Linear Circuit Analysis
1. Introduction
2. Basic Concepts
- Charge, current, and voltage
- Power and energy
- Linear circuits
- Linear components
- Loops and nodes
- Series and parallel
- R, L & C combinations
- V & I combinations
3. Simple Circuits
- Ohm's law
- Kirchhoff's current law
- Kirchhoff's voltage law
- Single loop circuits
- Single node-pair circuits
- Voltage division
- Current division
4. Nodal and Mesh Analysis
5. Additional Analysis Techniques
- Superposition
- Source transformation
- The $V_{test}/I_{test}$ method
- Norton equivalent
- Thévenin equivalent
- Max power transfer
6. AC Analysis
7. Magnetically Coupled Circuits
8. Polyphase Systems
9. Operational Amplifiers
10. Laplace Transforms
11. Time-Dependent Circuits
- Introduction
- Inductors and capacitors
- First-order transients
- Nodal analysis
- Mesh analysis
- Laplace transforms
- Additional techniques
12. Two-Port Networks
Appendix
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 $A=\infty$.
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.
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.
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.
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).
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).
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
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OpAmps with finite gain (A) in DC circuits
Circuit with 1 ideal OpAmp, follower (numerical)
Circuit with 1 ideal OpAmp (numerical)
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OpAmps with finite gain (A) and input resistance (Rin) in DC circuits
Circuit with 1 real OpAmp with finite A and Rin, follower (numerical)
Circuit with 1 real OpAmp with finite A and Rin (numerical)
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OpAmps with finite gain (A) in DC circuits
Circuit with 1 real OpAmp with finite A, follower with unit gain (numerical)
Circuit with 1 ideal OpAmp, follower (numerical)
Circuit with 1 ideal OpAmp, inverter (numerical)
Circuit with 1 ideal OpAmp (numerical)
Circuit with 2 ideal OpAmps in series (numerical)
Circuit with 2 ideal OpAmps in parallel (numerical)
Circuit with 1 real OpAmp with finite A, follower with unit gain (numerical gain calculation)
Circuit with 1 real OpAmp with finite A, follower (numerical gain calculation)
Circuit with 1 real OpAmp with finite A, inverter (numerical gain calculation)
Circuit with 2 real OpAmps with finite A in series (numerical gain calculation)
Circuit with 1 real OpAmp with finite A, follower with unit gain (analytical)
Circuit with 1 real OpAmp with finite A, follower (analytical)
Circuit with 1 real OpAmp with finite A, follower (numerical design)
Circuit with 1 real OpAmp with finite A, inverter (numerical design)
Circuit with 1 real OpAmp with finite A (numerical design)
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OpAmps with finite gain (A) and input resistance (Rin) in DC circuits
Circuit with 1 real OpAmp with finite A and Rin, follower with unit gain (numerical)
Circuit with 1 real OpAmp with finite A and Rin, follower (numerical)
Circuit with 1 real OpAmp with finite A and Rin, inverter (numerical)
Circuit with 1 real OpAmp with finite A and Rin (numerical)
Circuit with 2 real OpAmps with finite A and Rin in series (numerical)
Circuit with 2 real OpAmps with finite A and Rin in parallel (numerical)
Circuit with 1 real OpAmp with finite A and Rin, follower with unit gain (numerical gain calculation)
Circuit with 1 real OpAmp with finite A and Rin, follower (numerical gain calculation)
Circuit with 1 real OpAmp with finite A and Rin, inverter (numerical gain calculation)
Circuit with 2 real OpAmps with finite A and Rin in series (numerical gain calculation)
Circuit with 1 real OpAmp with finite A and Rin, follower with unit gain (analytical)
Circuit with 1 real OpAmp with finite A and Rin, follower (numerical design)
Circuit with 1 real OpAmp with finite A and Rin, inverter (numerical design)
Circuit with 1 real OpAmp with finite A and Rin (numerical design)