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Bipolar autoranging for a dc amplifier
N U C L E A R I N S T R U M E N T S AND METHODS
I26
(1975) 579-58I; © N O R T H - H O L L A N D P U B L I S H I N G CO.
BIPOLAR AUTORANGING FOR A DC AMPLIFIER O. LASSER
Atominstitut der Osterreichischen Hochschulen, Vienna, Austria Received 2 April 1975 Two possibilities o f a circuit and its performance for an autoranging dc amplifier for both polarities at the input are described, The circuit has been designed for an amplifier with a short
response time and can be adapted to the most various requirements of measuring technique.
1. Introduction
input current is unknown or the polarity changes during the measurement. In the latter case sometimes an exact registration of the zero crossing of the current is required. In the present work two possibilities of autoranging for a dc amplifier, constructed according to the above-mentioned principles, are outlined for bipolar input currents.
In engineering and research [e.g. reactor instrumentation 1,2)], the problem arises to measure exactly small currents which are changing very fast over quite a lot of decades. A dc amplifier with autoranging over several decades has already been developed for this purpose and published earlier3). A special feature of that amplifier set up is its construction in more stages, the amplification of which is altered by a factor of 10 if the input current exceeds a full decade. The multiplicate change of the amplification factor is achieved by individual comparators at each stage of the device. By means of that a simultaneous switching of several stages is possible in case of very fast input-current variations. Therefore the response time of the amplifier is drastically reduced at pulsed current operation. The amplifier mentioned allows the exact measurement of positive currents only. At special experiments the problem can arise that either the polarity of the
2. Circuit description Fig. 1 shows the input stage equipped with the matching comparators.
2.1. CASE NO. 1: THE SWITCH S IS IN POSITION 1 Suppose first of all the input current Ii, is positive. The current-to-voltage converter (stage I) delivers a negative voltage U6 (U6 = Uo, Uo = - I i , R1) to the both hysteresis comparators C - 1 , C - 2 which are formed by ordinary operational amplifiers. The hysteresis of these comparators is larger than the voltage
--c_~ R~
L~U.[
"i RZ
V~:-"ISV
,........
]2
' '~K ~
K
_](rII4v t
03
o3v
_U2
~
R7
Fig. 1. Input stage with comparators.
579
580
O. L A S S E R
change at the output of stage I when T1 is switched. Thus no instability occurs. The voltages + U1, - U 2 determine the trip-points of the comparators. As long as U~ lies below the triggering level of C - 2 , the outputs of C - 1 and C - 2 are almost compensated at point X (at the displayed voltages in fig. 1). Thus C - 1 , 2 , another operational amplifier, delivers a negative voltage to T1 whereby T t is not conducting. When /in exceeds a certain value (e.g. a full decade which will be assumed in the further consideration) and with it also U~, the comparator C - 2 is triggered so that a voltage of ca. + 5 V appears at X (D 1 is conducting). C-l,2 is also triggered and the positive output voltage delivered by C - 1 , 2 switches T1. By means of that the amplification of stage I is reduced, e.g. by a factor of I0 if R~' = (1/9)R~. Using operational amplifiers the switching transistor can be directly controlled, even at higher pinchoff-voltage, if the power supply is accordingly designed. The slightly negatively biased diode D 3 avoids a conducting gate-to-channel diode of T1 at a positive output voltage of C - 1 , 2 . When U~, according to a
S0
-10
reduction of Iin, reaches a value below the lower trippoint of C - 2, the original state of C - 2 and C - 1,2 is restored and a negative pulse, caused by C - 1 , 2 , turns off T 1 . At a negative input current the conditions are quite similar. When Iin exceeds a full decade the positive voltage U~ triggers C - 1 . Then a voltage of ca. - 2 V appears at point X and shifts the reference voltage of C - 1,2 to - 1 V by means of the conducting diode D E . Therefore C - 1 , 2 is triggered and a positive output pulse turns o n Z 1. The inputs of C - 1 and C - 2 should be symmetrical in their resistor combinations for temperature-independent trip-points as much as possible. The operation of the bipolar autoranging is being demonstrated in fig. 2. Suppose the input current is triangular and varies over one and a half decades. The course of the output voltage U~--Uo also lies symmetrically to the base-line between the extreme values Umax and Umin . The additional stages of the amplifier (if existing) can also be provided with such a or a similar bipolar autoranging device. It is essential that each amplifier stage is equipped with the adequate comparator circuitry of its own, to keep the response time as short as possible. The output signal of the amplifier is bipolar. A shortcoming of the discussed setup is the fact that for the evaluation of the results only half of the registration width is available for one polarity (see fig. 2). It also might be uneconomical for a multistage amplifier with many switching functions to construct each stage for both polarities. 2.2. CASE NO 2: THE SWITCH S IS IN POSITION 2 These disadvantages are overcome by an inverting
u;.u, q,,
o~
,•+
t~
IV
g
t Fig. 2. O p e r a t i o n o f the bipolar autoranging.
1OK
:o =
Fig. 3. Inverting stage.
IK
B I P O L A R A U T O R A N G I N G FOR A DC A M P L I F I E R
stage that operates after having switched S (fig. 1) in position 2. Fig. 3 shows the circuit diagram of the inverting stage. The comparator C - 3 receives a small positive reference voltage Ua, which is larger than the input off-set voltage of the operational amplifier, but smaller than the absolute value of the voltage appearing at the output of stage I after switching process. Further, if U3 is too large, the well-timed switching of another stage could be prevented in a multistage amplifier, as additional considerations show. At a negative voltage U 0 the transistor T 3 is turned on and short-circuits the output signal of the amplifier A2, whereas T2 is turned off. The latter has no influence upon the output of A 1. Therefore a negative voltage U~ appears in B, resulting from the output voltage at A~ and the ratio of the resistors R 8, R9, and R 11. At a small positive voltage U o the output voltage U~ will also be positive. When U o exceeds a certain positive value, C - 3 is triggered and turns off T3, while T2 is turned on. Now the amplifier A 2 operates and accordingly Ud is negative. For a stabile switching characteristic at slowly varying, noisy signals, C - 3 is provided with a small hysteresis over R12. An appropriate selection of the amplification factors of A~, A 2 and of the resistors Ra, R 9 , R i o , R~ t can always yield that LU~] = ] Uo]. In that case the course of Ud is shown in fig. 2 by a dashed line. When U o is negative, U~ is equal to U 0. At positive values of U o it can be seen, how the course of U~ crosses the base-line a little bit before the switching process occurs. This is practically no restriction of the registration width and has the advantage that the crossing of the base-line can be
581
registered without any disturbance by range switching. At the same time the transition of the polarity is clearly displayed by the positive parts of the output signal. If additional stages exist, the reference voltage U3, and with it the positive parts of U~, should be very small, so that no restriction of the registration width occurs. If the inverting stage is used, unipolarity exists concerning the range switching, so that in fig. 1 only one comparator ( C - 2 ) is necessary which can directly control the switching transistor. The same is valid also for the other stages (if existing). In this way an amplifier with autoranging for one polarity can be adapated to a bipolar-signal processing. The described inverting stage yields in the main a negative voltage U~. If the polarity of U 3 is changed, the character of U~ remains the same, but the positive parts of the output singal are corresponding to the negative voltage U 0. An exchange of the amplifiers A 1 and A2 results in a mainly positive voltage U~. Therefore the basic principle of this inverting stage is applicable to every kind of comparator and to every desired form of the output signal. The displayed circuits realize two possibilities of performing a d c amplifier with autoranging for both polarities. A combination of both circuits in an amplifier can be the solution for many measuring problems. References 1) H. B6ck et al., Nucl. Instr. and Meth. 123 (1975) 117. 2) H. B6ck, NucL Instr. and Meth. 125 (1975) 327. ~) O. Lasser and C. M. Fleck, NucL Instr. and Meth. 101 (1972) 527.
I26
(1975) 579-58I; © N O R T H - H O L L A N D P U B L I S H I N G CO.
BIPOLAR AUTORANGING FOR A DC AMPLIFIER O. LASSER
Atominstitut der Osterreichischen Hochschulen, Vienna, Austria Received 2 April 1975 Two possibilities o f a circuit and its performance for an autoranging dc amplifier for both polarities at the input are described, The circuit has been designed for an amplifier with a short
response time and can be adapted to the most various requirements of measuring technique.
1. Introduction
input current is unknown or the polarity changes during the measurement. In the latter case sometimes an exact registration of the zero crossing of the current is required. In the present work two possibilities of autoranging for a dc amplifier, constructed according to the above-mentioned principles, are outlined for bipolar input currents.
In engineering and research [e.g. reactor instrumentation 1,2)], the problem arises to measure exactly small currents which are changing very fast over quite a lot of decades. A dc amplifier with autoranging over several decades has already been developed for this purpose and published earlier3). A special feature of that amplifier set up is its construction in more stages, the amplification of which is altered by a factor of 10 if the input current exceeds a full decade. The multiplicate change of the amplification factor is achieved by individual comparators at each stage of the device. By means of that a simultaneous switching of several stages is possible in case of very fast input-current variations. Therefore the response time of the amplifier is drastically reduced at pulsed current operation. The amplifier mentioned allows the exact measurement of positive currents only. At special experiments the problem can arise that either the polarity of the
2. Circuit description Fig. 1 shows the input stage equipped with the matching comparators.
2.1. CASE NO. 1: THE SWITCH S IS IN POSITION 1 Suppose first of all the input current Ii, is positive. The current-to-voltage converter (stage I) delivers a negative voltage U6 (U6 = Uo, Uo = - I i , R1) to the both hysteresis comparators C - 1 , C - 2 which are formed by ordinary operational amplifiers. The hysteresis of these comparators is larger than the voltage
--c_~ R~
L~U.[
"i RZ
V~:-"ISV
,........
]2
' '~K ~
K
_](rII4v t
03
o3v
_U2
~
R7
Fig. 1. Input stage with comparators.
579
580
O. L A S S E R
change at the output of stage I when T1 is switched. Thus no instability occurs. The voltages + U1, - U 2 determine the trip-points of the comparators. As long as U~ lies below the triggering level of C - 2 , the outputs of C - 1 and C - 2 are almost compensated at point X (at the displayed voltages in fig. 1). Thus C - 1 , 2 , another operational amplifier, delivers a negative voltage to T1 whereby T t is not conducting. When /in exceeds a certain value (e.g. a full decade which will be assumed in the further consideration) and with it also U~, the comparator C - 2 is triggered so that a voltage of ca. + 5 V appears at X (D 1 is conducting). C-l,2 is also triggered and the positive output voltage delivered by C - 1 , 2 switches T1. By means of that the amplification of stage I is reduced, e.g. by a factor of I0 if R~' = (1/9)R~. Using operational amplifiers the switching transistor can be directly controlled, even at higher pinchoff-voltage, if the power supply is accordingly designed. The slightly negatively biased diode D 3 avoids a conducting gate-to-channel diode of T1 at a positive output voltage of C - 1 , 2 . When U~, according to a
S0
-10
reduction of Iin, reaches a value below the lower trippoint of C - 2, the original state of C - 2 and C - 1,2 is restored and a negative pulse, caused by C - 1 , 2 , turns off T 1 . At a negative input current the conditions are quite similar. When Iin exceeds a full decade the positive voltage U~ triggers C - 1 . Then a voltage of ca. - 2 V appears at point X and shifts the reference voltage of C - 1,2 to - 1 V by means of the conducting diode D E . Therefore C - 1 , 2 is triggered and a positive output pulse turns o n Z 1. The inputs of C - 1 and C - 2 should be symmetrical in their resistor combinations for temperature-independent trip-points as much as possible. The operation of the bipolar autoranging is being demonstrated in fig. 2. Suppose the input current is triangular and varies over one and a half decades. The course of the output voltage U~--Uo also lies symmetrically to the base-line between the extreme values Umax and Umin . The additional stages of the amplifier (if existing) can also be provided with such a or a similar bipolar autoranging device. It is essential that each amplifier stage is equipped with the adequate comparator circuitry of its own, to keep the response time as short as possible. The output signal of the amplifier is bipolar. A shortcoming of the discussed setup is the fact that for the evaluation of the results only half of the registration width is available for one polarity (see fig. 2). It also might be uneconomical for a multistage amplifier with many switching functions to construct each stage for both polarities. 2.2. CASE NO 2: THE SWITCH S IS IN POSITION 2 These disadvantages are overcome by an inverting
u;.u, q,,
o~
,•+
t~
IV
g
t Fig. 2. O p e r a t i o n o f the bipolar autoranging.
1OK
:o =
Fig. 3. Inverting stage.
IK
B I P O L A R A U T O R A N G I N G FOR A DC A M P L I F I E R
stage that operates after having switched S (fig. 1) in position 2. Fig. 3 shows the circuit diagram of the inverting stage. The comparator C - 3 receives a small positive reference voltage Ua, which is larger than the input off-set voltage of the operational amplifier, but smaller than the absolute value of the voltage appearing at the output of stage I after switching process. Further, if U3 is too large, the well-timed switching of another stage could be prevented in a multistage amplifier, as additional considerations show. At a negative voltage U 0 the transistor T 3 is turned on and short-circuits the output signal of the amplifier A2, whereas T2 is turned off. The latter has no influence upon the output of A 1. Therefore a negative voltage U~ appears in B, resulting from the output voltage at A~ and the ratio of the resistors R 8, R9, and R 11. At a small positive voltage U o the output voltage U~ will also be positive. When U o exceeds a certain positive value, C - 3 is triggered and turns off T3, while T2 is turned on. Now the amplifier A 2 operates and accordingly Ud is negative. For a stabile switching characteristic at slowly varying, noisy signals, C - 3 is provided with a small hysteresis over R12. An appropriate selection of the amplification factors of A~, A 2 and of the resistors Ra, R 9 , R i o , R~ t can always yield that LU~] = ] Uo]. In that case the course of Ud is shown in fig. 2 by a dashed line. When U o is negative, U~ is equal to U 0. At positive values of U o it can be seen, how the course of U~ crosses the base-line a little bit before the switching process occurs. This is practically no restriction of the registration width and has the advantage that the crossing of the base-line can be
581
registered without any disturbance by range switching. At the same time the transition of the polarity is clearly displayed by the positive parts of the output signal. If additional stages exist, the reference voltage U3, and with it the positive parts of U~, should be very small, so that no restriction of the registration width occurs. If the inverting stage is used, unipolarity exists concerning the range switching, so that in fig. 1 only one comparator ( C - 2 ) is necessary which can directly control the switching transistor. The same is valid also for the other stages (if existing). In this way an amplifier with autoranging for one polarity can be adapated to a bipolar-signal processing. The described inverting stage yields in the main a negative voltage U~. If the polarity of U 3 is changed, the character of U~ remains the same, but the positive parts of the output singal are corresponding to the negative voltage U 0. An exchange of the amplifiers A 1 and A2 results in a mainly positive voltage U~. Therefore the basic principle of this inverting stage is applicable to every kind of comparator and to every desired form of the output signal. The displayed circuits realize two possibilities of performing a d c amplifier with autoranging for both polarities. A combination of both circuits in an amplifier can be the solution for many measuring problems. References 1) H. B6ck et al., Nucl. Instr. and Meth. 123 (1975) 117. 2) H. B6ck, NucL Instr. and Meth. 125 (1975) 327. ~) O. Lasser and C. M. Fleck, NucL Instr. and Meth. 101 (1972) 527.