Op-Amp: Current to Voltage Converter (Transimpedance Amplifier) and it's applications

Summary

This video from the 'All About Electronics' YouTube channel explains how to design a current to voltage converter using an op-amp. It covers the basics of passive and active converters, their limitations, and applications in photodiode circuits and digital to analog converters, highlighting the importance of the transimpedance amplifier concept.

Takeaways

• 😀 The video discusses how to design a current to voltage converter using an operational amplifier (op-amp).
• 🔌 Current to voltage converters are useful in applications where a sensor or circuit component outputs current, such as photodiodes.
• 📈 These converters are essential for logging data from current-output devices using data acquisition systems that require voltage input.
• 🔗 The basic principle involves converting the input current into an output voltage, which is achieved by controlling the input current to manipulate the output voltage.
• 🔍 A simple passive circuit using a resistor can be used for current to voltage conversion, but it has limitations due to load resistance affecting the output voltage.
• 🚫 The passive converter's output voltage depends on the load resistance, which is not ideal for applications requiring an independent output voltage.
• 🔌 An active converter using an op-amp can overcome this issue by maintaining a consistent output voltage independent of the load resistance.
• 💡 The op-amp-based converter uses the inverting terminal for input current and a feedback resistor between the output and inverting terminal to achieve the conversion.
• 🌞 The video provides an example of using the converter with a photodiode, where the photocurrent is converted into a voltage proportional to the light intensity.
• 🔢 The converter is also applicable in digital to analog converters (DACs), where digital input bits are converted into an analog output voltage based on the current through resistors.
• 📚 The script concludes with an invitation for viewers to ask questions or provide feedback in the comments and encourages engagement through likes and subscriptions.

Q & A

What is the purpose of a current to voltage converter?

A current to voltage converter is used to convert an input current into an output voltage. This is useful in applications where a sensor or circuit component outputs data in terms of current, and the data acquisition system requires voltage input.

Why are current to voltage converters important in data logging scenarios?

Current to voltage converters are important in data logging scenarios because they allow the conversion of current outputs from sensors like photodiodes into voltage, which can be logged and monitored over time using data acquisition systems that typically accept voltage inputs.

What is a limitation of passive current to voltage converters?

A limitation of passive current to voltage converters is that the voltage across the load depends on the load resistance. This means that the converted voltage is not independent of the load resistance, leading to potential inaccuracies in the output voltage.

How does an active current to voltage converter overcome the limitations of passive converters?

An active current to voltage converter, typically using an operational amplifier (op-amp), overcomes the limitations of passive converters by maintaining the output voltage independent of the load resistance. This is achieved by using the op-amp's inverting terminal and feedback resistor to control the output voltage based on the input current.

What is the role of the feedback resistor in an op-amp based current to voltage converter?

The feedback resistor in an op-amp based current to voltage converter is crucial as it determines the output voltage. The output voltage is proportional to the input current multiplied by the value of the feedback resistor, ensuring that the output voltage is independent of the load resistance.

What is the term used to describe the ratio of output voltage to input current in a current to voltage converter?

The ratio of output voltage to input current in a current to voltage converter is described as transimpedance. This term is used because the ratio has the unit of impedance, reflecting the relationship between current and voltage in the converter.

How can a current to voltage converter be used in photodiode circuits?

In photodiode circuits, a current to voltage converter can be used to convert the photocurrent generated by the photodiode, which varies with light intensity, into a proportional voltage output. This allows for the measurement and monitoring of light intensity through voltage readings.

What is the significance of the virtual ground concept in op-amp based circuits?

The virtual ground concept in op-amp based circuits is significant because it allows the inverting terminal of the op-amp to be at zero voltage, which is crucial for maintaining the desired circuit operation. This concept ensures that the input current is accurately converted into the output voltage without being affected by the load resistance.

How does a current to voltage converter work in a digital to analog converter (DAC)?

In a digital to analog converter, a current to voltage converter is used to convert digital data bits into an analog output voltage. Each bit is treated as a current source, and the total current flowing through a feedback resistor determines the output voltage. The output voltage is proportional to the sum of the currents corresponding to the digital input bits.

What are some applications of current to voltage converters beyond photodiode circuits?

Beyond photodiode circuits, current to voltage converters are used in various applications such as in digital to analog converters, where they help convert digital data into an analog output voltage. They can also be used in sensor data acquisition systems, where they convert sensor outputs in current form into a voltage form that can be easily measured and recorded.

Outlines

00:00

🔌 Designing Current to Voltage Converters with Op-Amp

This paragraph introduces the concept of current to voltage converters and their applications. It explains that these converters are essential for converting current outputs from sensors or circuit components, such as photodiodes, into voltage signals that can be logged by data acquisition systems. The paragraph also highlights the limitations of passive circuit components in achieving this conversion and sets the stage for discussing the use of operational amplifiers (op-amps) to overcome these limitations. The key idea is that by controlling the input current, one can control the output voltage, making these converters useful in various applications.

05:02

🌞 Applications of Current to Voltage Converters in Photodiode Circuits

This paragraph delves into the practical applications of current to voltage converters, particularly in photodiode circuits. It describes how photocurrent generated by a photodiode can be converted into a voltage signal using a resistor and an op-amp. The explanation includes the calculation of the output voltage based on the photocurrent and the feedback resistor. Additionally, the paragraph discusses the use of current to voltage converters in digital to analog converters (DACs), where digital input bits are converted into an analog output voltage. The example given illustrates how a four-bit digital input sequence (1001) results in a specific output voltage, demonstrating the versatility of these converters in digital signal processing.

Keywords

💡Voltage to Current Converter

A voltage to current converter is an electronic circuit that converts an input voltage signal into an output current signal. This is the opposite of what is discussed in the video, but it's mentioned as a context to introduce the main topic. The video focuses on the current to voltage converter, which is used to convert current signals into voltage signals, which are more commonly accepted by data acquisition systems.

💡Current to Voltage Converter

A current to voltage converter is an electronic circuit that converts an input current signal into an output voltage signal. This is the main focus of the video. It is useful in applications where a sensor or circuit component outputs data in terms of current, and this current needs to be converted into a voltage that can be logged or processed by a data acquisition system.

💡Op-Amp

An operational amplifier, or op-amp, is a high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In the context of this video, an op-amp is used to design the current to voltage converter. It plays a crucial role in amplifying the input current signal and converting it into a proportional voltage signal.

💡Photodiode

A photodiode is a type of photodetector that generates a current when exposed to light. In the video, it is used as an example of a device that outputs a current signal. The current to voltage converter is shown to be particularly useful in converting the photocurrent from a photodiode into a voltage signal for data logging.

💡Data Acquisition System

A data acquisition system is a device or set of devices that gather and process data from various sources. In the video, it is mentioned that such systems typically accept data in terms of voltage, hence the need for a current to voltage converter to process current signals from sensors like photodiodes.

💡Passive Circuit Components

Passive circuit components are electronic components that do not require power to function. In the video, it is explained that a simple way to convert current to voltage is by using a resistor as a passive component. However, this method has limitations, such as the voltage across the load being dependent on the load resistance.

💡Transimpedance Amplifier

A transimpedance amplifier is a type of amplifier that converts an input current to an output voltage. The video explains that the current to voltage converter designed using an op-amp is an example of a transimpedance amplifier. The output voltage is proportional to the input current, and the term 'transimpedance' refers to the impedance that relates the output voltage to the input current.

💡Feedback Resistor

In the context of an op-amp circuit, a feedback resistor is used to provide negative feedback, which stabilizes the amplifier's gain. In the video, the feedback resistor is connected between the output and the inverting input terminal of the op-amp, and it is crucial for the conversion of the input current to the output voltage.

💡Digital to Analog Converter (DAC)

A digital to analog converter is a device that converts digital signals into analog signals. The video demonstrates how a current to voltage converter can be used in a DAC, where digital data bits are converted into an analog output voltage. The converter circuit uses the sum of currents corresponding to each bit to produce the final output voltage.

💡Virtual Ground

In an ideal op-amp circuit, the inverting input is often referred to as a 'virtual ground' because it is maintained at zero volts due to the high input impedance of the op-amp. The video mentions the concept of virtual ground in the context of the current to voltage converter, where the inverting terminal of the op-amp is at the same potential as the ground, facilitating the conversion process.

Highlights

Introduction to designing a current to voltage converter using an op-amp.

Explaining the utility of current to voltage converters in applications involving current output sensors.

Describing the concept of a current controlled voltage source in the context of current to voltage conversion.

Demonstrating a simple passive circuit for current to voltage conversion using a resistor.

Discussing the limitations of passive converters when connected to load resistance.

Introducing the active converter method to avoid issues with load resistance affecting the output voltage.

Using the virtual ground concept to explain the operation of the op-amp in a current to voltage converter.

Deriving the formula for the output voltage in terms of the input current and feedback resistor value.

Highlighting the independence of the output voltage from the load resistance in the active converter design.

Naming the current to voltage converter as a transimpedance amplifier due to its output voltage to input current ratio.

Exploring the use of current to voltage converters in photodiode circuits for light measurement.

Describing the application of current to voltage converters in digital to analog converters (DACs).

Calculating the output voltage for a given digital input sequence in a DAC using a current to voltage converter.

Summarizing the versatility of current to voltage converters in various electronic applications.

Inviting viewers to ask questions or provide suggestions in the comment section for further engagement.

Encouraging viewers to like and subscribe for more educational content on electronics.

Transcripts

Hey friends, Welcome to the YouTube channel ALL ABOUT ELECTRONICS.

So, in the previous video, we have seen that how to design the voltage to current converter

using the op-amp.

So, now in this video, let's understand how to design the current to voltage converter

using the op-amp.

Now, these type of current to voltage converter circuits are useful in many applications.

For example, let's say, you have one sensor or the circuit component which gives the output

in terms of the current.

For example, let's say you have a photodiode, which gives the output in terms of the current.

And let's say, you want to log the data of this photodiode for the entire day.

And let's assume that for that you are using the data acquisition system.

Now, most of the time these type of system used to accept the data in terms of the voltage.

So, you need to convert this current into the voltage.

And for this, this I to V or the current to voltage converters are useful.

Now, this current to voltage converter is also an example of a current controlled voltage

source, because if you observe over here, the input to this circuit is in terms of the

current.

While the output of this circuit is in terms of the voltage.

So, just by controlling the input current we can control the output voltage.

So, now let's understand, how we can design this current to voltage converter.

Now, before we see the op-amp based current to voltage converter, first of all, let's

see how we can design this converter using the passive circuit components.

And what are the limitations of this passive converter?

So, just by connecting the resistor across this current source we can convert this current

into the voltage.

And the voltage that is developed across this resistor can be given by this simple expression.

So, now suppose if we connect the load across this resistor R, then ideally the same voltage

should also appear across the load.

But whenever we connect the load to this resistor R, then some current will also flow through

this resistor RL.

And because of that, the voltage which appears across this resistor RL can be given by this

expression.

That is the input current, multiplied by the parallel combination of this R and RL.

So, as you can see over here, in case of this current to voltage converter, the voltage

which appears across the load will also depend upon this load resistance.

Now, ideally in this current to voltage converter, the converted voltage should be independent

of this load resistance.

So, unless the value of this load is much greater than this resistor R, the actual voltage

which appears across this load will be less than the voltage which is getting converted

by this converter.

So, this problem can be avoided by using this active converter.

So, here this input current is connected at the inverting terminal of this op-amp.

And the feedback resistor R is connected between the output terminal and the inverting input

terminal.

And here the non-inverting terminal is at a ground potential.

So, because of the virtual ground concept, this node will be also at zero voltage.

Now here, we are assuming that the op-amp is an ideal op-amp.

So, current is flowing into the op-amp terminals.

So, if we apply the KCL at this node A, then we can say that this input current Iin should

be equal to the current which is flowing through this resistor R.

And that will be equal to 0 minus Vout divided by this resistor R.

So, we can say that the output voltage vout will be equal to minus input current times

the value of this resistor R. So, as you can see over here the output voltage

Vout is proportional to the input current.

So, in this way, we can convert this input current into the output voltage.

Now, in this circuit even if you connect the load at the output terminal, then also the

output voltage will remain as it is.

So, basically, it is independent of the load resistance.

Now, this type of current to voltage converter is also known as the transimpedance amplifier.

Because here the current is applied as an input and the output of this amplifier is

in terms of the voltage.

So, here the ratio of this output voltage divided by this input current has a unit of

impedance.

And that's why this type of amplifier is known as the transimpedance amplifier.

So, now let's see some of the applications of this current to voltage converter.

So, this type of converter is particularly useful in the photodiode circuits.

So, as you can see over here the photodiode is connected at the inverting input terminal

of this op-amp.

And the non-inverting terminal is at a ground potential.

So, by applying the concept of virtual ground we can say that this inverting terminal is

also at a ground potential.

Now, depending upon the light which is falling on this photodiode, the photocurrent will

flow through this photodiode.

And if we apply the KCL at this node, then we can say that this photocurrent Iph will

be equal to the current IR which is flowing through this resistor R.

And that will be equal to Vout divided by this resistor R.

So, from this, we can say that the output voltage Vout will be equal to this photocurrent

Iph times this resistor R. So, in this way, we convert this photocurrent

into the output voltage.

So, similarly this current to voltage converter can also be used along with this photoresistor.

So, in case of this photoresistor, the value of the resistance will change according to

the light that is falling on it.

so, apart from these applications, this current to voltage converter is also used in digital

to analog converters.

So, here we have a four bit of digital to analog converter.

And to this converter, the four bits of digital data has been applied.

So, according to the applied digital bits, we will get the analog output voltage at the

output terminal.

Now, in this converter circuit, the logical one treated as 5V and the logical 0 is treated

as 0V.

And here, this D3 is the most significant bit of the applied input and D0 is the least

significant bit of the applied input.

So, let's say these four bits are applied to this DAC.

So, here this D0 is the logical 1.

So, here it is connected to the 5V.

And accordingly, some current I will flow through this resistor R.

And that current I will be equal to 5V divided by this 10 Kilo-Ohm resistor.

That is equal to 0.5 mA current.

So, because of this D0 bit, 0.5 mA of current will flow through this resistor R.

Now, similarly, if we consider this D1 bit, it is a logical 0.

So, it is connected to the ground.

So, no current will flow through this 5 Kilo-Ohm resistor.

Or we can say that the current I2 will be equal to 0 mA.

Similarly, if we consider this D2 bit, then it is also connected to the ground potential.

So, current I3 will also equal to 0 mA.

And now if we consider this most significant bit, that is D3, it represents the logical

1.

So, it is connected to the 5V and because of that, the current I4 will be equal to 5

V divided by 1.25 Kilo-Ohm resistor.

So, that will be equal to 4mA.

So, now the summation of these individual four currents will flow through this resistor

Rf.

Because here we are assuming that the op-amp is an ideal op-amp.

So, current is flowing into the op-amp terminals.

So, whenever we apply 1001 as an input, then the total current IT, will be equal to 0.5

mA plus 4mA.

That is equal to 4.5 mA.

Now, this current will flow through this resistor Rf.

And because of that, we will get the output voltage Vout as

minus IT times this feedback resistor Rf.

So, the output voltage Vout will be equal to (-4.5 mA) times this 1Kilo-Ohm resistor.

And that is equal to - 4.5 V.

So, this will be the output voltage whenever we apply this 1001 as the input sequence.

So, in this way, by using this current to voltage converter circuit, we can convert

the digital data into the analog output voltage.

And depending upon the applied input sequence, the output of this digital to analog converter

will change.

So, these are the some of the applications of this current to voltage converter.

So, I hope in this video you understood how we can design this current to voltage converter

using the op-amp and what are the different applications of this current to voltage converter.

So, if you have any question or suggestion, do let me know in the comment section below.

If you like this video, hit the like button and subscribe to the channel for more such

videos.