Voltage/current conversion circuit

A voltage/current (v/c) conversion circuit is designed for use in integrated circuit devices. Conventional designs of v/c conversion circuits require a relatively high value of load resistor, i.e. a steep v/c conversion slope, to generate high levels of output current. In place of the high load resistor which is expensive to fabricate with IC fabrication techniques, the source-drain resistance in an n-type MOSFET is utilized, and the output current level is adjusted by adjusting the potential of a bias input circuit supplied from the gate potential in an n-type MOSFET. The proposed configuration is ideally suited to IC fabrication processes, and the circuit is useful in a many applications requiring a wide range of high current levels from a conversion circuit having a low v/c conversion factor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a voltage/current conversion circuit 
suitable for applications in integrated circuits. 
2. Description of the Related Art 
An example of the conventional voltage/current (v/c) conversion circuits 
used generally for converting input voltage to output current is shown in 
FIG. 5. In this v/c conversion circuit shown in FIG. 5, if the gain of the 
operational amplifier (OP amp) is ideally high, a negative feed back is 
generated between the OP amp 2 and an n-type metal oxide semiconductor 
field-effect transistor (MOSFET) 5 serving as the active load so that the 
voltage input terminal 1a and the node point 1b are at the same potential. 
Denoting the potential at the voltage input terminal 1a as Vin volts, the 
resistance of the resistor element 46 as R ohms, and the potential at the 
node 1b as Vin volts, the current Ir flowing in the resistor element 46 is 
given by Vin/R in amperes. 
In FIG. 5, a p-type MOSFET 3 and another p-type MOSFET 4 constitute a 
current mirror so that the magnitude of the current flowing in p-type 
MOSFET 4 is the same as that flowing in p-type MOSFET 3. Therefore, the 
magnitude of the current flowing out of the output terminal 8, Iout, 
becomes the same as that flowing in the resistor element 46 given by Vin/R 
amperes. 
The v/c conversion circuit shown in FIG. 5 is characterized by a good 
linearity of the input/output relation, because the voltage/current 
conversion ratio between the input potential Vin and the output current 
Iout is determined by the value of the resistor R. However, when it is 
desired to provide a v/c conversion circuit that produces a small 
variation in the output current for a given variation in the input 
potential, i.e., to make a voltage/current conversion circuit with a low 
value of voltage/current conversion ratio, it is necessary to have a high 
resistor R. In IC production processes, high resistor elements are not 
desirable, because they require an increase in the substrate area or an 
increase in the number of fabrication steps. 
Also, the circuit shown in FIG. 5 does not permit a selection of the level 
of the output current, independently of the voltage/current conversion 
ratio. If the resistor value R is lowered to increase the output current 
level, the voltage/current conversion ratio inevitably becomes high (refer 
to FIG. 6, line b), and conversely, if the resistor value R is increased 
to decrease the voltage/current conversion ratio, the output current level 
becomes low (refer to FIG. 6, line c). Therefore, this type of circuitry 
is unable to produce a high level of output current, such as the one 
indicated by line a in FIG. 6, while providing a small variation in the 
output range and a low voltage/current conversion ratio, as expressed by 
the slope of the characteristic line. 
Other examples of such v/c conversion circuits include a circuit disclosed 
in a Japanese Patent Application, First Publication, H1-170206, FIG. 1, 
which behaves in the same manner as the circuit shown in FIG. 5, and a 
high resistor value is needed to realize a v/c conversion circuit having a 
small conversion factor. Also, once the conversion factor (i.e. resistor 
value) is decided in a device, its output current is determined uniquely, 
and it is not possible to generate variable levels of output current. A 
circuit disclosed in FIG. 4 of the above-mentioned reference, is reported 
to provide a low v/c conversion factor without using a high value 
resistor, by relying on a biasing current I.sub.E and a resistor R.sub.E 
so as to make the v/c conversion factor low; however, this circuit can 
produce only very low levels of current. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a voltage/current 
conversion circuit that enables to generate variable levels of output 
current, without using a high value resistor element. In place of a 
resistor, a transistor is used in the present circuit, either a metal 
oxide semiconductor field-effect transistor, known as a MOSFET, or a 
bipolar transistor for generating the voltage/current conversion 
relationship. 
Specifically, the present invention selects a first means related to a 
voltage/current conversion circuit comprising: a first active element 
functioning as a voltage/current conversion resistor; a comparator having 
a first input terminal connected to a voltage input terminal and a second 
input terminal connected to an output terminal of the first active 
element; a second active element having a control terminal connected to an 
output terminal of the comparator and an input terminal connected to the 
output terminal of the first active element so as to function as an active 
load; a third active element having an input terminal connected to a power 
input terminal, and an output terminal and a control terminal connected to 
an output terminal of the second active element; and a fourth active 
element having an input terminal connected to a power input terminal, a 
control terminal connected to the control terminal of the third active 
element, and an output terminal connected to a current output terminal so 
as to provide a flow of electrical current whose magnitude is proportional 
to a magnitude of electrical current flowing in the third active element. 
A second means related to the voltage/current conversion circuit is 
selected so that the first, second, third and fourth active elements, in 
the first means, correspond, respectively, to first, second, third and 
fourth transistors. 
A third means related to the voltage/current conversion circuit is selected 
so that the first transistor, in the second means, is a first n-type metal 
oxide semiconductor field-effect transistor, known as an n-type MOSFET, 
the second transistor is a second n-type MOSFET, the third transistor is a 
first p-type MOSFET and the fourth transistor is a second p-type MOSFET; 
wherein the input terminal corresponds to a source terminal in the first 
n-type MOSFET, in the second n-type MOSFET, in the first p-type MOSFET and 
in the second p-type MOSFET; the output terminal corresponds to a drain 
terminal in the first n-type MOSFET, in the second n-type MOSFET, in the 
first p-type MOSFET and in the second p-type MOSFET; and the control 
terminal corresponds to a gate terminal in the first n-type MOSFET, in the 
second n-type MOSFET, in the first p-type MOSFET and in the second p-type 
MOSFET. 
A fourth means related to the voltage/current conversion circuit is 
selected so that the comparator, in the first means, is an operational 
amplifier, wherein a first input terminal is a positive input terminal and 
a second terminal is a negative input terminal. 
A fifth means related to the voltage/current conversion circuit is selected 
so that the comparator, in the third means, is an operational amplifier, 
wherein a first input terminal is a positive input terminal and a second 
terminal is a negative input terminal. 
A sixth means related to the voltage/current conversion circuit is selected 
so that the first transistor, in the second means, is an n-p-n bipolar 
transistor whose emitter terminal is connected to a resistor, and the 
second transistor is an n-type MOSFET, the third transistor is a first 
p-type MOSFET and the fourth transistor is a second p-type MOSFET; wherein 
the input terminal corresponds to a source terminal in the n-type MOSFET, 
in the first p-type MOSFET and in the second p-type MOSFET; the output 
terminal corresponds to a gate terminal in the n-type MOSFET, in the first 
p-type MOSFET and in the second p-type MOSFET; and the control terminal 
corresponds to a drain terminal in the n-type MOSFET, in the first p-type 
MOSFET and in the second p-type MOSFET. 
A seventh means related to the voltage/current conversion circuit is 
selected so that, in the sixth means, the comparator is an operational 
amplifier, wherein a first input terminal is a positive input terminal and 
a second terminal is a negative input terminal. 
An eighth means related to the voltage/current conversion circuit is 
selected so that a voltage/current conversion circuit comprises: a first 
active element functioning as a voltage/current conversion resistor; a 
comparator having a first input terminal connected to a voltage input 
terminal and a second input terminal connected to an output terminal of 
the first active element; a second active element having a control 
terminal connected to an output terminal of the comparator, and an input 
terminal connected to the output terminal of the first active element so 
as to function as an active load; a third active element having an output 
terminal and a control terminal connected to an output terminal of the 
second active element; and a fourth active element having a control 
terminal connected to the control terminal of the third active element, 
and an output terminal connected to a current output terminal so as to 
provide a flow of electrical current whose magnitude is proportional to a 
magnitude of electrical current flowing in the third active element. 
A ninth means related to the voltage/current conversion circuit is selected 
so that the first, second, third and fourth active elements, in the eighth 
means, correspond, respectively, to first, second, third and fourth 
transistors. 
A tenth means related to the voltage/current conversion circuit is selected 
so that the first transistor, in the ninth means, is a first p-type metal 
oxide semiconductor field-effect transistor, known as an n-type MOSFET, 
the second transistor is a second p-type MOSFET, the third transistor is a 
first n-type MOSFET and the fourth transistor is a second n-type MOSFET; 
wherein the input terminal corresponds to a source terminal in the first 
p-type MOSFET, in the second p-type MOSFET, in the first n-type MOSFET and 
in the second n-type MOSFET; the output terminal corresponds to a drain 
terminal in the first p-type MOSFET, in the second p-type MOSFET, in the 
first n-type MOSFET and in the second n-type MOSFET; and the control 
terminal corresponds to a gate terminal in the first p-type MOSFET, in the 
second p-type MOSFET, in the first n-type MOSFET and in the second n-type 
MOSFET. 
An eleventh means related to the voltage/current conversion circuit is 
selected so that the comparator, in the eighth means, is an operational 
amplifier, wherein a first input terminal is a positive input terminal and 
a second terminal is a negative input terminal. 
An twelfth means related to the voltage/current conversion circuit is 
selected so that the comparator, in the tenth means, is an operational 
amplifier, wherein a first input terminal is a positive input terminal and 
a second terminal is a negative input terminal. 
A thirteenth means related to the voltage/current conversion circuit is 
selected so that the first transistor, in the ninth means, is a p-n-p 
bipolar transistor whose emitter terminal is connected to a resistor, the 
second transistor is a p-type metal oxide semiconductor field-effect 
transistor, known as a p-type MOSFET, the third transistor is a first 
n-type MOSFET and the fourth transistor is a second n-type MOSFET; wherein 
the input terminal corresponds to a source terminal in the p-type MOSFET, 
in the first n-type MOSFET and in the second n-type MOSFET; the output 
terminal corresponds to a drain terminal in the p-type MOSFET, in the 
first n-type MOSFET and in the second n-type MOSFET; and the control 
terminal corresponds to a gate terminal in the p-type MOSFET, in the first 
n-type MOSFET and in the second n-type MOSFET. 
A fourteenth means related to the voltage/current conversion circuit is 
selected so that the comparator, in the thirteenth means, is an 
operational amplifier, wherein a first input terminal is a positive input 
terminal and a second terminal is a negative input terminal. 
The present invention thus achieved the object of providing a 
voltage/current conversion circuit, suitable for IC fabrication processes, 
which is applicable to a variety of integrated circuit devices. The 
important concept behind the various approaches is that, by replacing a 
single-valued component (i.e. a high value resistor) in the conventional 
v/c conversion circuit with a multi-valued transistor circuit, the present 
invention has provided a v/c conversion circuit that can be fabricated by 
normal standard IC fabrication processes, and whose output current can be 
adjusted to a high level and controlled to within a small range of output 
current. Such a circuit should find ready uses in VCR controls?, CCD 
biasing? and other related applications that require fine current 
adjustments within a relatively high range of output current.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following, preferred embodiments of the v/c conversion circuits will 
be presented with reference to the drawings. 
A first embodiment is shown in FIG. 1. The positive input terminal of the 
OP amp 2 is connected to the voltage input terminal 1a of the v/c 
conversion circuit, the negative input terminal to the node 1b, and the 
output terminal to the gate terminal of the n-type MOSFET 5. The source 
terminal of the n-type MOSFET 5 is connected to the node 1b (negative 
input terminal of OP amp 2). The n-type MOSFET 6, which replaces a 
resistor in the conventional v/c conversion circuit, has its drain 
terminal connected to the node 1b, its gate terminal to the bias input 
terminal 7, and its source terminal to ground. The p-type MOSFET 3 and the 
p-type MOSFET 4 constitute a current mirror, and the source terminals of 
the p-type MOSFET 3 and the p-type MOSFET 4 are connected to their 
respective input power terminals. The gate terminal and the drain terminal 
of the p-type MOSFET 3 and the gate terminal of the p-type MOSFET 4 are 
connected to the drain terminal of the p-type MOSFET 5. The drain terminal 
of the p-type MOSFET 4 is connected to the current output terminal 8 of 
the v/c conversion circuit. 
Denoting A as the gain of the OP amp 2, Va as the potential at the voltage 
input terminal 1a and Vb as the potential at the node 1b, the output 
potential of the OP amp 2 (gate potential of the n-type MOSFET 5) is given 
by A.multidot.(Va-Vb). The drain current Id of the n-type MOSFET 5 is 
given by equation 1: 
##EQU1## 
where Vtn is a threshold voltage value of the n-type MOSFET 5, and Kn is 
an empirical constant determined by the transistor size and fabrication 
process. 
The resistance Rds between the source and drain of the n-type MOSFET 6 can 
be expressed by equation 2: 
EQU Id=Vb/Rds Eqn. 2 
From equations 1 and 2, equations 3 and 4 can be derived: 
EQU Vb/Rds=Kn.multidot.A.multidot.{Va-Vb-(Vb+Vtn)/A}!.sup.2 Eqn. 3 
EQU {Va-Vb-(Vb+Vtn)/A}.sup.2 =Vb/(Rds.multidot.Kn.multidot.A.sup.2)Eqn. 4 
If it can be assumed that the gain A of the OP amp 2 is sufficiently 
greater than the potential Vb, then the following equation can be derived: 
EQU (Va-Vb).sup.2 .apprxeq.0, leading to 
EQU Va.apprxeq.Vb Eqn. 5 
Because of the action of the OP amp 2 in conjunction with n-type MOSFET 5 
and n-type MOSFET 6, the potential Vb at the node 1b always follows the 
potential Va at the voltage input terminal 1a. If the potential at the 
voltage input terminal 1a is Va=Vin, then the potential at the node 1b 
will also be Vb=Vin. 
Because the current does not flow in the gate terminal of MOSFETs, the 
magnitude of the current flowing in the n-type MOSFET 5 and in the n-type 
MOSFET 6 is always equal to that of the current flowing in p-type MOSFET 
3. Also, because the p-type MOSFET 3 and the p-type MOSFET 4 form a 
current mirror, the potentials between the gate/source terminals in both 
transistors are always equal to each other, and if the transistor sizes 
are the same, the magnitudes of the current flowing in transistors 3 and 4 
are the same. In other words, the output current Iout flowing out of the 
current output terminal 8 always equals the current flowing in the n-type 
MOSFET 6. 
Thus, the voltage- current characteristics of the input voltage Vin at the 
voltage input terminal 1a to the output current Iout at the current output 
terminal 8 is the same as those between the potential Vb at the node 1b 
and the drain current Id of the n-type MOSFET 6. 
The voltage-current characteristic curve of the n-type MOSFET 6 is shown in 
FIG. 7. This relation is directly applicable to that for the input voltage 
Vin vs. output current Iout of the first embodiment v/c conversion 
circuit. 
When the potential at the bias input terminal 7 (gate potential of n-type 
MOSFET 6) is constant and the n-type MOSFET 6 is operating in its 
saturation region, changes in the drain current Id are small with respect 
to the changes in the potential Vb at the node 1b, and the same 
voltage-current characteristics, as the voltage-current characteristics 
based on a high value resistor element, are obtained. 
The slope of a voltage-current curve of a MOSFET operating in the 
saturation range is affected by the degree of channel length modulation 
effect, so that by increasing the gate length of MOSFET 6 so as to 
decrease the channel length modulation effect, it is possible to obtain a 
circuit having a lesser slope, i.e., a v/c conversion circuit exhibiting a 
low voltage/current conversion ratio. 
Furthermore, by changing the potential at the bias input terminal 7, it is 
possible to change the level of the drain current, with negligible change 
in the slope of the curve, thereby enabling to freely select any level of 
output current. 
FIG. 2 presents a second embodiment of the v/c conversion circuit. This 
circuit is obtained by replacing the n-type MOSFET 6 in the first 
embodiment circuit, shown in FIG. 1, with an n-p-n bipolar transistor 16 
and a resistor element 19. 
The n-p-n bipolar transistor 16 has its collector terminal connected to the 
node 1b, and the base terminal to the bias input terminal 7. One terminal 
of the resistor element 19 is connected to the emitter terminal of the 
n-p-n bipolar transistor 16 and the other terminal is grounded. 
As in the first embodiment circuit, the potential Va at the voltage input 
terminal 1a and the potential Vb at the node 1b are equal, and the 
collector current Ic flowing in the n-p-n bipolar transistor 16 is equal 
to the output current Iout flowing out from the current output terminal 8. 
Therefore, the characteristic curve of the input voltage Vin (potential at 
the voltage input terminal 1a) vs. output current Iout of the v/c 
conversion circuit, shown in FIG. 2, is the same as the voltage-current 
curve for the potential Vb at the node 1b vs. collector current Ic of the 
n-p-n bipolar transistor 16. 
FIG. 8 shows a voltage-current curve of the n-p-n bipolar transistor 16 
having a resistor element 19 connected between the emitter and the ground. 
The slope of the voltage-current curve in the saturation range is affected 
by the value of the resistor element 19 connected between the emitter and 
the ground. By increasing the value of the resistor element 19, the slope 
decreases so that a v/c conversion circuit having a low voltage/current 
conversion ratio can be realized. 
Although the aspect of the circuit that the resistor value determines the 
voltage/current conversion ratio is the same as in the conventional v/c 
conversion circuit, it should noted that the present conversion circuit 
shown in FIG. 2 differs from the conventional conversion circuit in the 
aspect that the resistor 19 is utilized in a series-arrangement with an 
n-p-n bipolar transistor 16, it means that a low voltage/current 
conversion ratio can be achieved with a value of the resistor element 19 
small enough (several tens of ohms to several hundreds of ohms) to be 
produced within the capability of ordinary IC fabrication processes. In 
other words, the v/c conversion circuit of the second embodiment can be 
produced without increasing neither the number of processing steps nor the 
substrate area. 
Similar to the first embodiment circuit, by changing the potential of the 
bias input terminal 7 (base current in the n-p-n bipolar transistor 16) 
the output current Iout can-be freely selected, independent of the v/c 
conversion factor. 
A third embodiment will be presented with reference to FIG. 3. The positive 
input terminal of the OP amp 22 is connected to the voltage input terminal 
21a of the v/c conversion circuit, the negative input terminal to the node 
21b, and the output terminal to the gate terminal of the p-type MOSFET 25. 
The source terminal of the p-type MOSFET 25 is connected to the node 21b 
(negative input terminal of the OP amp 22). For the p-type MOSFET 26, 
which replaces the function of a conventional v/c conversion resistor, the 
drain terminal is connected to the node 21b, and the gate terminal to the 
bias terminal 27, and the source terminal to a power source. The n-type 
MOSFET 23 and the n-type MOSFET 24 constitute a current mirror, and the 
source terminals of the n-type MOSFET 23 and the n-type MOSFET 24 are 
respectively grounded, the gate and drain terminals of the n-type MOSFET 
23 and the gate terminal of the n-type MOSFET 24 are connected to the 
drain terminal of the p-type MOSFET 25. The drain terminal of the n-type 
MOSFET 24 is connected to the output current terminal 28 of the v/c 
conversion circuit. 
Denoting A as the gain of the OP amp 22, Va as the potential at the voltage 
input terminal 21a and Vb as the potential at the node 21a, the output 
potential of the OP amp 22 (gate potential of the p-type MOSFET 25) is 
given by A.multidot.(Vb-Va). The drain current Id of the p-type MOSFET 25 
is given by equation 6: 
##EQU2## 
where Vtp is a threshold voltage value of the p-type MOSFET 25, and Kp is 
an empirical constant determined by the transistor size and fabrication 
process. 
Denoting VDD as the voltage at the power source terminal, Rds as the 
resistance between the source and drain of the p-type MOSFET 26, the drain 
current Id is expressed by equation 7: 
EQU Id=(VDD-Vb)/Rds Eqn. 7 
From equations 6 and 7, the following equation 8 can be derived: 
EQU (VDD-Vb)/Rds=Kp.multidot.A.multidot.{Vb-Va-(Vb+Vtp)/A}!.sup.2 
EQU {Vb-Va-(Vb+Vtp)/A}.sup.2 =(VDD-Vb)/(Rds.multidot.Kp.multidot.A.sup.2)Eqn. 8 
If it is assumed that the gain A of the OP amp 22 is sufficiently greater 
than the potential (VDD-Vb) and (Vb+Vtp), then the following equation can 
be derived: 
EQU (Vb-Va).sup.2 .apprxeq.0, leading to Eqn. 9 
EQU Va.apprxeq.Vb 
Because of the action of the OP amp 22 in conjunction with p-type MOSFET 25 
and the p-type MOSFET 26, the potential Vb at the node 21b always follows 
the potential Va at the voltage input terminal 21a. If the potential at 
the voltage input terminal 21a is Va=Vin, then the potential at the node 
21b will also be Vb=Vin. 
Because the current does not flow in the gate terminal of MOSFETs, the 
values of the current flowing in the p-type MOSFET 25 and the p-type 
MOSFET 26 are always equal to the value of the current flowing in n-type 
MOSFET 23. Also, because the n-type MOSFET 23 and the n-type MOSFET 24 
form a current mirror, the potentials between the gate/source terminals in 
both transistors 23, 24 are always equal to each other, and if the 
transistor sizes are the same, the magnitude of the current flowing in the 
two transistors 23 is the same as that flowing in the transistor 24. In 
other words, the output current Iout flowing out of the current output 
terminal 28 equals the current flowing in the p-type MOSFET 26. 
Thus, the voltage-current characteristics of the input voltage Vin at the 
voltage input terminal 21a to output current Iout at the current output 
terminal 28 are the same as those between the potential Vb at the node 21b 
and the drain current Id of the p-type MOSFET 26. 
The voltage-current characteristic curve of the p-type MOSFET 26 is shown 
in FIG. 9. This relation is directly applicable to that for the input 
voltage Vin vs. output current Iout of the third embodiment v/c conversion 
circuit. 
When the potential at the bias input terminal 27 (gate potential of p-type 
MOSFET 26) is constant and the p-type MOSFET 26 is operating in its 
saturation region, changes in the drain current Id are small with respect 
to the changes in the potential Vb at the node 21b, and the same 
voltage-current relationship, as the voltage-current relationship based on 
a high value resistor element, is obtained. 
The slope of the voltage-current curve of a MOSFET operating in the 
saturation range is affected by the degree of channel length modulation 
effect, so that by increasing the gate length of MOSFET 26 so as to 
decrease the channel length modulation effect, it is possible to obtain a 
circuit having a lesser slope, i.e., a v/c conversion circuit exhibiting a 
low voltage/current conversion ratio. 
Furthermore, by changing the potential at the bias input terminal 27, it is 
possible to change the level of the drain current, with negligible change 
in the slope of the curve, thereby enabling to freely select a desired 
level of output current. 
A fourth embodiment will be presented with reference to FIG. 4. The fourth 
embodiment conversion circuit is obtained by replacing the p-type MOSFET 
26 in the third embodiment, shown in FIG. 3, with a p-n-p bipolar 
transistor 36 and a resistor element 39. 
The n-p-n bipolar transistor 36 has its collector terminal connected to the 
node 21b, and the base terminal to the bias input terminal 27. One 
terminal of the resistor element 39 is connected to the emitter terminal 
of the n-p-n bipolar transistor 39 and the other terminal is connected to 
a power source. 
As in the third embodiment circuit, the potential Va at the input terminal 
21a and the potential Vb at the node 21b are equal, and the collector 
current Ic flowing in the n-p-n bipolar transistor 36 is equal to the 
output current Iout flowing out from the current output terminal 28. 
Therefore, the characteristic curve of the input voltage Vin vs. output 
current Iout of the v/c conversion circuit of the fourth embodiment is the 
same as that of the potential Vb at the node 21b vs. collector current Ic 
in the n-p-n bipolar transistor 36. 
FIG. 10 shows a voltage-current curve of the n-p-n bipolar transistor 36 
having a resistor element 39 connected between the emitter and the power 
input terminal. The slope of the voltage-current curve in the saturation 
range is affected by the value of the resistor element 39 connected 
between the emitter and the power input terminal. By increasing the value 
of the resistor element 39, the slope decreases so that a v/c conversion 
circuit having a low voltage/current conversion ratio can be realized. 
Although the aspect of the circuit that the resistor value determines the 
voltage/current conversion ratio is the same as in the conventional v/c 
conversion circuit, it should noted that the present conversion circuit 
shown in FIG. 4 differs from the conventional conversion circuit, because 
the v/c conversion resistor 39 is utilized in a series-arrangement with an 
n-p-n bipolar transistor 36, it means that a low voltage/current 
conversion ratio can be achieved with a value of the resistor element 39 
small enough (several tens of ohms to several hundreds of ohms) to be 
produced within the capability of ordinary IC fabrication processes. In 
other words, the v/c conversion circuit of the second embodiment can be 
produced without increasing neither the number of processing steps nor the 
substrate area. 
Similar to the third embodiment circuit, by changing the potential of the 
bias input terminal 27 (base current in the n-p-n bipolar transistor 26) 
the level of the output current Iout can be freely selected, independent 
of the voltage/current conversion ratio.