Input Impedance and Output Impedance | Examples, Formula
We know that the concept of impedance is very important. We studies about it from basic electrical circuits to advanced electronic circuits. Now, in this article, we are going to learn about Input Impedance and Output Impedance. You may hear so many times these terms especially when you learning about electronic amplifiers.
Anyway, here we will discuss the actual concept of input and output impedance that may be related to an amplifier or any other circuit or device. Once you clear your knowledge about these two particular terms, you will never face any problems or confusion while understanding any amplifier or advanced electronic circuit.
First of all, let's remind the concept of Impedance. Impedance is nothing but a property of an electrical or electronic circuitry or network that opposes the flow of electrical current. Impedance is same as the resistance just the difference is we use Resistance in a DC circuit and Impedance in an AC circuit.
Also, remember that resistance may be the internal physical property of any object or material. But the impedance never be only a physical property of an object or material. Impedance produces due to only the temporary characteristics of any electrical network or the combination of internal resistance and temporary characteristics of any network. Theoretically, impedance is the ratio between voltage and current.
Concept of Input Impedance and Output Impedance
In this section, we will discuss why we use the terms input Impedance and Output Impedance. And where we use these terms.
We know that every electronic circuit we design for a particular requirement has an input and output. For example, we design an electronic amplifier circuit to amplify the electronic signal. The amplifier circuit has an input where it receives a low or weak electronic signal(which is nothing but electric current or voltage) and it also has an output where it provides the amplified signal(which is also electric current or voltage). Similarly, there are so many circuits that also have inputs and outputs.
So it is clear that an electronic circuit will consume some current or voltage at its input and it delivers some current and voltage in its output load. And this total concept of input-output voltage and current describes the input and output impedance. Because the impedance is nothing but the ratio between voltage and current.
What is Input Impedance?
Input impedance is nothing but the opposition property of an electrical or electronic network that controls its input voltage and current. Theoretically, we can say the input impedance is the ratio between the input voltage and input current of that network. The input impedance will decide how much voltage or current will be consumed by the circuit or network while the source is constant.
The input impedance mainly blocks the input current. For example, when a circuit is connected to a source, the source voltage will apply across the input of that circuit. But the voltage across the input of that circuit and the source voltage may not be the same because the source has some series internal resistance itself so a small voltage will drop and the circuit will get a lower voltage than the source voltage.
Now if the circuit has a very low input impedance, the current flow will be more so the voltage drop across the series internal resistance of the source will be more, so the voltage across the input of the circuit will be very lower than the source voltage.
But if the circuit has very high input impedance, the current flow will be less so the voltage drop across the series internal resistance of the source also be less. So the input voltage of the circuit will be high, in fact, it may almost equal to the source voltage.
Now you may understand how the input impedance of a circuit controls its input voltage and current. By the way, remember that the input impedance of any circuit depends upon the design and component rating of that circuit. So a manufacturer set a required input impedance by its design.
Now the question is whether the impedance should be low or high. Actually, it depends upon for which purpose the circuit is designed. If a circuit is designed to sense the current flow it must have a very low input impedance so it can sense or consume electric current in its input as much as possible. But if the circuit is designed to sense the voltage, then the input impedance should be very high because the current circulating will be less and the circuit can sense as much as possible the source voltage in its input.
Input Impedance of an Amplifier
The opposition property of the amplifier that controls the input voltage and current of that amplifier is called the Input impedance of the Amplifier. Or we can say the input impedance of an amplifier is the ratio between the input voltage and input current of that amplifier.
Generally, an amplifier is designed in such a way that it will have a very high input impedance. Even an ideal amplifier has an infinite input impedance.
What is Output Impedance?
Output Impedance is the opposition property of an electrical circuit or network that controls its output current and voltage across the load. Theoretically, we can say the output impedance of any network is the ratio between the output voltage and output current of that network. The output impedance decides maximum how much current and voltage are possible across the load.
The output impedance mainly blocks the output current. Can you remember that we earlier told you that all the voltage sources have their own internal series resistance? Here, the circuit provides an output to the load so it acts as a source of voltage. So it also has its own internal resistance. From the point of view of that circuit, this internal resistance is the output impedance of that circuit. The output impedance of this circuit also depends upon its design and internal component rating.
Now if the output impedance of the circuit is very high, the voltage drop also be very high and the voltage across the load will be very less. But if the output impedance is very low the voltage drop also be very less so the maximum voltage(approximately equal to the actual output of the circuit) will be available across the load.
So if there is a requirement for high output current then the output impedance of the circuit should be very high. But if there is a requirement for a high output voltage then the output impedance of the circuit should be very low.
Output Impedance of an Amplifier
The opposition property of the amplifier that controls the output voltage and current of that amplifier is called the Output impedance of the Amplifier. Or we can say the Output impedance of an amplifier is the ratio between the output voltage and output current of that amplifier.
Generally, an amplifier is designed in such a way that it will have a very low output impedance. Even an ideal amplifier has a zero output impedance.
Examples
Examples of High Input Impedance: Ideal Amplifier, Ideal Voltmeter, CMOS Inverter, Transistor in Common Collector (CC) Configuration, Electric guitar, karaoke microphone, etc.
Examples of Low Input Impedance: Current Meter, Multimeter, OpAmp as Current to Voltage Converter, Transistor in Common Base(CB) Configuration, etc.
Examples of High Output Impedance: Voltage Buffers, Transistors in Common Base(CB) Configuration, Current Series Feedback Amplifiers, etc.
Examples of Low Output Impedance: Ideal Amplifier, CMOS Inverter, Transistor in Common Collector (CC) Configuration, Constant Voltage Power supply, etc
Formula
Input Impedance Formula,
Zin = Vin/Iin
Here, Zin - Input Impedance, Vin - Input Voltage, Iin - Input Current
Output Impedance Formula,
Zout = Vout/Iout
Here, Zout - Output Impedance, Vout - Output Voltage, Iout - Output Current
Calculation
Example. 1
Input Impedance, Zin = 5 ohm, Input Voltage, Vin = 10V, Find out Input Current Iin =?
Ans: Input Current, Iin = Vin/Zin = 10/5 = 2A
Example. 2
Output Impedance, Zout = 5 ohm, Iout =3A, Find out Output Voltage, Vout =?
Ans: Output Voltage, Vout = Zout x Iout = 5 x 3 = 15V
Input Impedance and Output Impedance | Examples, Formula
Reviewed by Author
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May 11, 2023
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Reviewed by Author
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May 11, 2023
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