Linear Circuit Analysis


Non-Ideal Operational Amplifiers

An operational amplifier (or OpAmp) is a voltage amplifier with a differential input, a single-ended output, and very high gain. Operational amplifiers are widely used in electronics for signal amplification, filter design (for instance to avoid using bulky inductors), analog calculators, analog-to-digital converters, differentiators, integrators, etc.

The equivalent circuit of an OpAmp is shown in Fig. 1, where $R_{in}$ is the input resistance, $R_{out}$ is the output resistance, and $A$ is the gain. Practical (or non-ideal) OpAmps have finite input resistance, non-zero output resistance, and/or the dependent voltage source has a finite gain. In contrast, ideal OpAmps have $R_{in}=\infty$, $R_{out}=0$, and $A=\infty$.

+ Vin Rin AVin Rout +
Fig. 1. Equivalent circuit of a non-ideal OpAmp in which the input resistance $R_{in}$, output resistance $R_{out}$, and gain $A$ are finite.

Sometimes, only one of the three parameters describing the OpAmp is finite. For instance, the input resistance of the OpAmp represented in Fig. 3 is infinite, the output resistance in zero, but the gain is finite. In this case, the OpAmp is still called non-ideal.

AVin + + Vin
Fig. 2. Equivalent circuit of a non-ideal OpAmp in which $R_{in}=\infty$, $R_{out}=0$ and $A$ is finite.

OpAmps are usually represented in simplified form, as shown in Fig. 3, where the power supply is assumed to be part of the device itself. Since the OpAmp is a four-terminal device, the algebraic sum of the currents flowing out through its four terminals must equal zero in accordance with (Kirchhoff's current law).

However, to simplify notations, the ground electrode is often omitted when representing OpAmps (see the right-side diagram in Fig. 3). This representation is somewhat misleading, as it may suggest that the algebraic sum of the currents flowing out of the three visible terminals is zero, which is generally not the case. Consequently, even when an OpAmp is depicted with only three electrodes, it is implicitly understood that a fourth terminal (the ground electrode) is present to satisfy Kirchhoff's current law.

A1 A2
Fig. 3. Simplified representations of an OpAmp.

Table 1 shows the values of the input resistance, output resistance and gain for a few OpAmps available commercially. In addition to these parameters manufacturers also specify parameters such maxim voltage, operating range of temperatures, cutoff frequency, transient response time, input noise, etc.

Table 1. Common electric components used in linear circuits.
Manufacturer Part No. $A$ $R_{in}$ $R_{out}$ Applications
ST Microelectronics LM324 $10^6$ 2.6 MΩ 20 Ω Low power, general OpAmp
Texas Instruments LM741A $2\times10^5$ 2 MΩ 150 Ω General OpAmp
Texas Instruments OPA132P $10^7$ 10,000 GΩ 100 Ω High-speed OpAmp
How to Solve Problems with Non-Ideal OpAmps

To solve problems with non-ideal OpAmps it is always recommended to replace the OpAmp with its equivalent circuit shown in Fig. 1. Then, one can use nodal analysis to compute the sought variables.

Inverter

A standard configuration for OpAmps is the inverting amplifier shown in Fig. 4. If the input and output resistances of the OpAmp are $R_{in}$ and $R_{out}$ respectively, and the gain is $A$, it is relatively simple to show that $$\begin{equation}\frac{V_{out}}{V_{in}}=-\frac{1}{\frac{{\color{blue}R_{out}}+R_2}{{\color{blue}A} R_2 - {\color{blue}R_{out}}} \left(1+\frac{R_1}{R_2}+\frac{R_1}{{\color{blue}R_{in}}}\right) + \frac{R_1}{R_2}} \end{equation}$$ In the limit when $R_{in}=\infty$ and $R_{out}=0$, the above equation simplifies to $$\begin{equation}\frac{V_{out}}{V_{in}}=-\frac{1}{\frac{1}{{\color{blue}A}} \left(1+\frac{R_1}{R_2}\right) + \frac{R_1}{R_2}} \end{equation}$$

In the limit when $R_{in}=\infty$, $R_{out}=0$ and $A=\infty$ (this is the case of ideal OpAmps) the above equation simplifies to $$\begin{equation}\frac{V_{out}}{V_{in}}=-\frac{R_2}{R_1} \end{equation}$$ which is the gain of an inverter with ideal OpAmp (see next section in this WebBook).

R2 R1 A + Vout RL Vin
Fig. 4. Inverting amplifier.
Follower

Another standard configuration for OpAmps is the non-inverting amplifier (follower) shown in Fig. 5. If the input and output resistances of the OpAmp are $R_{in}$ and $R_{out}$ respectively, and the gain is $A$, one can show that $$\begin{equation}\frac{V_{out}}{V_{in}}=\frac{\frac{1}{{\color{blue}R_{in}}}+A \frac{\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{{\color{blue}R_{in}}}}{\frac{{\color{blue}R_{out}}}{R_2}-A} } {\left(\frac{1}{{\color{blue}R_{in}}} + \frac{1}{R_2}\right) \frac{\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{{\color{blue}R_{in}}}}{\frac{1}{R_2}-\frac{A}{{\color{blue}R_{out}}}} - \frac{1}{R_2}} \end{equation}$$ If $R_{in}=\infty$ (this approximation works well for OpAmps made with FETs), the above equation simplifies to $$\begin{equation}\frac{V_{out}}{V_{in}}= \frac{1}{ \frac{1+\frac{{\color{blue}R_{out}}}{R_2}}{A}+ \frac{1}{\beta}}\end{equation}$$ where we denoted $\beta=1+\frac{R_2}{R_1}$. If also $R_{out}=0$, the above equation simplifies to $$\begin{equation}\frac{V_{out}}{V_{in}}= \frac{1}{ \frac{1}{A}+ \frac{1}{\beta}}\end{equation}$$ Finally, if $R_{in}=\infty$, $R_{out}=0$ and $A=\infty$ (i.e. in the case of ideal OpAmps) we get $$\begin{equation}\frac{V_{out}}{V_{in}}=1+\frac{R_2}{R_1} \end{equation}$$ which is the gain of a follower with ideal OpAmp (see next section in this WebBook).

A R1 R2 + Vout RL Vin
Fig. 5. Non-inverting amplifier.
Other Configurations

Ideal OpAmps have many important applications in electronics, audio and video-frequency pre-amplifiers and buffers, differential amplifiers, precision rectifiers, peak detectors, voltage and current regulators, analog calculators, analog-to-digital and digital-to-analog converters, clippers and clampers. When used in combination with inductors and capacitors one can build integrators, differentiators, filters, and oscillators.

Examples of Solved Problems
See also

Ideal Op-Amps
Nodal analysis

Read more

Operational amplifier