In the last article we have seen how to convert voltage into current using voltage to current converter. Today we will see how to convert current into voltage form using current to voltage converter.

As we know we can not send output voltage of sensor to a long distance because of addition of noise. So we first convert it into current and then send to the destination. But at the destination we have to convert that current into its original form i.e. in voltage form. Using current to voltage converter we can easily convert the current into voltage.

Following figure shows the circuit diagram of the current to voltage converter. It uses simple operational amplifier and a feedback resistance.

The output voltage of operational amplifier is directly proportional to the current given to the inverting terminal of the op amp.

The value of the output voltage is given by the following equation

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Hi friends, in this post we will see how to convert voltage into current using simple circuitry. In most of the cases we get the output of measuring devices in the form of voltage. It is not good to transmit this output voltage to the destination directly. Due to addition of noise and wire impedance the output voltage may get corrupted. So in such cases we have convert that voltage into current form. So let us see voltage to current converter.

Voltage to Current Converter using Op Amp

Following circuit shows the voltage to current converter using operational amplifier. It consist of simple resistance connected to the inverting and non inverting terminals of op amp.

In this circuit the load is grounded and the current through the load can be calculated as follows.

The current through the load is given by,

The gain of the amplifier is

So

Substituting this value in above equation we get,

Thus the current is directly proportional to the applied voltage and the resistance used in the circuit. it should be noted that all the resistances used in the circuit are equal to R.

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Integrator is a circuit whose output is proportional with the area of the input waveform. Following figure shows the op-amp as integrator circuit. We know that the RC circuit itself acts as a simple integrator. But in such circuits the output waveform is not exactly linear triangular waveform as it should be. In op-amp integrating circuit the inverting terminal is virtual grounded by the differential op-amp input circuit as shown in figure. Input current is equal to Vin/R1. As the input impedance of the op-amp is infinite all of I1 will flow to the capacitor.

op amp as an integrator

Working of op-amp integrator:

Let us consider R1 is kept constant. Assume that the input voltage (Vin) is kept constant for a given period of time, hence the value of the I1 is also constant. Because of high impedance all of I1 flows through capacitor so that capacitor starts charging by a constant current source. When input voltage (Vin) is kept constant, the capacitor will charge and discharge at linear rate. The output of this op-amp integrator is shown below.

op amp integrator output waveforms

From the above output waveforms we can observed that when input voltage goes positive (from –V to +V) the output is a negative ramp. Similarly when the input goes negative (from +V to -V) the output is a positive ramp. This implies that the op-amp integrator output is 180 degree out of phase with the input.

We will see another application of operational amplifier i.e. op-amp audio amplifier in the next article.

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In this section we are going to learn a basic op-amp application as a comparator. Comparator is a circuit used for comparing two voltages (either DC or both AC or one DC & one AC) and indicating the relationship between those voltages.

Generally comparators are used to compare either:

a) Two changing voltages to each other, example: two different sinusoidal waveforms.

b) A changing voltage to a set DC reference voltage.

The circuit diagram of op-amp comparator is sown below. There is no feedback path present in the circuit. To understand the working of op-amp comparator let us consider a sinusoidal input voltage is applied to the non-inverting terminal where as a fixed DC voltage (V reference) is applied to the inverting terminal.

Circuit diagram:

In this example we are going to compare sinusoidal voltage with fixed dc voltage using op-amp comparator. The working of this comparator is explained as follows.

Working:

As long as the input voltage is below the reference voltage (which is connected to the non-inverting terminal) , the comparator output is approximately “-Vmax” volts. When input voltage equals to reference voltage or exceeds, the output voltage of the comparator becomes “+Vmax” volts. Thus op-amp comparator shows the relationship between the magnitudes of two voltages applied to its input. Following figure shows the polarity (or magnitude) relationship between two voltages.

Now let us discuss one special case of op-amp comparator. If we apply reference voltage (Vref) to the inverting terminal and made it ground so that V ref =0V. Now in this case the output of the comparator will be “- V max” volts as long as the input voltage is below 0V. When input voltage exceeds 0V the output of the comparator switches to “+V max” volts.

Such special case op-amp comparator is called as zero-level detector. This zero-level detector circuit can be used to obtain square waveform from a sinusoidal waveform.

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In the case of amplifiers, the term open loop indicates that there is no connection, either direct or via another network, exists between the output and input terminals. That is the output signal is not fed back in any form as part of the input signal, and the loop that has would be formed with feedback is open.

When op-amp is connected in open loop configuration, it acts as high gain amplifier. There exist three open loop op-amp configurations as follows:

1) Differential amplifier

2) Inverting amplifier

3) Noninverting amplifier

These three configurations are classed according to the number of inputs used and the terminal to which the input is applied when a single input is used.

The differential amplifier:

The following figure shows the differential amplifier in which input signals Vin1 and Vin2 are applied to the positive and negative terminal of the op-amp respectively. As the op-amp amplifies the difference between the two input signals, this configuration is called ‘differential amplifier’.

The op-amp is called versatile device because it amplifies both AC as well as DC input signals. The voltage drop across Rin1 and Rin2 can be neglected since they are very small as compared to input resistance of op-amp. Which then implies that V1=Vin1 and V2=Vin2.

Thus from the equation Vo=A(Vid)=A(V1-V2)

We get

Vo=A(Vin1-Vin2)

Thus the output voltage is equal to the voltage gain A times the difference between input voltages. The polarity of the output voltage depends upon the difference (Vin1-Vin2). In open loop configurations, the gain A is commonly referred as open-loop gain.

The Inverting Amplifier:

In inverting amplifier there is only one input which is applied to the inverting input terminal (which is negative). The Noninverting terminal is grounded.

Since V1=0V and V2=Vin

From the equation Vo=A(Vid)=A(Vin1-Vin2)

We get Vo=-AVin

The negative sign shows that the output voltage is out of phase with respect to input by 180 degrees or is of opposite polarity.

The Noninverting Amplifier:

The following figure shows the Noninverting amplifier. In this configuration, input is applied to the Noninverting input terminal, and the inverting terminal is connected to the ground.

Here we have V1=Vin and V2=0V.

Therefore according to the equation Vo=A(Vid)=A(Vin1-Vin2)

We get Vo=AVin

It implies that the output voltage is voltage gain A times the applied voltage.

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