Op-Amp Characteristics for Simulation, Models, and PCB Design

In our world of social technologies, nearly everything that someone writes or says—or thinks that they have heard or said—becomes amplified. Some Twitter accounts have as many as 100 million followers. Influence marketing has replaced conventional marketing techniques by identifying individuals who have influence over potential customers. Everything comes back to amplifying the brand or the message.

In electronics, we work with different types of amplifiers every day. Versatility and simplicity have made operational amplifiers the backbone of many electronic circuits. The stability of those circuits contrasts against the instability of social network amplification.

Operational Amplifier Characteristics

When we work with operational amplifiers (op-amps), we consider voltage amplification and the use of capacitors and resistors as feedback components. If we break an operational amplifier into individual parts, we have:

• A multi-stage, high-gain, direct-coupled amplifier

• An inverting input

• A non-inverting input, and

• A single-ended output.

The analysis of an operational amplifier considers voltage, current, and impedance at the input and output terminals, gain bandwidth product, and the gain at the output terminals. Before going any further, though, we should define a few terms. An op-amp may have a voltage common between either of its inputs and ground called the common-mode voltage. The common-mode voltage gain represents the amount of amplification applied to the common-mode voltage.

We can measure the performance of an op-amp by how much the device amplifies the difference between the two input voltages divided by the common-mode voltage gain.  Dividing the differential voltage gain by the common-mode gain gives us the common-mode rejection ratio (CMRR) or:

CMRR = differential Av / common-mode Av

The input offset voltage (Vio) of an operational amplifier is the differential input voltage required to make the output voltage equal zero. An output dc offset voltage is the output voltage that results from a zero input voltage.

With feedback controlling the operating characteristics of an op-amp, the linear device not only amplifies but also function as filters and signal conditioners or perform mathematical functions. Most circuits use negative feedback—or the return of part of the 180o out-of-phase output signal to the input signal—at either the inverting or the non-inverting input. We refer to an op-amp circuit that uses feedback as operating with a closed loop.  An operational amplifier with no feedback at the inputs operates with an open loop. Closed loop operation yields precisely controllable gain while open loop gain ranges from 20,000 to 100,000.

Op-Amp Analysis: Nothing is Ideal

When we analyze any type of device, we need to establish a benchmark. With operational amplifiers, we use Ideal Operational Analysis to set the perfect benchmark parameters. Ideal Operational Analysis assumes that the current flow to the input leads equals zero. From there, we assume that the device has infinite gain. That is, an ideal operational amplifier can drive an output voltage to any value that satisfies the input conditions.

Let’s think about this for a moment in terms of the ideal operational amplifier. With no current and no signal at the input leads of the op-amp, we can also assume that zero voltage exists at the inputs. Both leads remain at the same potential. The condition of zero current at the input leads also indicates that the ideal operational amplifier has infinite input impedance.

At this point, our analysis of the ideal op-amp becomes extremely interesting. Because an ideal op-amp has a zero output impedance, it can drive any load without an output impedance dropping a load across it.  An ideal operational amplifier also has a flat frequency response. As a result, any increase or decrease in frequency has no impact on gain.

Now that we have completed a quick ideal operational analysis, let’s summarize the ideal operational amplifier characteristics.

Op-Amp Characteristics: Here’s the Real Deal

The cool part of using Ideal Operational Analysis as a benchmark is that some op-amps can come close to near-ideal operating characteristics. For example, most operational amplifiers have an output impedance of less than an ohm at low current. With some practical op-amps, we can use negative feedback to approach ideal input resistance, output resistance, and bandwidth characteristics.

However, key differences between ideal and practical op-amps exist.

Every practical operational amplifier has imperfections and limitations. Every day, practical op-amps consume power, have a measurable input bias current and high to very high input impedances, limited gain, and an input offset voltage. In real life, parameters such as gain occur as a function of frequency. Even with those limitations, practical op-amps provide performance characteristics that match or surpass the requirements of most circuits.

Understanding the functionality behind op-amps enables your designing

While the inputs of an ideal operational amplifier do not draw current, a small amount of dc bias current enters both of the inputs (IB+ and IB-) of a practical amplifier. The values of the bias currents range from 60 Femto Amps (fA) or 60 x 10-15 to 10 nanoamps. When designing your circuit, you should consider that even small amounts of bias current can create problems and could use an op-amp that includes some type of internal bias current compensation. Without the cancellation, the bias current flows in external impedances and produces voltages that add to system errors. With the cancellation, the circuit also has low voltage noise, a low offset, and minimal drift but may experience current noise.

Practical op-amps also have an input offset current (IOS) that is the difference between IB+ and IB-. The input offset current only becomes a factor when the two bias currents match. If the operational amplifier has internal bias current compensation, the offset current will have the same magnitude as the bias current.

When we refer back to the ideal op-amp characteristics, we also find that input and output voltages equal zero. In contrast to the ideal input and output voltages that equal zero, a typical op-amp has an input offset voltage of approximately two millivolts. The small differential voltage applied to the inputs force the output of the op-amp to zero. Amplification of that very small voltage produces a larger output voltage and introduces dc error.

Practical operational amplifiers have an open-loop voltage gain that equals the ratio of the change in output voltage to a change in input voltage without feedback. In contrast to the ideal op-amp, a practical op-amp has a high voltage gain for low frequency inputs. As the input frequency increases, the voltage gain decreases.

Now, let’s compare the ideal operating characteristics with the characteristics of a general purpose practical op-amp.

Op-Amp Circuit Analysis

Working with or toward an ideal operational amplifier is great; however, what if you had to use a non-ideal op-amp? Utilizing strong circuit analysis techniques can ensure that your op-amp usage doesn't char your circuit and instead continues functioning appropriately and optimally within the overall current and voltage flow. 

Particularly for op-amps, we'll be looking at bias, or steady state operating characteristics with no signal being applied, and gain, where you'll most likely be running into open-loop or closed-loop voltage gain. 

DC Bias in Op-Amps

In order to determine your DC bias, you'll need to know the input bias current of the specific op-amp you're using. Typically you'll be able to find this in your datasheet, and if not it should be a fairly quick simulation. After knowing the input bias, you can then find the DC output bias voltage. This can change depending on if you're using an inverting or non-inverting op-amp, with the respective characteristics depending on feedback resistors to lead to proportional voltage gains.  

Input and Output Bias of Op-Amps

While above we explained ideal op-amp input and outputs, importantly, there's additional characteristics involved in non-ideal op-amps. Things like voltage follower circuits and unity voltage gains allow you to design more nuanced and complicated circuits the more you come to understand the relationships op-amps have to the current and voltage flow in your circuit. 

Op-Amp Voltage Gain and Current Flow

You can usually look at current flow in your circuit through the input resistor and feedback resistors that your op-amp comes between. You may even have the luxury of working in or toward circuits that utilize resistors in a way so as to create a virtual ground. 

Operational Amplifier Characteristics and PCB Design

Understanding the differences between ideal and practical characteristics allows you to select the operational amplifier characteristics that deliver the best performance by accurately matching the needs of your application. Without the knowledge of how operational amplifiers work in low-noise, high speed applications, circuit performance will suffer. Modeling a practical op-amp illustrates the non-ideal characteristics.

When you design your PCB, remember that an op-amp is analog. Your layout should partition the op-amp circuitry in a separate section of the board. The use of high-speed analog circuits not only impacts the layout of the board but also the selection of materials used for the PCB. PSpice can simulate and analyze the effect of any of these practical Op Amp characteristics on the overall functioning of your circuit, too.

When it comes to designing around some finnicky components which alter the state of your power and signals, you’ll want to be able to trust your simulation, analysis, and modelling efforts. With OrCAD's PSpice you’re capable of choosing component models from a library of over 34,000 listings and accurately assess model parameters for your design.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.