode to the base of an output transistor which is complementary to the transistors of the channels and the emitter-collector path of which is connected on the one hand to the first pole of the voltage source and on the other via a constant current source to ground, that the analog voltage input of each channel is connected to the base of the transistor via a diode, and that there is connected in parallel with the emitter-collector path of the transistor of each channel a controllable switch whose control input is connected to the control input of the channel.

In the analog voltage switch made according to the invention the transistors of the unselected channels are short-circuited by the parallel-connected switches. n the other hand, the switch of the selected channel is open so that its transistor is connected in series with a constant current source to the voltage source and controls the complementary output transistor which in turn is connected in series with a constant current source to the voltage source. In this connection there is a diode and the input voltage is also applied to the base of the transistor of the selected channel via a diode. With integrated construction of the circuit, the latter may be so designed that the current amplifications of the com plementary transistors and the forward voltage of the diodes are of substantially the same magnitude and the two constant current sources supply substantially the same current. It is thus possible to achieve that the temperature-dependent deviations of the channel transistor and of the output transistor on the one hand and the temperature-dependent deviations of the input diode and the decoupling diode on the other cancel each other out. The circuit is then largely independent of temperature.

An advantageous further development of the invention resides in that the controllable switch in each channel is formed by a second transistor which is complementary to the first transistor and connected in parallel therewith and the base of which is connected to the control input.

The invention will be explained by way of example with reference to the drawings, wherein:

FIG. 1 shows the basic circuit diagram of an analog voltage switch according to the invention having two channels,

FIG. 2 is a more detailed circuit diagram of an analog voltage switch according to the invention having four channels, and

FIG. 3 is a schematic circuit diagram for explaining the mode of operation.

The analog voltage switch illustrated in FIG. 1 contains in the first channel two complementary transistors T11 and T12 which are connected in parallel to each other in such a manner that the collector of the pnp transistor T11 and the emitter of the npn transistor T12 are connected to ground. The emitter of the pnp transistor T11 and the collector of the npn transistor T12 are connected on the one hand by a constant current source Q11 to a terminal K1 to which the positive pole of a DC voltage source is connected whose negative pole is grounded; on the other hand they are connected via a diode D12 to the base of an output transistor T1. The collector of the output transistor T1 is connected to the terminal Kl while its emitter is connected via a constant current source 01 to ground. Connected to the emitter is the output terminal A1 of the analog voltage switch.

The base of the transistor T11 is connected via a diode D11 to an input terminal E11 to which the analog voltage of the channel 1 is applied. The base of the transistor T12 is connected to a terminal E12 to which is fed the control signal for the channel 1.

The channel 2 is constructed in identical manner as the channel 1 from the circuit elements T21, T22, E21, E33, D21, D22 and Q21.

The diodes D12 and D22 serve for mutual decoupling of the channels.

. In the quiescent state of the analog voltage switch control signals are fed to the input terminals E12 and E22 and keep the corresponding transistor T12 or T22 saturated so that its collector is practically at ground potential. The emitter of the associated transistors T11 and T21 is thus also held at ground potential and consequently non-conductive. To transmit to the output, for example, the analog voltage applied to the input E11, the control signal fed to the input E12 is varied in such a manner that the transistor T12 becomes nonconductive. The circuit then assumes the position illustrated in FIG. 3.

In FIG. 3 the non-conductive transistor T12 has been omitted because its impedance can be considered to be infinitely large. The channel 2 is also omitted because it is completely disconnected by the decoupling diode D22.

The emitter of the transistor T11 assumes the following voltage:

a'ru "E rnu ser" I- wherein:

U511]! emitter voltage of the transistor T11; U the analog voltage applied to the input terminal E11; U forward voltage of the diode D11; U base-emitter voltage of the transistor T11. The base of the output transistor T1 is driven via the diode D12. The voltage U, at the output A1 is then:

U4 ,4 (.UJA SLUQW wherein:

U forward voltage of the diode D12 U base-emitter voltage of the transistor T1. Inserting equation (I) in equation (2) gives:

UA:UE 30 FDII FDl2+ !lET1l HET1 The emitters of the transistors T11 and T1 are connected to constant current sources supplying approximately the same current. Thus, as is apparent, the output voltage U A corresponds exactly to the input voltage U if firstly the forward voltages of the diodes D11 and D12 are of the same magnitude and secondly the pnp transistor T11 and the npn transistor T1 have the same current amplification. This can easily be achieved with integrated construction of the circuit.

If furthermore the temperature dependence of the circuit elements is sustantially the same, which is also the case with an integrated circuit, the temperature drift of the diode D11 will be compensated by that of the diode D12 and the temperature drift of the transistor T11 by that of the transistor T1.

FIG. 2 shows a more exact circuit diagram of an analog voltage switch constructed according to this principle and comprising four channels. The

channels

1 and 2 consist of the same circuit elements as in FIG. 1; the only difference is that a resistor R12 is inserted in the connection between the input terminal E12 and the base of the transistor T12 and a resistor R22 is inserted in the connection between the input terminal E22 and the base of the transistor T22. The

channels

3 and 4 are made up in similar manner from the circuit elements T31, T32, E31, E32, D31, D32, R32, and T41, T42, E41, E42, D41, D42 and R42.

The constant current source of the transistor T1 is formed by an npn transistor T2 to whose base a constant voltage is applied from a voltage divider which is connected between the DC voltage terminal K1 and ground and consists of a pnp transistor T3 and an npn transistor T4 connected as diode by shorting the basecollector junction. In the same manner the constant current source of each channel is formed by a pnp transistor T13, T23, T33 or T43 to whose base is applied a constant voltage. This constant voltage for the bases of these transistors is supplied by a common voltage divider connected between the DC voltage terminal K1 and ground and consisting of the series circuit of a pnp transistor T5 connected as diode, an npn transistor T6 and a resistor R1. This voltage divider also supplies the base bias for the transistor T3. The base bias for the transistor T6 is supplied by a voltage divider which is formed by the series connection of two resistors R2 and R3 and a transistor T7 connected as diode.

In a practical construction of the 4-channelanalog voltage switch shown in FIG. 2 it was possible to obtain the following values:

DC voltage U at the terminal Kl: 33V

input voltage range: 2 28 V input current: 10 p. A

output impedance: lkQ.

The transistors T13, T23, T33, T43 and T2 connected as constant current sources each supplied a current of p. A. A current of 400 p. A was taken off via the voltage divider formed by the transistors T5 and T6.

As is apparent, the entire circuit consists solely of transistors, diodes and resistors and it is therefore ideally suited for construction as an integrated semiconductor circuit. 7

What is claimed is:

1. An analog voltage switch comprising:

a plurality of channels, each of said channels having:

an analog voltage input and a control input for selecting the channel; a pair of transistors of complementary conductivity types, each having a collector, base and emitter; a common connection between the emitter of one of said pair of transistors and the collector of the other one of said pair of transistors; the collector of said one transistor and the emitter of said other transistor connected to a first reference potential; a constant current source for said channel comprising a third transistor having a collector, base and emitter, the collector-emitter path of said third transistor connected in series between a second reference voltage and said common connection of said pair of transistors; first means including a first diode connecting said analog voltage input to the base of a normally nonconductive one of said pair of transistors, said first diode being poled in the same direction as the base-emitter junction of said normally non-conductive transistor; second means connecting said control input to the base of a normally conductive one of said pair of transistors, and a second diode connected to said common connection of said pair of transistors and poled in an opposite direction from that of the base-emitter junction of said normally non-conductive one of said pair of transistors; an output transistor connected to an analog voltage output common to all said channels and of complementary conductivity type with respect to that of the normally non-conductive transistor of each channel, said output transistor having a collector, base and emitter; the emitter-collector path of said output transistor connected to a second constant current source comprising a fourth transistor having a collector, base and emitter, the emitter-collector path of said fourth transistor connected in series with the emitter-collector path of said output transistor; means for applying a constant bias to the base of each of said third transistors and to the base of said fourth transistor; the normally-conductive transistor of each said pair of transistors of said channels being responsive to a respective control signal applied to the control input of a particular channel in selecting that channel by changing the conductive state of the normally non-conductive transistor of said selected channel to a conductive state thereby transmitting an analog voltage signal received at said analog voltage input of the selected channel substantially without loss to said analog voltage output; and wherein all of said transistors, diodes and resistors comprise circuit elements of an integrated circuit wherein any temperature drift of said first diode in each channel is substantially compensated by corresponding temperature drift of said second diode in that channel; and any temperature drift of each normally nonconductive transistor of said pair of transistors of each channel is substantially compensated by corresponding temperature drift of said output transistor.

2. An analog voltage switch according to claim 1, wherein the said means for applying a constant bias to the base of each of said third transistors and to said fourth transistor comprises a first potential divider connected between said first and second reference potentials; said potential divider including a fifth transistor having a collector, base and emitter, the collector of said fifth transistor connected by a diode to said second reference potential, the emitter of said fifth transistor connected by a resistor to said first reference potential, and the base of said fifth transistor connected to a tap on a resistor-diode potential divider connected between said first and second reference potentials; the collector of said fifth transistor further being connected to the base of each of said third transistors to apply said constant bias thereto; a sixth transistor having a collector, base and emitter, the emitter-collector path of said sixth transistor connected to said second reference potential and by a third diode to said first reference potential, a junction between said third diode and said sixth transistor being connected to the base of said fourth transistor to supply said constant bias thereto.

Claims

1. An analog voltage switch comprising: a plurality of channels, each of said channels having: an analog voltage input and a control input for selecting the channel; a pair of transistors of complementary conductivity types, each having a collector, base and emitter; a common connection between the emitter of one of said pair of transistors and the collector of the other one of said pair of transistors; the collector of said one transistor and the emitter of said other transistor connected to a first reference potential; a constant current source for said channel comprising a third transistor having a collector, base and emitter, the collector-emitter path of said third transistor connected in series between a second reference voltage and said common connection of said pair of transistors; first means including a first diode connecting said analog voltage input to the base of a normally nonconductive one of said pair of transistors, said first diode being poled in the same direction as the base-emitter junction of said normally non-conductive transistor; second means connecting said control input to the base of a normally conductive one of said pair of transistors, and a second diode connected to said common connection of said pair of transistors and poled in an opposite direction from that of the base-emitter junction of said normally non-conductive one of said pair of transistors; an output transistor connected to an analog voltage output common to all said channels and of complementary conductivity type with respect to that of the normally non-conductive transistor of each channel, said output transistor having a collector, base and emitter; the emitter-collector path of said output transistor connected to a second constant current source comprising a fourth transistor having a collector, base and emitter, the emitter-collector path of said fourth transistor connected in series with the emitter-collector path of said output transistor; means for applying a constant bias to the base of each of said third transistors and to the base of said fourth transistor; the normally-conductive transistor of each said pair of transistors of said channels being responsive to a respective control signal applied to the control input of a particular channel in selecting that channel by changing the conductive state of the normally nonconductive transistor of said selected channel to a conductive state thereby transmitting an analog voltage signal received at said analog voltage input of the selected channel substantially without loss to said analog voltage output; and wherein all of said transistors, diodes and resistors comprise circuit elements of an integrated circuit wherein any temperature drift of said first diode in each channel is substantially compensated by corresponding temperature drift of said second diode in that channel; and any temperature drift of each normally nonconductive transistor of said pair of transistors of each channel is substantially compensated by corresponding temperature drift of said output transistor.