 Currenttovoltage converter

In electronics, a transimpedance amplifier is an amplifier that converts current to voltage. Its input ideally has zero impedance, and the input signal is a current. Its output may have low impedance, or in highfrequency applications, may be matched to a driven transmission line; the output signal is measured as a voltage.
Because the output is a voltage and the input is a current, the gain, or ratio of output to input, is expressed in units of ohms.
When constructed as a simple operational amplifier circuit (right), the gain is equal to the negative of feedback resistance.
Transimpedance amplifiers are commonly used in receivers for optical communications to convert the current generated by a photodetector into a voltage signal for further amplification.
Contents
 1 Application
 2 The basic idea behind the passive version
 3 Improvement: Active currenttovoltage converter
 4 See also
 5 References
 6 External links
Application
Transimpedance amplifiers are commonly used in receivers for optical communications. The current generated by a photodetector generates photo voltage, but in a nonlinear fashion. Therefore the amplifier has to prevent any large voltage by its low input impedance and generate either a 50 Ohm signal (considered low impedance by many) to drive a coaxial cable or a voltage signal for further amplification. But note that the most linear amplification is current amplification by a bipolar transistor, so you may want to amplify before the impedance conversion.
The basic idea behind the passive version
Nonelectrical domain: Flow causes pressure
In physical terms, there are many situations where a pressurelike quantity induces flow of a substance through an impediment. However, there are also corresponding situations where a flowlike quantity induces pressure at an impediment: mechanical (if one tries to stop a moving car with his body, the "flowing" car exerts pressure on him, the impediment), pneumatic (pinch a hose in the middle and you will see that a pressure appears at the pinch point).
In this arrangement, the flow, pressure, and impedimentlike attributes are interrelated. Usually, the output pressurelike variable is proportional to the input flowlike one; in this way, the flowlike quantity creates (is converted to) a pressurelike one.
To induce pressure, an impediment must be put in the way of a flowing quantity.
Electrical domain: Current causes voltage
Building the circuit. Similarly, in electricity, if a current I_{IN} flows through a resistor R (Fig. 2), the latter impedes (resists) the current; as a result, a proportional voltage drop V_{R} = R.I_{IN} appears across the resistor according to currentcausesvoltage formulation of Ohm's law (V = R.I). In this currentsupplied circuit, the voltage drop V_{R} acts as an output voltage V_{OUT} (the voltage drop V_{R} is created not by the resistor; it is created by the excitation voltage source inside the input current source). In this way, the current I_{IN} is converted to a proportional voltage V_{OUT}; the resistor R serves as a currenttovoltage converter  a linear circuit with transfer ratio k = V_{OUT}/I_{IN} [V/mA] having dimensions of resistance.
Circuit operation. Fig. 2 represents graphically the circuit operation by using a current loop and voltage bars. The thickness of the current loop is proportional to the magnitude of the current and the height of the voltage bars is proportional to the corresponding voltages (see also an interactive animation).
A graphoanalytical interpretation of the circuit (and of the Ohm's law) is shown on Fig. 3. As the current through and the voltage across the two components (the current source and the resistor) are the same, their IV curves are superimposed on a common coordinate system. The intersection of the two lines is the operating point A; it represents the present magnitudes of the current I_{A} and the voltage V_{A}. When the current I_{IN} of the input current source varies, its IV curve moves vertically (see also an interactive animation). As a result, the working point A slides over the IV curve of the resistor R; its slope represents the converter's ratio.
Fig. 4 shows another attractive graphical interpretation of Ohm's law  the voltage diagram (the voltage distribution along the resistive film inside a linear resistor). When the input current varies, the local voltages along the resistive film vary decreasing gradually from left to right (see also another interactive animation). In this arrangement, the angle α represents the input current I_{IN}.
Passive version applications
ItoV converter acting as an output device
Currentcontrolled voltage source. Although there are enough constant voltage sources in nature (primary and secondary batteries), if a current source is available but there is a need of a voltage source, it may be built. For this purpose, a currenttovoltage converter has to be connected after the current source, according to the building formula below:
Voltage source = Current source + Currenttovoltage converter
The simplest implementation of this idea is shown in Fig. 5 where a resistor R is connected in parallel to the input current source I_{IN} (the Norton's idea in electricity).
If the load is ideal (that is, it has an infinite resistance), a constant voltage V_{OUT} = R.I_{IN} will be generated. This voltage will affect the current, if the input current source is imperfect (see the section below about passive version imperfections).
Compound passive converters: Similarly, in the popular passive circuits of capacitive differentiator, inductive integrator, antilogarithmic converter, etc., the resistor acts as a currenttovoltage converter:
VtoV CR differentiator = VtoI C differentiator + ItoV converter
VtoV LR integrator = VtoI L integrator + ItoV converter
VtoV DR antilog converter = VtoI D antilog converter + ItoV
For example, a classic capacitiveresistive differentiator is built on Fig. 6 by using the simpler voltagetocurrent capacitive differentiator (a bare capacitor) and a currenttovoltage converter.
In these circuits, the resistor R acting as a currenttovoltage converter introduces some voltage drop V_{R}, which affects the excitation voltage V_{IN}. As a result, the current decreases and an error appears (see the section about passive version imperfections).
Transistor collector resistor. A transistor is a currentcontrolling device. Therefore, to obtain a voltage as an output, a collector resistor is connected in the output circuit of the transistor stage (Fig. 7). Examples of this technique are the commonemitter, commonbase and differential amplifier, a transistor switch, etc.
Voltageoutput transistor = Currentoutput transistor + ItoV converter
The transistor's collector resistor acts as a currenttovoltage converter.
Since the voltage drop V_{Rc} is floating, usually the complementary (to the power supply) voltage drop V_{CE} is used as an output. As a result, these transistor circuits are inverting (when the input voltage rises, the output voltage drops and v.v.)
A similar technique is used to obtain a voltage in the transistor emitter (see the section below about negative feedback current source). Examples of this technique are all the transistor circuits using series negative feedback.
The transistor's emitter resistor acts as a currenttovoltage converter.
ItoV converter acting as an input device
Compound ammeter. Today's measuring instruments (DVM's, analogtodigital converters, etc.) are mainly voltmeters. If there is a need to measure a current, a simple currenttovoltage converter (a shunt resistor) is connected before the voltmeter (Fig. 8). This ammeter is a composed device consisting of two components:
Compound ammeter = Currenttovoltage converter + voltmeter
The shunt resistor of a composed ammeter acts as a currenttovoltage converter.
Although the active version is the perfect current measurement solution, the popular multimeters use the passive version to measure big currents (see the section about power considerations below).
ItoV converter as a part of negative feedback VtoI converters
Negative feedback systems have the unique property to reverse the causality in the electronic converters connected in the feedback loop. Examples: an opamp noninverting amplifier is actually a reversed voltage divider, an opamp integrator is a reversed differentiator and v.v., an opamp logarithmic converter is a reversed antilogarithmic converter and v.v., etc.
Similarly, an opamp voltagetocurrent converter (a voltagecontrolled constant current source) built by using a negative feedback is actually a reversed currenttovoltage converter. This powerful idea is implemented on Fig. 9 (a transistor version of a current source) and on Fig. 10 (an opamp version of a current source) where a currenttovoltage converter (the bare resistor R) is connected in the negative feedback loop. The voltage drop V_{R} proportional to the load current I is compared with the input voltage V_{Z}. For this purpose, the two voltages are connected in series and their difference dV = V_{Z}  V_{R} is applied to the input part of the regulating element (the baseemitter junction of the transistor T or the differential input of the opamp OA). As a result, the regulating element establishes the current I = V_{R}/R ≈ V_{Z}/R by changing its output resistance so that to zero the voltage difference dV. In this way, the output current is proportional to the input voltage; the whole circuit acts as a voltagetocurrent converter.
Passive version imperfections
The passive currenttovoltage converter (as all the passive circuits) is imperfect because of two reasons:
Resistor R. The voltage drop V_{R} affects the input current I_{IN} as the resistor R consumes energy from the input source (Fig. 11). A contradiction exists in this circuit: from one side, the voltage drop V_{R} is useful as it serves as an output voltage; from the other side, this voltage drop is harmful as it effectively modifies the actual currentcreating voltage V_{Ri}. In this arrangement, the voltage difference V_{IN}  V_{R} determines the current instead the voltage V_{IN} (the resistor Ri actually acts as the opposite voltagetocurrent converter). As a result, the current decreases.
Load resistance. In addition, if the load has some finite resistance (instead of infinite resistance), a part of the current I_{IN} will diverts through it. As a result, both the current I_{IN} and the voltage V_{OUT} decrease. The problem is again that the load consumes energy from the passive circuit (click Imperfections in [1]).
Improvement: Active currenttovoltage converter
The basic idea behind the active version
Nonelectrical domain: Removing disturbance by equivalent "antidisturbance"
The active version of the currenttovoltage converter is based on a wellknown technique from human routine, where we compensate the undesirable effects caused by ourselves using equivalent "antiquantities". This idea is implemented by using an additional power source, which "helps" the main source by compensating the local losses caused by the internal undesired quantity (conversely, in the opposite active voltagetocurrent converter, the additional power source compensates the losses caused by the external quantity). Example: if we have broken our window in winter, we turn on a heater that compensates the thermal losses; and v.v., in summer, we turn on an airconditioner. More examples: if our car has come into collision with other car, the insurance company compensates the damages we caused to the other car. If we cause trouble to others, we apologize. If we spend money from an account, we deposit funds. (See virtual ground page for more examples.) In all these cases, we prepared "standby" resources to use if there is a need to compensate internal losses.
Electrical domain: Removing voltage by equivalent "antivoltage"
Electrical implementation. To show how this powerful basic idea is applied to improve the passive currenttovoltage converter, first, an equivalent electrical circuit is used (Fig. 12). In this active currenttovoltage converter, the voltage drop V_{R} across the internal resistor R is compensated by adding the same voltage V_{H} = V_{R} to the input voltage V_{IN} [2]. For this purpose, an additional following voltage source B_{H} is connected in series with the resistor. It "helps" the input voltage source; as a result, the undesired voltage V_{R} and the resistance R disappear (the point A becomes a virtual ground).
Active ItoV converter = passive ItoV converter + "helping" voltage source
Where to take an output from? The magnitude of the compensating quantity is frequently used to measure indirectly the initial quantity (an example  weighing by using scales). This idea is applied in the circuit of active currenttovoltage converter by connecting the load to the compensating voltage source B_{H} instead to the resistor. There are two advantages of this arrangement: first, the load is connected to the common ground; second, it consumes energy from the additional source instead from the input source. Therefore, it might possess small resistance.
Opamp implementation
The basic idea above is implemented in the opamp currenttovoltage converter (Fig. 13, 14) [3]. In this circuit, the output of the operational amplifier is connected in series with the input voltage source; the opamp's inverting input is connected to point A. As a result, the opamp's output voltage and the input voltage are summed.
From other viewpoint, the output of the operational amplifier is connected in series with the resistor R in the place of the compensating voltage source B_{H} from Fig. 12. As a result, the opamp's output voltage and the voltage drop V_{R} are subtracted; the potential of the point A represents the result of this subtraction (it behaves as a virtual ground).
Opamp ItoV converter = passive ItoV converter + "helping" opamp
Opamp circuit operation
Zero input voltage results in no voltage drops or currents in the circuit (click Exploring in [4]).
Positive input voltage. If the input voltage V_{IN} increases above the ground, an input current I_{IN} begins flowing through the resistor R. As a result, a voltage drop V_{R} appears across the resistor and the point A begins raising its potential (the input source "pulls" the point A up toward the positive voltage V_{IN}). Only, the opamp "observes" that and immediately reacts: it decreases its output voltage under the ground sucking the current. Figuratively speaking, the opamp "pulls" the point A down toward the negative voltage V until it manages to zero its potential (to establish a virtual ground). It does this work by connecting a portion of the voltage produced by the negative power supply V in series with the input voltage V_{IN}. The two voltage sources are connected in series, in the same direction (traversing the loop clockwise, the signs are  V_{IN} +,  V_{OA} +) so that their voltages are added. However, regarding to the ground, they have opposite polarities.
Negative input voltage. If the input voltage V_{IN} decreases under the ground, the input current flows through the resistor R in opposite direction (Fig. 15). As a result, a voltage drop V_{R} appears across the resistor again and the point A begins dropping its potential (now, the input source "pulls" the point A down toward the negative voltage V_{IN}). The opamp "observes"' that and immediately reacts: it increases its output voltage above the ground "pushing out" the current. Now, the opamp "pulls" the point A up toward the positive voltage +V until it manages to zero again the potential V_{A} (the virtual ground). For this purpose, the opamp puts a portion of the voltage produced by the positive power supply +V in series with the input voltage V_{IN}. The two voltage sources are connected again, in the same direction (traversing the loop clockwise, + V_{IN} , + V_{OA} ) so that their voltages are added. However, regarding to the ground, they have opposite polarities as above.
Conclusion. In the circuit of an opamp currenttovoltage converter, the opamp adds as much voltage to the voltage of the input source as it loses across the resistor. The opamp compensates the local losses caused by this internal resistor (conversely, in the opposite opamp voltagetocurrent converter, the opamp compensates the losses caused by the external load).
ItoV converters versus transimpedance amplifiers
The active currenttovoltage converter is an amplifier with current input and voltage output. The gain of this amplifier is represented by the resistance R (K = V_{OUT}/I_{IN} = R); it is expressed in units of ohms. That is why this circuit is named transresistance amplifier or more generally, transimpedance amplifier [5]. Both terms are used to designate the circuit considered.
See also
References
External links
Categories: Analog circuits
 Electronic amplifiers
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