What is input impedance?

This is the resistance offered to the flow of current by both static and dynamic resistance into the network which is external to the source. The properties of the source to load would be the same if the load side network would be replaced with a device where output resistance is equal to the input resistance of the equivalent circuit. Therefore, the voltage and current through the input terminals would be the same as the chosen equivalent load network.

The input impedance and output impedance decide how the current and voltage change in a circuit.

Calculation

Ohm's law can be used to calculate the transfer function if we were about to create with same characteristics across the input side by placing the input resistance across the load side of the network and the output resistance in series with the source.

Electrical efficiency

The values of $Z_{in}$ and $Z_{out}$ are often used to find the efficiency of electrical networks and each stage is evaluated independently by breaking multiple stages. The electrical losses are minimized when $Z_{out}$ is insignificant in comparison to the $Z_{in}$. In this case, $Z_{in}>>Z_{out}$ which is the input resistance is much larger than the output impedance.

Power factor

In AC-type circuits, the losses which are caused due to reactive components can be insignificant. This is termed as phase imbalance where the current is not in phase with the potential. So because of this reason, the product of current and potential is less than the original value if current and potential were in phase with each other. In the case of the DC source, the correction of the power factor is not required because there is no impact by the reactive circuits.

Power transfer

When the resistance of the source is equal to the resistance of the load, then the power is said to be transferred in the maximum amount. The power factor is corrected by canceling the reactance of the circuit. The circuit is called the complex conjugate when this occurs. The efficiency of the circuit is not maximized by this but only power transfer is maximized. The circuit runs at half of the efficiency when power transfer is optimized.

Input impedance of an amplifier

The input impedance is the input characteristics of an amplifier with regards to the voltage from supply and current from the current source looking into the input terminals of an amplifier. The input impedance is one of the important parameters in the design of amplifiers. It characterizes the amplifier in accordance with the effective output and input impedances including their power and current ratings. The input impedance is denoted as $Z_{in}$ and output impedance is denoted as $Z_{out}$.

To minimize distortion signal, an amplifier’s impedance value is very important for the analysis of cascading amplifier stages together one after another. The $Z_{in}$ is the resistance seen by the source. It can have an adverse loading effect if the value is too low and it can affect the previous stage, possibly affecting the output signal level and the frequency-dependent response of the stage. The common emitter and common collector amplifiers generally have high $Z_{in}$, any arbitrary common-mode gain, and low output impedance but if the $Z_{in}$ is lower than the desired value then the output impedance of the previous stage can be adjusted to neutralize it or stages of buffer amplifier can be used. The amplifier circuit must have current amplification along with voltage amplification. The circuit can have power amplification too. A perfect amplifier should have infinite $Z_{in}$ and zero output impedance that is every amplifier must have these three parameters which are high input impedance, low output impedance, and bandwidth.

Input and output impedance

An amplifier circuit can be considered or imagined as a black box having two input and two output terminals. Here the resistor $Z_S$ is the output resistor and the resistor $Z_L$ is the input resistor in the first figure.

If we look into these circuit terminals then we will see that the input and output impedance are the ratios of voltage to the current flowing in and out of these terminals. The $Z_{in}$ depends upon the source but the output impedance depends on the load impedance. The amplified input signals are alternating currents with the equivalent circuit of amplifier representing a load.  The input impedance can be from ohms (Ω) to Kilo-ohms (kΩ) for bipolar transistors and Mega-ohms (MΩ) for FET-based circuit transistors.   

The $Z_{in}$ is quoted at a specified frequency of the signal. The $Z_{in}$ is the ratio of small-signal input sine wave voltage to the current flowing into the input terminals at a specified range of frequency response. The impedance of the output is also quoted at a specified signal frequency. Also, if the capacitance and inductance are present in the circuit then they are affected with frequency so $Z_{in}$ also changes.

Importance of impedances

The concept of resistance of input and output is really important for matching impedances. Impedance matching is the concept which states that the power transfer is maximized when power is transferred from an internal source resistance to load resistance that is when $Z_S = Z_L$.

Input impedance of transmission line and operational amplifier

The input resistance of the transmission line is the resistance seen by any entering signal into it. There is no attenuation if the line is ideal to signal amplitudes. When the line is of infinite length then the input impedance is equal to characteristic impedance. The characteristic impedance is defined as the ratio of voltage to the current of the traveling wave entering the line. This is an important parameter in the design of these lines.

The source impedance seen by the driving device of the op-amp is the $Z_{in}$ that should be supplied by the signal source. For non-inverting mode, the input resistance is the resistance of the input terminal modified by feedback effects whereas for inverting mode it may be tough to attain high resistance because of leakage currents in the wiring or circuit boards. 

Context and Applications

This topic is significant in the professional exams for Under-Graduate, Graduate, and Post-Graduate courses.

• Bachelors in Electrical Engineering
• Masters in Electrical Engineering

Practice Problems

1. What is the value of input impedance for a matched line?

1. Load impedance
2. Characteristic resistance
3. Output impedance
4. Voltage amplifier resistance

Answer: Option b

Explanation: The value of input impedance for a matched line will be equal to characteristic resistance because a  matched line refers to the same characteristic and input impedance.

2. What kind of resistance is seen by the source?

1. Input
2. Output
3. Both
4. None

Answer: Option a

Explanation: The resistance seen by the source is the input terminal impedance.

3. What is important to minimize distortion?

1. Amplifier’s impedance value
2. Circuit resonance
3.  Supply voltage value
4. Circuit reactance value

Answer: Option a

Explanation: To minimize distortion signal, an amplifier’s impedance value is very important for the analysis of cascading amplifier stages together one after another

4. On what the input impedance depends?

1. Source
2. Thevenin voltage input
3. Both
4. None

Answer: Option a

Explanation: The input impedance depends on the source because the source is connected to the input terminals only.

5. When power transfer is maximized?

1. $R_L>R_S$
2. $R_L=R_S$
3. $R_L<R_S$
4. None

Answer: Option b

Explanation: The power transfer is maximized when $R_L=R_S$  

Related Concepts

• Output impedance
• Transistors
• Voltage divider