High Input Impedance and Low Output Impedance in Op Amp Circuit

Introduction

The input and output impedance of an amplifier is the ratio of voltage to current flowing in or out of these terminals. The input impedance may depend upon the source supply feeding the amplifier while the output impedance may also vary according to the load impedance, RL across the output terminals. Op amps are supposed to have zero output impedance, or very low. However, op amp input impedance and output impedance are important factors in the design of any circuit.

op amp exam

Catalog

Introduction

Ⅰ Input Impedance and Output Impedance

1.1 Impedance Overview

1.2 Input Impedance of Op-Amp

1.3 Output Impedance of Op-Amp

1.4 Ideal Op Amp Impedance

Ⅱ High Input Impedance and Low Output Impedance Effect

Ⅲ How to Calculate Input Impedance and Output Impedance

Ⅳ Conclusion

Ⅰ Input Impedance and Output Impedance

1.1 Impedance Overview

In circuits, impedance is the relationship between voltage and current. It's a combination of resistance (frequency-independent, resistors) and reactance (frequency-dependant, inductors and capacitors). Why op amp has input and output impedance? Let’s give you some ideas. The input impedance of the op-amp looks like the load impedance to whatever is proving the signal to the op-amp. The output impedance of the op-amp looks like the source impedance to whatever is receiving the signal from the op-amp. But what do they mean, and why they are useful?

1.2 Input Impedance of Op-Amp

It is assumed that the input impedance of an ideal op-amp is infinite, but this is not the case in real life. A small amount of current is decreased by any electrical input, source or sink. This can be modelled as a resistor, as seen in the figure below, connected parallel to the input.
The input impedance is represented in the above diagram as a resistor, since this is valid in most cases. The inputs, however, also have a tiny capacitance. At lower frequencies, this is not a concern as it just has the effect of minimizing rise and fall times. However, this capacitance can provide a substantial load for AC signals at high enough frequencies and hinder the rise and fall times, which can also contribute to distortion of the signal.

1.3 Output Impedance of Op-Amp

You should be able to push any amount of current through any load using an ideal amplifier. With a good output driver phase, this is possible, but in terms of how much current it can produce, a bare amplifier itself has some limitations. For instance, only 40mA/20mA can source/sink the popular LM358 op-amp. As shown in the figure below, this restriction of the output drive can be considered as a resistor in series with an ideal output.
Since the output is only seen on the other side of the resistor, there is a large voltage drop across the resistor if the output is overloaded, and the output is not the actual output given by the amplifier. This can be countered by the addition of an output stage that further amplifies the signal and makes it ideal for large loads to be powered.

1.4 Ideal Op Amp Impedance

As for an ideal op amp, an ideal op-amp has infinite input impedance. This means that there can be no current into or out of the inverting and non-inverting input terminals, because the current flow into the input leads is zero. An ideal op-amp has zero output impedance. This means that the output voltage is independent of output current. So the ideal op amp can drive any load without an output impedance dropping voltage across it. The short summary: input impedance is "high" (ideally infinite), output impedance is "low" (ideally zero).

op amp impedance matching

Op Amp Impedance Matching

Ⅱ High Input Impedance and Low Output Impedance Effect

The high impedance ensures that it draws very little current. It is the amplifier's task to convert a low energy, voltage-driven signal into a higher-voltage output signal. Low impedance circuits can be dangerous because of the high current draw that they produce. Op amps avoid this by having very high input impedance. In other words, Op amps need high input impedance because they are voltage-gain devices. In order for voltage to drop across the input, the impedance has to be very high, as ohm's law states, V=IR. It's also important to prevent the loading effect. If the impedance were small, the current draw would be high.

Ⅲ How to Calculate Input Impedance and Output Impedance

Impedance is represented by the ratio of the current variation ΔI to the voltage variation ΔV. The variation in the input bias current is measured against the variation in the input common-mode voltage range.

Input Impedance and Output Impedance of Amplifier

Given the gain, source impedance, and output impedance, the formulas for the input and output voltages of an amplifier can be determined using the voltage divider principle.
(Zin/(Rs + Zin)) Vin = Vsource ......(1)
Where the reference voltage the amplifier sees is Vin, the input voltage is Vsource, the input impedance is Zin, and the source impedance is Rs.
It is possible to calculate the output load voltage similarly:
Vload = Vout • (Rload/(Rload + Zout)) ......(2)
Where the voltage across the load is Vload, Vout is the amplifier's output, Rload is the load resistance, and Zout is the amplifier's output impedance. It is also possible to replace Vout with input voltage gain times.
You can also measure the output impedance as a Thevenin equivalent circuit:
Zout = Vo/Is ......(3)
Where Vo is the output voltage when the output is open circuit and when the output is short, the output current is I. A linear relationship between the output voltage and current is assumed by this formula.

Ⅳ Conclusion

Op amps are generally employed in situations where input impedance should be huge relative to other impedances on the input pin, and where the feedback path should make the effective output impedance infinitesimal relative to external impedances. The nature of the application will determine how good the op amp must be to meet these requirements. The input and output impedance of amplifiers is a product of input and output parasitic resistance and capacitance. It is also given with formulas for the same. Knowing these limits and how to solve them contributes to the efficient design of amplifiers.