Current Converter with floating loads (V/I):
to current converter in which load resistor RL is floating (not
connected to ground). Vin is applied to the non- inverting input
terminal, and the feedback voltage across R1 devices the inverting
input terminal. This circuit is also called as a current – series negative
feedback amplifier. Because the feedback voltage across R1 (applied
Non-inverting terminal) depends on the output current i0 and is in
series with the input difference voltage Vid.
KVL for the input loop,
Vid=Vf and IB
= 0 , vi=RLi0 = where = i0=vi/RL
the fig input voltage Vin is converted into output current of Vin/RL
[Vin -> i0]. In other words, input volt appears across
R1. If RL is a precision resistor, the output current
= Vin/R1) will be precisely fixed
voltage ac and dc voltmeters
and Zener diode testers.
is the other type V – I converter, in which one terminal of the load is
connected to ground.
analysis of the circuit can be done by following 2 steps.
To determine the voltage V1 at the non-inverting (+) terminals and
To establish relationship between V1 and the load current IL.
Applying KCL at node a,
(Vi +Va)/R + (V0
–Va )/R= IL
= (Vi +Vo - IL R)/2 and gain =1+ R/R= 2.
∴Vi =ILR ; IL=Vi/R
– loop gain A of the op-amp is very large. Input impedance of the op amp is
output voltage V0 = -RF Iin.
the gain of this converter is equal to -RF. The magnitude of the gain (i.e.) is
called as sensitivity of I to V converter.
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.
keeping RF variable, it is possible to vary the sensitivity as per the
some applications, it is necessary to have matched diodes with equal voltage
drops at a particular value of diode current. The circuit can be used in
finding matched diodes and is obtained from fig (V-I converter with floating
load) by replacing RL with a diode.
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 = Vin/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.
output voltage is equal to (Vin +VD) V0= Vin
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.
the switch position 2) when the switch is in position 2, the circuit becomes a
Zener diode tester. The circuit can be used to find the breakdown voltage of
Zener diodes. The Zener current is set at a constant value by Vin and R1. If
this current is larger than the knee current (IZK ) of the Zener,
the Zener blocks (Vz ) volts. For Ex: IZK = 1mA , VZ =
6.2V, Vin = 1V, R1 = 100Ω Since the current through the Zener is ,
I0 = Vin/R1 = 1/100 =10mA > IZK the voltage across the
Zener will be approximately equal to 6.2V.
circuit becomes a LED when the switch is in position 3. LED current is set at a
constant value by Vin and R1. LEDs can be tested for
brightness one after another at this current.
LEDs with equal brightness at a specific value of current are useful as
indicates and display devices in digital applications.
of the most common uses of the current to voltage converter is
to analog Converter (DAC)
current through Photo detector. Such as photo cell, photo diodes and
devices produce a current that is proportional to an incident energy or light
can be used to detect the light.
photodiodes, photovoltaic cells give an output current that depends on the
intensity of light and independent of the load. The current through these
devices can be converted to voltage by I – V 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. Ex of Photovoltaic
Cell: used in space applications and watches.