Transimpedance amplifier
From Wikipedia, the free encyclopedia
A transimpedance amplifier is a circuit that performs current to voltage transfomation and is sometimes known simply as a current-to-voltage converter.
Contents |
[edit] Background
Three kinds of devices are used in electronics: generators (having only outputs), converters (having inputs and outputs) and loads (having only inputs). Most frequently, they use voltage as input/output quantity.
In some cases, there is a need for converters having current input and voltage output (current-to-voltage converters). A typical situation is the measuring of a current (for example, of a real voltage source) using instruments having voltage inputs.
Ideal current-to-voltage converters have zero input resistance (impedance), so that they actually short the input source. Therefore, in this case, the input source has to have some resistance; ideally, it has to behave as a constant current source. Otherwise, the current-to-voltage converter will saturate.
[edit] Passive current-to-voltage converter
[edit] Basic idea
The passive version of the current-to-voltage converter is based on the popular impediment causes pressure phenomenon: when something moving encounters an impediment, a pressure appears. Analogies: mechanical (if we try to stop a moving car with our body, it exerts pressure to us), fluid (if we pinch the water hose, we will see that a pressure appears across the bottleneck), human (if we stay in someone's way, a "pressure" appears) etc.
A conclusion: in order to induce a pressure, an impediment has to be applied.
[edit] Implementation
In the electricity, this basic idea is represented by Norton's equivalent circuit and Ohm's law written as V = I.R. According to them, a resistor R can act as a current-to-voltage converter (resistance causes voltage principle) [1]. It impedes (resists) the current flowing through it; as a result, a voltage drop VR = IINR appears across the resistor (the voltage drop is created not by the resistor; it is created by the input voltage source VIN). In this arrangement, the voltage drop VR acts as an output voltage VOUT.
[edit] Imperfections
The passive current-to-voltage converter (as all the passive circuits) is imperfect because of two reasons:
Resistor R. The voltage drop VR interferes in the input current IIN as the resistor R consumes energy from the input source. A contradiction exists in this circuit: from one side, the voltage drop VR is useful as it serves as an output voltage; from the other side, this voltage drop is harmful as it effectively modifies the actual current-creating voltage VRi. In this arrangement, the voltage difference VIN - VR determines the current instead the voltage VIN (the resistor Ri actually acts as a voltage-to-current converter). As a result, the current decreases.
Load resistance. In addition, if the load has some resistance (instead to have infinite resistance), a part of the current IIN will diverts through it. As a result, both the current IIN and the voltage VOUT decrease. The problem is again that the load consumes energy from the passive circuit (click Imperfections in [2])..
[edit] Active current-to-voltage converters
[edit] Basic idea
The active version of the current-to-voltage converter is based on well-known technique, which is frequently used where the undesirable quantities are compensated by equivalent "anti-quantities" and v.v.
This idea is implemented by using an additional power source, which "helps" the main source by compensating the local losses caused by the undesired quantity. An example: in order to keep a desired temperature in the room, an additional heater is installed; it "helps" the main thermal source (the sun).
[edit] An equivalent electrical circuit
In order to show how this basic idea is applied to improve the passive current-to-voltage converter, first, an equivalent electrical circuit is used.
In the active current-to-voltage converter, the voltage drop VR across the resistor R is compensated by adding the same voltage VH = VR to the input voltage VIN [3]. For this purpose, an additional following voltage source BH is connected in series with the resistor. It "helps" the input voltage source; as a result, the undesired voltage VR and the resistance R disappear (the point A becomes virtual ground).
The magnitude of the compensating quantity is frequently used to measure the initial quantity (an example - weighing by using scales). This idea is applied in the circuit of active current-to-voltage converter by connecting the load to the compensating voltage source BH 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.
[edit] Op-amp implementation
The basic idea above is implemented in the op-amp current-to-voltage converter [4]. In this circuit, the output of the operational amplifier is connected in series with the resistor R in the place of the compensating voltage source BH; the op-amp's input is connected to point A. As a result, the op-amp's output voltage VOA and the voltage drop VR are subtracted; the potential of the point A represents the result of this subtraction.
[edit] Circuit operation
Zero input voltage results in no voltage drops or currents in the circuit (click Exploring in [5]).
Positive input voltage. If the input voltage VIN increases above the ground, an input current IIN begins flowing through the resistor R. As a result, a voltage drop VR appears across the resistor and the point A begins raising its potential. Only, the op-amp "observes" that and immediately reacts: it decreases its output voltage under the ground sucking the current until it manages to zero the potential of the point A (virtual ground). The op-amp does this work by connecting a portion of the voltage produced by the negative power supply -V in series with the input voltage VIN. The two voltage sources are connected in series, in the same direction (traversing the loop clockwise, - VIN +, - VOA +) so that their voltages are added. Regarding to the ground, they have opposite polarities.
Negative input voltage. If the input voltage VIN decreases under the ground, the input current flows through the resistor R in opposite direction. As a result, a voltage drop VR appears across the resistor again and the point A begins dropping its potential. The op-amp "observes"' that and immediately reacts: it increases its output voltage above the ground "pushing out" the current until it manages to zero the potential VA (virtual ground). The op-amp achieves this by connecting a portion of the voltage produced by the positive power supply +V in series with the input voltage VIN. The two voltage sources are connected again, in the same direction (traversing the loop clockwise, + VIN -, + VOA -) so that their voltages are added. Regarding to the ground, they have opposite polarities as above.
Conclusion: In the circuit of an op-amp current-to-voltage converter, the op-amp adds as much voltage to the voltage of the input source as it loses across the resistor.
[edit] I-to-V converters versus transimpedance amplifiers
The active current-to-voltage converter is an amplifier with current input and voltage output. The gain of this amplifier is represented by the resistance R (K = -VOUT/IIN = R); it is expressed in units of ohms. That is why, this circuit is named transresistance amplifier or more general, transimpedance amplifier [6]. The both terms are used to designate the circuit considered.
Its input ideally has low impedance, and the input signal is a current. Its output may have low impedance, or in high-frequency applications, may be matched to a driven transmission line; the output signal is measured as a voltage.
[edit] Applications
Transimpedance amplifiers are commonly used in receivers for optical communications. The current generated by a photodetector generates photo voltage, but in a nonlinear fashion. Therefor the amplifier has to prevent any large voltage by its low input impedance and generate either a 50 Ohm signal (which is by the way 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 circuit considered is also used as a main part of more complex op-amp inverting circuits with (parallel) negative feedback: inverting amplifier, CR differentiator, LR integrator, inverting summer etc. An example - op-amp inverting amplifier - is showed on the picture to right.
[edit] See also
- Virtual ground considers an important property of the negative feedback circuits
[edit] External links
- Reinventing transimpedance amplifier in eight consecutive logically connected steps
- Current-to-voltage passive converter - an animated flash tutorial about the passive circuit version
- Reinventing op-amp inverting summer - an animated flash tutorial about the active version (presented as a part of a voltage summer)
- Secrets of parallel negative feedback circuits reveals the philosophy of this class of circuits
- What's All This Transimpedance Amplifier Stuff, Anyhow?
