Op-amp Current to Voltage Converter

Current to Voltage Converter

In such a converter, the output voltage is proportional to the input current. It accepts an input current I¡ and yields an output voltage V_o such that V_o = A I_i, where A is the gain of the circuit. Since A is measured in ohms, it is more appropriate to denote gain by the symbol R. Because of this, I-V converters are also called transresistance amplifiers.

Fig. 2.26.1 shows the current to voltage converter.

The node A is a virtual ground as node B is grounded.

Key Point Thus, output voltage is proportional to the input current and circuit works as a current to voltage converter.

This circuit is also referred as Current Controlled Voltage Source (CCVS). If the resistance in circuit is replaced by the impedance Z, the circuit is called transimpedance amplifier.

1. Applications of I/V Converter

One of the most frequent applications of I/V converters is in connection with current type photodetectors such as photodiodes, photoFETs and photomultipliers. Another common application is I/V conversion of current output digital to analog converter. In this section we are going to study few applications of I/V converters.

a. Photodiode Detector

Fig. 2.26.2 shows the connection diagram of one of the most widely used photodetectors, photodiode.

The photodiode produces electrical current in response to incident light. This current flows through R. The voltage across R, the output voltage is proportional to the diode current.

b. PhotoFET Detector

Fig. 2.26.3 shows another photodetector circuit with photoFET.

The photoFET is similar to a conventional junction FET, with the exception of a lens for focusing light onto the gate junction. On application of light, photons enter the gate area and excite valence electrons into the conduction band.

The photon-excited current carriers cause a small current ∆ I_G resulting in large current change ∆ I_D The change in ID results change the voltage drop across R and hence the output voltage. Therefore the op-amp circuit acts as an I/V converter and gives the indication of light interms of voltage. 

Review Question

1. With the help of circuit diagram explain the operation of I to V converter and its applications.

May-13, Dec.-15, Marks 8