GB2131639A - Linear bipolar transistor circuits - Google Patents

Abstract

A linear bipolar transistor circuit has at least one bipolar transistor (Q1) capable of having voltages greater than the sustaining voltage applied across it, and means to vary automatically the operating point of the bipolar transistor when the voltage applied across the transistor is greater than the sustaining voltage, so that the transistor operates satisfactorily in the manner of a linear amplifier irrespective of the voltage applied across it, usually the means being arranged to cause the ratio of the emitter and base currents, or voltage, to alter in the appropriate way. The means may comprise a feedback connection (D3, D4, Q4, Q2), whereby, when the transistor is an NPN transistor, at least a portion of, any base current flowing out of the transistor is substracted from the instantaneous current flowing in the emitter circuit, when a voltage greater than the sustaining voltage is applied across the transistor. <IMAGE>

Description

SPECIFICATION Linear bipolar transistor circuits This invention relatesto linear bipolartransistor circuits.

Within each such linearcircuitthere is, orthere can be considered to be, at least one constituent bipolar transistor required, in response to the receipt of analogue input signals, to provide output signals having a parameterwith instantaneous magnitudes exactly proportional to the corresponding instan taneousmagnitudes of a parameter of the input signals, and in this specification, and the accompanying claims, each such bipolar transistor will be referredtoas operating inthemannerofalinear amplifier.

Four a bipolartransistor, in any circuit configuration, ifthe instantaneous collector-emitter voltage is increased from zero, eventually the transistor breaks down, there being avalanche multiplication of holeelectron pairs within the transistor. Usually, the voltage applied between the collector and the emitter is measured ata collector currentsuch that the slope of the collectorvoltage-collector current characteristic in the breakdown region is positive. This voltage, measured in the breakdown region, is less than the breakdown voltage, and is called the sustaining voltage ofthe transistor.

Previously, it has been considered thatthe maximum possiblevoltagewhich could be applied betweenthecollectorandemitter, andforthe satisfactoryoperation of the bipolar transistor, within any circuit configuration, has been the sustaining voltage ofthetransistor.

It is an object ofthe present invention to provide linear bipoiartransistor circuits, in each ofwhich circuits there is, or there can be considered to be, a constituent bipolar transistor, at least capable of, operating in the required manner of a linear amplifier with voltages greaterthan the sustaining voltage applied between the collector and emitter thereof.

The avalanche multiplication effect can be considered as being represented by a term (m), where (m) has a variablevalue. As the instantaneous voltage applied between the collector and emitter of a bipolar transistor is increased from zero, the value of (m) increases steadily. For a transistor having a gain in the common emitter mode, p, of one hundred, and a current amplification factor in the common base mode, oc, of approximately unity, when (m) is 1.01, the avalanche multiplication effect first becomes dominant, and the instantaneous voltage applied between the collector and emitter is the sustaining voltage. If the instantaneous voltage applied between the collector and emitter is increased still further, the value of (m)ontinuesto rise.

For an NPN transistor, when the avalanche multi ptication effect first becomes dominantthe transistor base ceases to draw current, and if the instantaneous collector-emitter voltage is increased beyond the sustaining voltage, a current flows out ofthe base. The higherthe value of (m) the greater the current flow out of the base.

Four a PNPtransistor, when the avalanche multiplication effectfirst becomes dominant a currentflows into the base, and the higherthe value of (m) the greater the current flow into the base. For convenience, in this specification and the accompanying claims, only N PN transistors, operated so that the avalanche multiplication effect can become dominant, will be considered, but it will be understood that references to such NPN transistors include references to PN P transistors operated so that the avalanche multiplication effect can become dominant.

It is conventional to represent the manner of operation of a bipolar transistor, underanypredeter- mined operating conditions, by a family of characteristics, representative ofthe inter-relationships ofthe electrode voltages and currents. Any instantaneous predetermined operating conditions are referred to as the operating pointof the transistor, and are repre- sented by the corresponding points within the family of characteristics, when the transistor input signal is zero. The operating point corresponds to the average electrode voltages and currents underthe predetermined operating conditions.

According to the present invention a linear bipolar transistor circuit has at least one constituent bipolar transistor, or what can be considered to be such a bipolar transistor, with means to cause the operating point ofthe bipolartransistorto vary automatically in a required way, in response to instantaneous magnitudes ofthe avalanche multiplication effect (m) in the bipolar transistor, when this effect becomes dominant, with a voltage greaterthan the sustaining voltage being applied between the emitter and the collector of the bipolar transistor, by causing the corresponding instantaneous ratios of the emitter and base currents, orvoltages, to vary, so that the bipolar transistor operates satisfactorily in the manner of a linear amplifier irrespective ofthevoltage applied betweenthe emitter and the collector.

The means to vary the operating point ofthe bipolar transistor may include a feedback connection between the base and the emitter circuit of the bipolar transistor. Thereby, a current at least proportional to the instantaneous base current may be subtracted fromthe instantaneous current otherwiseflowing in the emitter circuit of the bipolar transistor, the current flowing in the emitter circuit when the avalanche multiplication effect is not dominant being, at least proportional to, the input signal currentto the linear bipolartransistor circuit.In one such arrangement, when the circuit has unity current gain, there is provided a first current mirror between the emitter circuit of the bipolar transistor and the input ofthe linear bipolartransistor circuit, and the feedback connection includes a second current mirror coupled to the first current mirror.

In another such arrangement, when the circuit has a current gain greater than unity, the feedback connec- tion comprises a second bipolar transistor connected in series with said bipolartransistor, and a third bipolartransistor connected to the base of said bipolar transistor, the base of the third bipolartransistor is connected to a point between said bipolartransistor and the second bipolartransistor, and both the second and third bipolartransistors are in series with an input bipolartransistorforthe circuit.

According to another aspect the present invention comprises a semiconductor device having a linear bipolar transistor circuit of any one of the different forms referred to above embodied within a monolithic semiconductor body, the appropriate electrode voltages to be applied to constituent regions of the semiconductor body.

Conveniently, the linear bipolar transistor circuit is embodied in the monolithic semiconductor body by employing the so-called collector -diffusion-isolation process, the semiconductor body initially comprising an epitaxial layer of one conductivitytype upon a substrate of the same conductivity type, and each constituent bipolar transistor of the circuit having a collector comprising both a buried layer of the opposite conductivity type at a portion of the interface between the substrate and the epitaxial layer, together with an isolation barrier forthe transistor extending through the epitaxial layer to contactthe buried layer, and the base of the transistor including an unmodified region of the epitaxial layer surrounded by the isolation barrier.

The present invention will now be described by way ofexamplewith reference to the accompanying drawings, in which Figure lisa diagram of one embodiment of a linear bipolar transistor circuit in accordance with the present invention,the circuit including a bipolar transistor capable of operating satisfactorily, in the manner of a linear amplifier, with voltages greater than thesustaining voltage applied between the emitterandcollector,there being a feedback connection between the base and the emitter circuit ofthe bipolartransistor, and the current gain ofthe circuit being unity, and Figure 2 is an alternative construction to that of the circuit of Figure l,thebipolartransistor being in cascodewith a second bipolartransistor, and the current gain ofthe circuit being greater than unity.

If the operation of an NPN bipolartransistor, in any circuit configuration, is considered, the currentflowing in the collector circuit associated with the bipolar transistor is ami5, where is is the currentflowing in the emiiter circuit (oc) is the current amplification factor in the common base mode, and (m) is a term representing the avalanche multiplication of hole-electron pairs within thetransistor. The corresponding base current is:- - - (m-l) a T where ss is thetransistor gain in the common emitter mode of operation.

The value of the avalanche multiplication effect term (m) varies with changes in the voltage applied between the emitter and collector of the transistor, (m) increasing as the applied voltage increases. If p is 100, and a is approximately unity, when m = 1.01, the negative term of the base current equation first becomes dominant. This onset of the dominance of the avalanche multiplication effect is indicated by the transistor base ceasing to draw cu ri ent. When this occurs the voltage applied between the emitterand collector is referred to as the sustaining voltage. If the voltage applied between the collector and emitter is increased still further, the value of (m) continues to rise, and a currentflows out of the base.The higherthe voltage applied between the collectorand emitter the greaterthe current flowing out of the base. Thus, the dominance ofthe avalanche multiplication effect (m) may be detected, for example, by detecting the direction of the base current, orthe base voltage.

Previously, the sustaining voltage has been considered to bethe maximum acceptablevoltage capable of being applied between thecollectorand emitter of the transistor, for the transistorto have a satisfactory manner of operation, within any circuit configuration.

However, within one embodiment of a linear bipolar transistor circuit in accordance with the present invention, and shown in Figure 1, with an NPN bipolar transistor operating in the manner of a linear amplifier, when the sense ofthe current flow in the base circuit associated with thetransistor is detected as changing, when the voltage applied across the transistor is the sustaining voltage, in response, means ofthe transistor circuit is caused to vary automaticallythe operating point of the transistor in a required way.In particular,the means capable of varying the operating of the bipolar transistor com- prises a feedback connection between the base and the emitter circuit, whereby, if the voltage applied between the collector and emitter is increased beyond the sustaining voltage, the instantaneous current flowing in the emitter circuit is varied by subtracting therefrom the instantaneous current flowing out of the base. Hence, the voltage applied across the transistor can be greater than the sustaining voltage, and the transistor continues to operate satisfactorily in the mannerofa linear amplifier. When the voltage applied across the transistor is less than the sustaining voltagethe means capable of varying the operating point ofthe transistor is inoperable.

Such a bipolartransistor isthe NPN transistor Q1 in the linear bipolartransisor circuit illustrated in Figure 1. The collector is connected, via a resistor Tri, to a rail 10 maintained at a potential V5. A current source 11, comprising a mirroring arrangement, is connected to the emitter circuit associated with the transistor Q1.

The current source 11 comprises an N PN transistor Q2 connected in series between the transistor Q1 and a rail 12 maintained at zero potential. The base ofthe transistor Q2 is also connectedtothe rail 1 via a diode D1. The input 14 tothe circuit, to reeieve analogue input signals, is connected to a point between the base of the transistor Q2 and the diode Dl. With the transistor circuit unmodified, the instantaneous current input jlN, supplied to the input 14, is mirrored in the emitter circuit ofthe transistor Q î . The output 16 of the circuit is connected to a pointbetweenthe transistor Q1 and the resistor R.

The voltage across the transistor Q1 increases as the input current jlN decreases, and the voltage across the transistor Q1 is capable of approaching Vs, the potential of the rail 10. If it is desired thatthe maximum possible voltage across the transistor Q1 can be greaterthan the sustaining voltage, with the transistor circuit unmodified, there is a particularvalueforthe current flowing in the emitter circuit at which the sustaining voltage is applied across the transistor. As the current in the emitter circuit decreases still further, the transistor operates in a manner unacceptable for a linear amplifier.

Thus, previously, such a linear bipolar transistor circuit has only been operated in a satisfactory manner by the rail 10 being at a potential Vs at most equal to the sustaining voltage; and by the base current only being drawn from biassing means, for exarrzple, in the illustrated arrangement, the base of the transistor Q1 being connected to a rail 20 maijitainedata reference potential VrlEFvia an NPN transistor03. However, with the modified linear bipolartransistor circuit in accordance with-the present invention, when the base does not draw any current, when the voltage applied between the emitter and collector is the sustaining voltage, the transistor 03 is switched OFF.If the voltage applied between the collector and emitter is increased beyond the sustaining voltage, by decreasing the current in the emitter circuit, current flows out ofthe base. Such base current B is supplied toafeedback connection, including a current mirroring arrangement 22, comprising an NPN transistor 04, the emitter ofwhich is connected to the rail 12, and the base of which is connected to the rail 12 via a diode D2. The baseof the transistor Q1 is connected to a point between the diode D2 andthe base of the transistor 04, via a diode D3.The biassing means is completed bytwo resistors R2 and R3, and two diodes D4 and D5, being connected in series between the rails 20 and 12, the base ofthe transistor 03 being connected to a point between the two resistors R2 and R3, which point is maintained at a potential of approximately 1.75 volt, or 2.5 xVbe of each transistor. The current mirroring arrangement 22 is coupled to the current mirroring arrangement 11,sothatthe current in the emitter circuit ofthetransistor 01 is not the input current IN, but is instead (i1N - jB) Hence, the collector of the transistorQ4Ofthe current mirroring arrangement 22 is connected to the base ofthe transistor Q2 of the currentmirroring arrangement 11.

By the subtraction of any instantaneous current iB flowing out ofthe base of the transistor Q1 from the instantaneous input current ilN,the current is in the emitter circuit associated with the transistor Q1 is decreasedfurther, and the operating point ofthe transistor Q1 is varied automatically, so that the transistor Q1 operates satisfactorily in the manner of a linear amplifier, even though a voltage greaterthan the sustaining voltage is applied across it. In particular,the instantaneous ratio ofthe emitter and base currents is varied in the appropriate way in accordance with the instantaneous value ofthe avalanche multiplication effect (m).

The feedback connection is inoperable when the voltage applied across the bipolartransistor Ol is less than the sustaining voltage, current being supplied to the base of the transistor 01 via thetransistor Q3.

Thus, swings of the input current ilN are reproduced in a linear manner atthe output 16; and the potential of the output 16 may risetothe maximum possiblevalue Vs, even though this voltage is greater than the sustaining voltage ofthetransistor0l.

The transistor Q1 either operates satisfactorily, or is capableofoperating satisfactorily, in the manner a linear amplifier, when voltages up to the collectorbase breakdown voltage ofthe transistor are applied across the transistor 01.

Care may be required to avoidthe linear bipolar transistor circuit oscillating undesirably as the voltage across the transistor Q1 is increased significantly beyondthesustaining voltage.

The illustrated linear bipolartransistor circuit has a current gain of unity associated therewith.

It is not essentialthat the input current 11N is the current supplied bythe current mirror 11 to the emitter circuit associated with the transistor 01, but instead the current supplied may be niN, where (n) is any convenient number. However, it is required that the current supplied by the current mirror 22 to the emitter circuit associated with the transistor Q1 is Bin. In such a circuit the areas of the emitters of the transistors 02 and 04, respectively, have the appropriate values in relation to the areas of the cathodes of the diodes D1 and D2.

Within another embodiment of a linear bipolar transistor circuit in accordance with the present invention, having a current gain greater than unity associated therewith, when the sense of the current flow in the base circuit of a bipolartransistor, required to operate in the manner of a linear amplifier, is detected as changing, when the voltage applied across the transistor is at least the sustaining voltage, a feedback connection, provided in the transistor circuit causes automatically a portion of the instantaneous current flowing out ofthe base of the transistorto be subtracted from the instantaneous current otherwise flowing in the emittercircuitofthe transistor. Such a bipola rtransistor circuit is shown in Figure 2.Hence, when the voltage applied across the transistor is greaterthan the sustaining voltage, the transistor continues to operate satisfactorily, in the manner of a linear amplifier.

In the bipolartransistor circuit of Figure 2,the collector ofafirst NPN bipolar transistor 011, to operate in the manner of a linear amplifier, is connected, via a resistor R1 1,toa rail 10 maintained at a potential V5, greater than the sustaining voltage of the firsttransistor. The output 16 of the circuit is connected to a point between the firsttransistor Q11 and the resistor Rl 1.The base of the first transistor is connected to biassing means, indicated generally at 24, via a diode D1 1 ,the output potential ofthe biassing means being VREF- The first transistor Q1 1 is in cascode with a second, input, NPN transistor Q1 the emitter circuit of the firsttransistor Q11 being coupled to the collector circuit ofthe second transistor 012. The emitter of the second transistor 012 is connected to a rail 12 maintained at zero potential, and the input 14 ofthe circuit is connected to the base ofthetransistor 012.

An analogue input signal voltage is applied to the input 14, and it is required thatthe signal is amplified, ion a linear manner, atthe output 16.

An instantaneous current jlN flows into the base of the second transistor 2, and a corresponding instantaneous current ic flows in the collector circuit of the second transistor 012. With the bipolar transistor circuit unmodified, this current inflows in the emitter circuit ofthe first transistor 01 and because the potential Vs ofthe rail loins greater than the sustaining voltage ofthe first transistor, there is a value for the current ic aspic decreases, at which the manner of operation of the first transistor becomes unacceptable for a linear amplifier, when the voltage applied across the first transistor is the sustaining voltage, and the base of thefi rst transistor Q7 I ceases to draw current.

If the voltage applies across the firsttransistor is increased beyond the sustaining voltage, a current flows out of the base of the first transistor. As the voltage applied across the firsttransistor increases beyond the sustaining voltage,the currentflowing out ofthe base ofthefirsttransistor increases.

However, as indicated above, and in accordance with the present invention, a feedback connection is provided between the base and the emittercircuitof the firsttransistor 011. The feedback connection comprises an NPN transistor Q13 connected in series between the first and second transistors Q11 and Q12, and another NPN transistor Q14 is connected between a point between the diode DI 1 and a point between the emitter of the transistor 013 and the collector of the transistor 012. The base of the transistor Q14 is connected to a point X, between the emitter of the transistor Q11 and the collector of the transistor 013.

The base ofthe transistor Q13 is connected to biassing means, indicated generally at 26, and is maintained at a voltage of VREF- 1 volt, i.e. at a potential greater than 1 x Vbe of any constituent transistor below the reference potential VREF, comprising the output of the biassing means 24.

In the operation of the circuit, when any current flows out ofthe base ofthe first transistor 01 when the voltage applied across this transistor is greater than the sustaining voltage, the biassing means 24 becomes inoperable; and automatically a portion of this instantaneous current is supplied bythefeedback connection to be subtracted from the instantaneous current ic otherwise flowing in the emitter circuit of the first transistor.

If 1L is the currentflowing in both the emitter circuit, andthe collector circuit, ofthetransistor0l3then:- miL= ic Assume m,the avalanche mulitplication factor of the transistor Q11, has a value of 3, the voltage Vs applied across the transistor Q11 beign greaterthan the sustaining voltage. The current flowing in the collector circuit of the transistor Q11 remainsic; and a current 2icl3flows out ofthe base of the transistor 011, and in the emittercircuitofthe transistor Q1 4. Thus, the currentisflowing in the emittercircuitofthetransistor 011 is not ic, but ic/3.

The operating point of the transistor Q11 is varied automatically, so that the transistor operates satisfactorily in the manner of a linear amplifier, even though a voltage greater than the sustaining voltage is applied across it. In particular, the instantaneous ratio ofthe emitter and base currents associated with the transistor Q11 is varied in the appropriate way in accordance with the instantaneous value of the avalanche multiplication effect (m).

If potential atthe output terminal 16, and, hence, also the potential of the base of the transistor Q11, tendsto rise, inadvertently, the currentflowing in the collector circuit of the transistor Q14 tends to rise correspondingly.This causes the potential ofthe base of the transistor Q14 to tend to follow the potential of the base ofthe transistor Q11; and in particularthe potential of the pointX, between the emitter circuit of the transistor 011 and the collector circuit ofthe transistor 013, tends to followthe potential of the base ofthetransistor 011. Thetendency of the potential ofthe emitter circuit of thetransistorQ11 1 to rise, causes the current 4flowing in the emitter circuit to fall.In particular, any increase of the current flowing in the emitter circuit ofthe transistor Q14 is equal to the corresponding decrease of the current flowing in the emitter circuit of the transistor 013. Hence, the current icflowing in the collector circuits of the transistors Q11 and 012, remains constant, irrespective of any increase ofthe potential at the output terminal 16. If the potential atthe output terminal 16 tendsto fall, the voltages, and currents, around the feedback connection of the circuit arrangement of Figure2 again tend to balance, in a mannercomplementaryto that indicated above.In particular, in spite of any fluctuation in the potential at the output terminal 16, the current icflowing in the collector circuits ofthe transistors 011 and Qf2, remain constant.

When the voltage applied across the firsttransistor Q11 is less than the sustaining voltage, the feedback connection is inoperable; and the biassing means 24 supplies cu rrentto the base ofthe transistor.

The voltage applied across the first transistor may have any value less than the collector-base breakdown voltage associated with the transistor.

Modifications of linear bipolartransistor circuits in accordance with the present invention are possible.

The operating point of a bipolartransistor required to operate in the manner of a linear amplifier may be varied by providing any convenient circuit construction to cause the ratio of the emitter and base currents to alter in the appropriate way.

The operating point ofthe bipolartransistor may be varied by altering the ratio ofthe emitter and base voltages.

Claims (10)

1. A linear bipolartransistor circuit having at least one constituent bipolartransistor, with means to cause the operating pointofthe bipolartransistorto vary automatically in a required way, in response to instantaneous magnitudes of the avalanche multiplication effect (m) in the bipolartransistor, when this effect becomes dominant, with a voltage greater than the sustaining voltage being applied between the emitter and the collector ofthe bipolartransistor, by causing the corresponding instantaneous ratios of the emitter and base currents, or voltages, to vary, so that the bipolartransistor operates satisfactorily in the manner of a linear amplifier irrespective of the voltage applied between the emitter and the collector.

2. A circuit as claimed in claim 1 in which the means to varythe operating pointofthe bipolar transistor includes a feedback connection between the base andthe emitter circuit ofthe bipolar transistor.

3. Acircuitas claimedin claims 3 in which the means to varythe operating point ofthe bipolar transistor is arranged to cause a current at least proportional to the instantaneous base current to be subtracted from the instantaneous current otherwise flowing in the emitter circuit ofthe bipolartransistor, the current flowing in the emitter circuit whenthe avalanche multiplication effect is not dominant being, at (east proportional to, the input signal currentto the linearbipolartransistor circuit.

4. A circuit as claimed in claim 3 in which a first current mirror is provided between the emitter circuit ofthe bipolartransistor and the input of the linear bipolartransistor circuit, and the feedback connection includes a second current mirror coupled to thefirst current mirror.

5. A circuitas claimed in claim 3 in which the feedback connection comprises a second bipolar transistor connected in series with said bipolar transistor, and a third bipolar transistor connected to the base of said bipolar transistor, the base ofthe third bipolar transistor is connected to a point between said bipolartransistor and the second bipolartransistor, and both the second and third bipolar transistors are in series with an input bipolartransistor for the circuit.

6. A circuit as claimed in any one of the preceding claims in which biassing means is connected to the base ofthe bipolartransistor, current flowing therebetween when the voltage applied across the bipolar transistor is lessthan the sustaining voltage, the biassing means being rendered inoperable when the voltage applied across the bipolar transistor is greater than the sustaining voltage.

7. A circuit as claimed in any one of the preceding claims in which each constituent bipolar transistor is an NPN transistor, with current flowing out of the base of said bipolartransistorwhen a voltage greaterthan the sustaining voltage is applied across said bipolar transistor.

8. Asemiconductordevice having a linear bipolar transistor circuit as claimed in any one ofthe preceding claims embodied within a monolithic semiconductor body, the appropriate electrode voltages to be applied to constituent regions ofthe semiconductor body.

9. A device as claimed in claim 8 in which the linear bipolartransistorcircuit is embodied inthe monolithic semiconductor body by employing the so-called collector-diffusion-isolation process, the semiconductor body initially comprising an epitaxial layer of one conductivity type upon a substrate of the same conductivity type, and each constituent bipolar tran- sistorofthe circuit having a collector comprising both a buried layer of the opposite conductivity type at a portion ofthe interface between the substrate and the epitaxial layer,togetherwith an isolation barrierfor thetransistor extending through the epitaxial layerto contact the buried layer, and the base of the transistor including an unmodified region ofthe epitaxial layer surrounded by the isolation barrier.

10. A linear bipolar transistor circuit substantially as described herein with reference to Figure 1, or Figure 2, of the accompanying drawings.