OpAmp 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 (opamps), 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 multistage, highgain, directcoupled amplifier

An inverting input

A noninverting input, and

A singleended 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 opamp may have a voltage common between either of its inputs and ground called the commonmode voltage. The commonmode voltage gain represents the amount of amplification applied to the commonmode voltage.
We can measure the performance of an opamp by how much the device amplifies the difference between the two input voltages divided by the commonmode voltage gain. Dividing the differential voltage gain by the commonmode gain gives us the commonmode rejection ratio (CMRR) or:
CMRR = differential Av / commonmode 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 opamp, 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 outofphase output signal to the input signal—at either the inverting or the noninverting input. We refer to an opamp 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.
OpAmp 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 opamp, 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 opamp becomes extremely interesting. Because an ideal opamp 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.
Input bias current 
0 
Input offset voltage 
0 
Bandwidth 
∞ 
Input impedance 
∞ 
Output impedance 
0 
Voltage gain 
∞ 
OpAmp Characteristics: Here’s the Real Deal
The cool part of using Ideal Operational Analysis as a benchmark is that some opamps can come close to nearideal operating characteristics. For example, most operational amplifiers have an output impedance of less than an ohm at low current. With some practical opamps, we can use negative feedback to approach ideal input resistance, output resistance, and bandwidth characteristics.
However, key differences between ideal and practical opamps exist.
Every practical operational amplifier has imperfections and limitations. Every day, practical opamps 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 opamps provide performance characteristics that match or surpass the requirements of most circuits.
Understanding the functionality behind opamps 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 1015 to 10 nanoamps. When designing your circuit, you should consider that even small amounts of bias current can create problems and could use an opamp 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 opamps 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 opamp 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 opamp has an input offset voltage of approximately two millivolts. The small differential voltage applied to the inputs force the output of the opamp to zero. Amplification of that very small voltage produces a larger output voltage and introduces dc error.
Practical operational amplifiers have an openloop 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 opamp, a practical opamp 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 opamp.
Ideal Operating Amplifier Characteristics 
Practical Operating Amplifier Characteristics 

Input bias current 
0 
Input bias current 
60 fA to 10 nA 
Bandwidth 
∞ 
Bandwidth 
Several MHz 
Input Offset Voltage 
0 
Input Offset Voltage 
Few mV 
Input impedance 
∞ 
Input impedance 
Mega Ohms 
Output impedance 
0 
Output impedance 
Few ohms 
Voltage gain 
∞ 
Voltage gain 
1 x 105 
OpAmp Circuit Analysis
Working with or toward an ideal operational amplifier is great; however, what if you had to use a nonideal opamp? Utilizing strong circuit analysis techniques can ensure that your opamp usage doesn't char your circuit and instead continues functioning appropriately and optimally within the overall current and voltage flow.
Particularly for opamps, 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 openloop or closedloop voltage gain.
DC Bias in OpAmps
In order to determine your DC bias, you'll need to know the input bias current of the specific opamp 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 noninverting opamp, with the respective characteristics depending on feedback resistors to lead to proportional voltage gains.
Input and Output Bias of OpAmps
While above we explained ideal opamp input and outputs, importantly, there's additional characteristics involved in nonideal opamps. 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 opamps have to the current and voltage flow in your circuit.
OpAmp Voltage Gain and Current Flow
You can usually look at current flow in your circuit through the input resistor and feedback resistors that your opamp 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 lownoise, high speed applications, circuit performance will suffer. Modeling a practical opamp illustrates the nonideal characteristics.
When you design your PCB, remember that an opamp is analog. Your layout should partition the opamp circuitry in a separate section of the board. The use of highspeed 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.