Voltage to current converting circuit

A voltage to current converting circuit can operate at low voltages and throughout a high range of voltages without distortion of the output signal current. An input-output current mirror circuit has a current input transistor and a current output transistor having their bases connected. The collector of the current input transistor is connected to an input resistor which accepts the input signal voltage. The collector of the current input transistor is also connected to the bases of the current input and output transistors through a non-inverting current amplifier having a low input impedance. The collector of the current input transistor is further connected to a source of a constant DC reference current I having a high output impedance. The input signal voltage e is converted to a current signal by the input resistor and applied to the collector of the current input transistor. The resulting current I-i which appears at the collector of the current output transistor is used by an output circuit to develop a first output signal of I-i. The output circuit also generates an intermediate current of 2I and provides a second output signal of I+i obtained by subtracting I-i from the intermediate current 2I.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a voltage to current converting circuit 
and, more particularly, to a voltage to current converting circuit capable 
of operation over a wide range with low distortion and low power 
requirements. 
2. Description of the Prior Art 
A typical voltage to current converting circuit uses a differential 
amplifier to convert a signal voltage e into a balanced signal current. 
One such prior art circuit simply connects the ends of the signal source to 
the inputs of a differential amplifier and takes the current in one of the 
outputs of the amplifier as the signal current. This circuit is only 
useful for signal voltages having a maximum range of 10 mv. Signals with 
voltages having a greater range cause distortion of the output signal that 
is unacceptable for most applications. 
One solution to that problem uses one or more current sources and resistors 
connected to the emitters of the differential amplifier transistors. The 
maximum range of the circuit is increased, but so is the power required 
for its operation. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a voltage to current 
converting circuit that overcomes the shortcomings of the prior art. 
It is another object of the present invention to provide a voltage to 
current converting circuit that is capable of handling an input signal 
with a wide range while having low power requirements. 
It is a further object of the present invention to provide such a voltage 
to current converting circuit that can be fabricated as an integrated 
circuit. 
In accordance with an aspect of the present invention, a voltage to current 
converting circuit for converting an input signal voltage at an input 
terminal into an output signal current comprises input-output current 
mirror means including a current input transistor and at least one current 
output transistor having the base thereof connected to the base of the 
current input transistor and the collector thereof being an output for 
connection to an output means for developing the output signal current, 
and an input resistor connected between the input terminal and the 
collector of the current input transistor. A noninverting current 
amplifier has an input connected to the collector of the current input 
transistor and an output connected to the bases of the current input and 
output transistors. A constant reference current source is connected to 
the collector of the current input transistor for providing a 
substantially constant reference current thereto. 
Those and other objects, features and advantages of the present invention 
will be apparent from the detailed description of preferred embodiments of 
the invention, which follows.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention will be better understood by first gaining an 
appreciation of the shortcomings of known voltage to current converting 
circuits. 
FIG. 1 shows a simple voltage to current converting circuit that can be 
fabricated as or part of an integrated circuit. The converting circuit 
includes differentially connected bipolar transistors Q.sub.R and Q.sub.L. 
The commonly connected emitters of the transistors Q.sub.R and Q.sub.L are 
connected to a constant current source 10 that provides a current I. The 
base of the transistor Q.sub.L receives one end of a signal source 20 and 
the base of the transistor Q.sub.R receives the other end of the signal 
source 20. The signal source 20 provides an input signal voltage e. The 
collectors of the transistors Q.sub.R and Q.sub.L are connected to a 
reference potential +V.sub.cc at a reference terminal T.sub.2. A load 
resistor R.sub.L is connected between the collector of the transistor 
Q.sub.R and the reference terminal T.sub.2. An output terminal T.sub.0 is 
located between the collector of the transistor Q.sub.R and the load 
resistor R.sub.L. An output signal current I-i appears at the output 
terminal T.sub.0 in accordance with the input signal voltage e. 
The principal shortcoming of the converting circuit of FIG. 1 is in the 
small range of variation in the input signal voltage e that it can 
tolerate. If the potential of the signal e varies more than 10 mV, the 
output signal current will be distorted. Thus, the circuit shown in FIG. 1 
is suitable only for input signals having a small range. 
The prior art converting circuits shown in FIGS. 2 and 3 have been proposed 
to solve that particular problem. They include the same elements as the 
converting circuits shown in FIG. 1. However, they also include, in the 
case of FIG. 2, two emitter resistors R.sub.1 and R.sub.2 connected 
between the current source 10 and, respectively, the emitters of the 
transistors Q.sub.L and Q.sub.R. In FIG. 3, two current sources 10.sub.L 
and 10.sub.R are connected directly to the emitters of the transistors 
Q.sub.L and Q.sub.R, respectively. A resistor R.sub.0 is connected between 
the emitters of the transistors Q.sub.L and Q.sub.R. 
With the differential amplifiers shown in FIGS. 2 and 3, distortion in the 
output signal current will be eliminated if the DC and AC potentials 
across the emitter resistors, either R.sub.0, or R.sub.1 and R.sub.2, are 
maintained at levels exceeding the input signal voltage e. For example, if 
the circuit is to handle input signal voltages up to twice the 
peak-to-peak voltage, then the potentials at the collectors of the 
transistors Q.sub.R and Q.sub.L must be at least 2.5 volts. In that case 
the circuit component that is to use the output signal current must then 
have a power source that supplies a voltage of at least 3 volts. 
Therefore, although they provide a distortion-free output signal 
throughout a higher range of input signals, the circuits shown in FIGS. 2 
and 3 require relatively high voltages for operation. 
FIG. 4 depicts a voltage to current converting circuit in accordance with 
the present invention. The circuit in FIG. 4 comprises a current input 
transistor Q.sub.1 and a current output transistor Q.sub.2 that have their 
bases connected together. The emitters of the transistors Q.sub.1 and 
Q.sub.2 are connected to a reference terminal T.sub.2 so as to have a 
reference potential +V.sub.cc applied thereto. The collector of the 
current input transistor Q.sub.1 is connected to the input terminal 
T.sub.1, which receives the input signal e, and to a reference-current 
transistor Q.sub.3. The base of the referencecurrent transistor Q.sub.3 is 
connected to the base of an intermediate-current transistor Q.sub.4. The 
commonly connected bases of the transistors Q.sub.3 and Q.sub.4 are 
connected to a DC bias voltage source V.sub.3. The emitters of the 
transistors Q.sub.3 and Q.sub.4 are connected to ground through 
reference-current and intermediate-current resistors R.sub.3 and R.sub.4, 
respectively. The emitters of the transistors Q.sub.3 and Q.sub.4 thus are 
disposed to have reference potentials applied thereto depending on the 
resistors R.sub.3 and R.sub.4. 
As shown in FIG. 4, the collectors of the transistors Q.sub.1 and Q.sub.3 
receive the input signal voltage e through an input resistor R.sub.10. A 
resistor R.sub.11 is connected between the transistor Q.sub.1 and the 
reference terminal T.sub.2 and a capacitor is connected as shown in FIG. 4 
in parallel with the resistor R.sub.11 to damp oscillations at the 
collector of the transistor Q.sub.1. A resistor R.sub.12 is connected 
between the reference terminal T.sub.2 and the collector of the transistor 
Q.sub.2. 
A differential amplifier 11, an equalizing current mirror 13 and a resistor 
R.sub.14 are connected as shown to construct a non-inverting current 
amplifier 15. More particularly, the differential amplifier 11 comprises 
an amplifier input transistor Q.sub.11 having its emitter connected to the 
emitter of an amplifier output transistor Q.sub.12. The commonly connected 
emitters of the transistors Q.sub.11 and Q.sub.12 are grounded through the 
amplifier resistor R.sub.14. It will be understood throughout this 
description that the connections to ground can be made through a single 
ground terminal, although for the sake of clarity, FIG. 4 shows a 
plurality of individual connections to ground. The collectors of the 
amplifier transistors Q.sub.11 and Q.sub.12 are connected to the 
collectors of the equalizing input and output transistors Q.sub.13 and 
Q.sub.14, respectively. The emitters of the transistors Q.sub.13 and 
Q.sub.14 are connected to the reference terminal T.sub.2 and the bases of 
the transistors Q.sub.13 and Q.sub.14 are connected. The collector of the 
transistor Q.sub.13 is connected to its base so that the transistor 
Q.sub.13 comprises the input transistor of the equalizing current mirror 
13. As is well known, if the input and output transistors Q.sub.13 and 
Q.sub.14 of the current mirror 13 are complementary, that is, have the 
same effective emitter area, then the collector currents of the 
transistors Q.sub.13 and Q.sub.14 will differ only by the negligible 
amount 2I.sub.B `the base current of the transistors`. In any event, the 
commonly connected collectors of the transistors Q.sub.12 and Q.sub.14 are 
connected to the commonly connected bases of the current input and output 
transistors Q.sub.1 and Q.sub.2. 
FIG. 5 depicts schematically functional equivalents of portions of the 
circuit shown in FIG. 4 which have thus far been described. The current 
input and output transistors Q.sub.1 and Q.sub.2 comprise an input-output 
current mirror means 1 in which the base and collector of the input 
transistor, here the current input transistor Q.sub.1, are connected 
through a non-inverting current amplifier 15. Or, if that connection is 
considered in relation to the amplifier 15, the amplifier 15 is connected 
with 100% negative current feed-back. The input impedance of the amplifier 
15 is thus negligible relative to the resistance r.sub.10 the input 
resistance R.sub.10 and can be disregarded. 
The resistors R.sub.3 and R.sub.4 are provided with properties such that if 
the DC current at the collector of the reference-current transistor 
Q.sub.3 is I, the DC current at the collector of the intermediate-current 
transistor Q.sub.4 is 2I. The transistor Q.sub.3 thus functions as a 
constant reference current source and the output impedance at its 
collector is relatively high. Thus, the current at the collector of the 
input transistor Q.sub.1 is I-i, where i is the current through the input 
resistor R.sub.10, as shown in FIG. 4. Because transistors Q.sub.1 and 
Q.sub.2 are connected in the inputoutput current mirror means 1, the 
collector current at the current output transistor Q.sub.2 is also I-i. 
The collector of that transistor is thus an output that provides a current 
to an output circu:t to generate a balanced output signal of I+i and I-i. 
The output circuit shown in FIG. 5 represents various components in FIG. 4. 
In particular, the various current mirrors 21, 24 and 26 and the first and 
second current supplying circuits 30 and 40 comprise an output stage of 
the output means 50 represented schematically in FIG. 5. As will be 
apparent as this description proceeds, the output means also can be 
considered to include the intermediate-current source comprised of the 
transistor Q.sub.4 and the resistor R.sub.4. 
A primary current mirror means 21 includes a transistor Q.sub.21 as a 
primary input transistor. The primary current mirror 21 also includes a 
primary output transistor Q.sub.22 and a secondary output transistor 
Q.sub.23, which have their bases connected to the base of the transistor 
Q.sub.21, and the resistors R.sub.21, R.sub.22 and R.sub.23 connect the 
respective emitters of the transistors Q.sub.21, Q.sub.22 and Q.sub.23 to 
ground. The bases of the transistors Q.sub.21, Q.sub.22 and Q.sub.23 are 
connected to the collector of the primary input transistor Q.sub.21. 
A feeding current mirror means 24 includes a feeding input transistor 
Q.sub.24 that has its base and collector connected. The collector of the 
transistor Q.sub.24 is also connected to the collector of the 
intermediate-current transistor Q.sub.4. A feeding output transistor 
Q.sub.25 has its base connected to the base of the transistor Q.sub.24 and 
its collector connected to the collector of the secondary output 
transistor Q.sub.23. The emitters of the feeding transistors Q.sub.24 and 
Q.sub.25 are connected through resistors R.sub.24 and R.sub.25, 
respectively, to the reference terminal T.sub.2. 
A subtracting current mirror means 26 comprises a subtracting input 
transistor Q.sub.26 that has its collector connected to the collectors of 
the transistors Q.sub.23 and Q.sub.25 and to its base. The base of the 
transistor Q.sub.26 is connected to the base of a subtracting output 
transistor Q.sub.27. The emitters of the transistors Q.sub.26 and Q.sub.27 
are connected to ground through the resistors R.sub.26 and R.sub.27, 
respectively. 
A first current supplying circuit 30 is connected between the reference 
terminal T.sub.2 and the collector of the primary output transistor 
Q.sub.22. A second current supplying circuit 40 is connected between the 
reference terminal T.sub.2 and the collector of the subtracting output 
transistor Q.sub.27. The current supplying circuits supply the output 
current signal to the circuit that is going to use it. 
The voltage to current converting circuit provides first and second outputs 
I+i and I-i as follows. As previously described, the current I-i flows in 
the collector of the transistor Q.sub.2 when an input signal e is present 
at the input terminal T.sub.1. That current I-i thus also flows in the 
collector of the transistor Q.sub.21 and thus in the collectors of the 
transistors Q.sub.22 and Q.sub.23. The collector of the primary output 
transistor Q.sub.22 provides that current as the first output to the first 
current supplying circuit 30. From the circuit 30, the first output can be 
provided to another circuit. 
The current supplying circuits 30 and 40 are provided as shown in FIG. 4 
when the voltage to current converting circuit is fabricated as an 
integrated circuit. By connecting them to the reference terminal and 
making them part of the same integrated circuit, the entire integrated 
voltage to current converting circuit is more easily and conveniently 
connected to the apparatus that is to utilize its outputs. Of course, the 
outputs could be taken directly at the collectors of the transistors 
Q.sub.22 and Q.sub.27 and the circuits 30 and 40 could be omitted. 
It will also be recalled that, because of the values of the resistors 
R.sub.3 and R.sub.4, the current 2I is present at the collector of the 
transistor Q.sub.4. The feeding current mirror 24 provides that same 
current in the collector of the secondary output transistor Q.sub.25. 
Since the current I-i is present at the collector of the transistor 
Q.sub.23, the current I+i [2I-(I-i)] is present at the collector of the 
transistor Q.sub.26. Thus, the current I+i is present at the collector of 
the subtracting output transistor Q.sub.27. The collector of the 
subtracting output transistor Q.sub.27 provides that current to the 
current supplying circuit 40. 
In accordance with the present invention the current i is given by the 
equation i=e/r10, and thus the input signal voltage e can be increased 
arbitrarily. Furthermore, since the amplifier 15 has 100% negative 
feedback, distortion in the output current is extremely low. 
Moreover, the input voltage e is converted to the signal current i by the 
resistor R.sub.10. The operation of the voltage to current converting 
circuit of the present invention does not depend on the value of the 
predetermined reference potential +V.sub.cc. Experiments have shown that 
the present invention operates satisfactorily with the bias voltage 
provided by the source V.sub.3 =1 volt and the predetermined reference 
potential +V.sub.cc applied to the terminal T.sub.2 at 1.8 volts. 
It is also possible with the present invention to increase the number of 
output stages. Additional output transistors for the current mirrors 21 
and 26, additional feeding current mirrors 24 and current supplying 
circuits 30 and 40 can be provided to generate additional outputs. It is 
also possible to provide additional current output transistors connected 
to the base of the transistor Q.sub.2 when additional outputs are desired. 
The configuration of the present invention also permits fabrication of the 
entire circuit as an integrated circuit with the desired number of output 
stages. In addition, maxiumum flexibility is possible because the current 
generating portion of the circuit can be fabricated as one integrated 
circuit with the capability of accepting different output stage 
configurations also formed as integrated circuits. 
The present invention thus provides a voltage to current converting circuit 
that can convert input signal voltage e into an output signal current over 
a large range of input voltages, including low voltages, without 
distortion, and which is capable of having as many outputs as are required 
for a particular application. 
Although a specific embodiment of the invention has been described in 
detail herein with reference to the accompanying drawings, it is to be 
understood that the invention is not limited to that embodiment, and that 
various changes and modifications other than those specifically mentioned 
can be effected therein by one skilled in the art without departing from 
the scope or spirit of the invention as defined in the appended claims.