Current to Voltage Converter, 1. Sensitivity of the Current to Voltage Converter, 2. Applications of Current to Voltage Converter with Floating Load

Current to Voltage Converter

A current to voltage converter, also known as an I to V converter, is an electronic circuit that receives current as its input and outputs voltage. The current to voltage converter based on op-amps is covered in this section.

1.   Open – loop gain a of the op-amp is very large.  V₁»V₂    @@@@ ^`1^a

2.   Input impedance of the op-amp is very high. (i.e) the currents entering into the 2 input terminals is very small. I_B=I_B2

1. Sensitivity of the Current to Voltage Converter

1.        The output voltage V₀  = -R_FI_in.

2.        Hence the gain of this converter is equal to -RF. The magnitude of the gain (i.e) is also called as sensitivity of I to V converter.

3.        The amount of change in output volt ∆V0 for a given change in the input current ∆Iin is decide by the sensitivity of I-V converter.

4.        By keeping RF   variable, it is possible to vary the sensitivity as per the requirements.

2. Applications of VI Converter with Floating Load

1. Diode Match finder

In certain applications, matched diodes with equal voltage drops at a specific value of diode current are required. Replace RL with a diode to obtain the circuit from fig (V-I converter with floating load), which can be used to find matched diodes. When the switch is in position 1: (Diode Match Finder) Rectifier diode (IN 4001) is placed in the f/b loop, the current through this loop is set by input voltage Vin and Resistor R1 . For Vin = 1V and R1 = 100Ω, the current through this

I₀ = V_in/R₁ = 1/100 = 10mA.

As long as V₀ and R₁ constant, I0 will be constant. The Voltage drop across the diode can be found either by measuring the volt across it or o/p voltage. The output voltage is equal to (V_in + V_D ) V₀ = Vin + VD . To avoid an error in output voltage the op-amp should be initially nulled. Thus the matched diodes can be found by connecting diodes one after another in the feedback path and measuring voltage across them.

2. Zener diode Tester

(When the switch position 2)

The circuit transforms into a zener diode tester when the switch is in the second position. The circuit can be used to determine the zener diode breakdown voltage. Vin and R1 control the zener current to maintain a constant value. The zener blocks (Vz) volts if this current is greater than the zener’s knee current (IZK).

For Ex:

I_ZK = 1mA , V_Z = 6.2V, V_in = 1v , R₁ = 100Ω Since the current through the zener is , I₀ = V_in/R₁ = 1/100 =10mA > I_ZK the voltage across the zener will be approximately equal to 6.2V.

3. When the switch is in position 3: (LED)

When the switch is in the third position, the circuit turns into an LED. Vin and R1 control the LED current to maintain a constant value. At this current, it is possible to check the brightness of one LED after another. In digital applications, matched LEDs with equal brightness at a certain value of current are useful as indicators and displays.

3. Applications of IV Converter

One of the most common use of the current to voltage converter is

1.        Digital to analog Converter (DAC)

2.        Sensing current through Photodetector. Such as photocell, photodiodes and photovoltaic cells.

Photoconductive devices produce a current that is proportional to an incident energy or light (i.e) It can be used to detect the light.

4. DAC using IV converter

It displays a DAC and a current-to-voltage converter combined. The 8-bit binary signal is the DAC’s input, and V0 is the analogue output of the current to voltage converter that corresponds to it. The binary inputs’ logic state (0 or 1) determines the value of current I0, the DAC’s output, as shown by the equation below.

This means I0 is zero when all inputs are logic 0.

I0 is max when all inputs are logic 1.

The variations in I0 can be converted into a desired o/p voltage range by selecting a proper value for RF. since, V₀ = I₀ R_F

Where I₀ is given by eqn (1). So It is common to parallel RF with capacitance C to minimize the overshoot. In the fig the o/p voltage of the current to voltage converter is positive because the direction of input current I₀ is opposite to that in the basic I – V Converter.

Detecting current through photosensitive devices:

Photocells, photodiodes, photovoltaic cells give an output current that depends on the intensity of light and independent of the load. So The current through this device can be converted to voltage by Current to Voltage Converter and it can be used as a measure of the amount of light. In this fig photocell is connected to the I – V Converter. Photocell is a passive transducer; it requires an external dc voltage (Vdc). The dc voltage can be eliminated if a photovoltaic cell is used instead of a photocell. The Photovoltaic Cell is a semiconductor device that converts the radiant energy to electrical power. It is a self-generating circuit because it does not require dc voltage externally. So Ex of Photovoltaic Cell : used in space applications and watches.