Denis Lachapelle, ing.

Francis Thiffault ing.

**Introduction**

Operational amplifiers have existed for over 50 years and each year there are many new op-amp introduced on the market, the questions are: Why so many op-amp are needed? Which op-amp to select?

The first monolithic op-amp appeared in the sixties with part numbers like uA702, uA709, LM101, uA741. But even before monolithic op-amps there were op-amps built with discrete transistors and even tubes. Since these early days of monolithic op-amp, new op-amps are introduced to the market every year.

Op-amp function is quite simple, as shown in Figure 1, it provides a differential input, a high-gain, and an output. Even if the function looks very simple op-amp can be used for realizing many types of circuit blocks such as inverting amplifier, non-inverting amplifier, adder, integrator, differentiator, non-linear gain block, multiplier, and you can possibly imagine more.

*Figure 1, Modèle d’ampli opérationnel.*

**Feedback**

All the types of circuit blocks listed above are based on the theory of feedback and more particularly negative feedback, as shown in Figure 2. The negative feedback principle is to take a portion of the output and to subtract it back from the input.

*Figure 2, Feedback.*

The equation of the output can be derived as :

Afb: Close-Loop gain.

Vout: Output Voltage.

Vin: Input Voltage.

Aol: Open-Loop Gain.

β: Feedback Path Gain

And if AOL is very large the equation simplifies to AFB ≈ 1 / β, notice that AOL is no more present in this equation. In the context of op-amp the AOL is the open loop gain of the op-amp. So according to this equation, the gain of the op-amp if large enough does not have impact on the circuit block function.

**Follower**

The most basic op-amp based circuit is probably the follower, which has a gain of one, as shown in Figure 3. In this case β is equal to 1 and the gain is one. So, what this circuit does, it has a gain of 1?

*Figure 3, Suiveur.*

This circuit is of great importance since it provides a high input impedance and a low output impedance, so it is used to adapt signal impedance. As example if we need to measure the voltage provided by a voltage divider and convert it to digital, in most case we cannot drive the ADC directly with the voltage divider, we insert a follower in between the voltage divider and the ADC input.

In this case we select an op-amp with a high input impedance, much larger than the Thevenin equivalent of the voltage divider and a low output impedance to make sure the output is not affected by the ADC input impedance. We also select an op-amp with the proper bandwidth and a small DC offset in between the plus and minus pins to limit DC error.

Now just with this simple circuit, the follower, we can list five parameters that are important for an op-amp, the Table 1 lists these five parameters and their respective value for three op-amp, two dating from the sixties and another more modern (2023).

*Table 1, Basic Op-Amp Parameters*

UA702 | UA741 | TLV365 | |
---|---|---|---|

Open loop gain | 66 dB | 94 dB | 120 dB |

Input impedance | 50kohm | 6Mohm | 5pF (en anglais seulement) |

Output impedance | 300ohm | 70ohm | 40ohm |

Bandwidth | 20 MHz | 1Mhz | 50 MHz |

Offset voltage | 0,5 mV | 0,8 mV | 0,4 mV |

Note that the UA702 is uncompensated compared to the other two op-amps, this explains why the gain bandwidth is so large compared to the UA741.

By just considering the follower circuit we understand that a simple op-amp has many parameters, we should understand each of them to select the proper op-amp for each particular application.

A high open loop gain will minimize the approximation error caused by the simplification of the negative feedback loop equation to AFB ≈ 1 / β. A high input impedance ensures the op-amp input has a minimum effect on the input voltage source. A low output impedance will reduce the gain error caused by the load on the op-amp output and will reduce the instability risk caused by capacitive load. A high bandwidth will flatten the frequency response. A low input offset voltage will reduce the DC error.

So, it is easy to imagine that manufacturers try to optimize each of these parameters to fit a particular class of applications, and since their processes differ and evolve, they come with better performances op-amp in each of their categories year after year.

**Other Parameters**

There are many more parameters than these five. For each application we consider many other parameters, the following paragraphs list some of these parameters including the few first we already discussed about.

### Open loop gain (dB)

Open loop gain is particularly important to lower gain error, note that gain error will increase with frequency since the gain margin get lower. Note that open loop gain is measured at very low frequency.

### Gain-Bandwidth Product (MHz)

The gain-bandwidth product is the frequency at which the gain is one or is 100 times the frequency at which the gain is 100.

### Input Offset voltage (mV, uV)

Input offset voltage is the DC voltage between the +/- differential input. Ideally, we would seek to have zero offset voltage, but in practice it is not possible. The input offset voltage is reflected at the output affected by the close loop DC gain.

### Input Offset Voltage Drift (uV/C)

The input offset voltage drift is important especially if you implement a circuit to trim the offset during calibration. In this case the offset of almost zero at the calibration temperature, but the offset will build up with temperature drift.

### Input impedance (Ohm, pF)

Input impedance is an important factor to consider, the choice really depends on the application as example if the driving circuit is a low impedance microphone a few 100kohm input impedance will be fine as opposed to application like electrometer measuring charge, then an op-amp with input impedance in the order of Tohm will be necessary.

### Input bias current (nA, pA, fA)

Input bias current may be reflected as a voltage output offset if the DC resistive path seen by each of the plus and minus input differs. Circuit topology should take care of this.

### Input Offset Current (nA, pA, fA)

Even if the DC resistive paths are equal, a voltage output offset will exist due to Input offset current, but the offset is generally much lower than the bias current.

### Input Common Mode Voltage Range (V)

The input common mode voltage is the range in which the op-amp will behave as expected and linearly, outside this range the gain may fall abruptly and in some of the first op-amp versions the gain was changing polarity. Many op-amps have input common mode voltage range from rail-to-rail and some have range over or below one rail or both rails.

### Common Mode Rejection Ratio (dB)

This is the ability to reject signal apply to both plus and minus pin simultaneously. This parameter will impact circuit rejection to noise. A low CMRR will also contribute to the gain error.

### Input Voltage Noise, Noise Density (uV, nV/√Hz)

Input voltage noise shall be seen as a noise voltage source in between the plus and minus input. It is specified as voltage or voltage density over frequency.

### Input Current Noise, Noise Density (nA, fA/√Hz)

Input current noise shall be seen as a current source in parallel with the plus and minus inputs. It is specified as current or current density over frequency. Proper circuit design will minimize the total contribution of the voltage noise source and current noise source.

### Output Voltage Range (V)

Output voltage range is limited by the supply range and internal op-amp design. Some op-amps have a range limit to 1 or 2 volts within the rails while some have range near one or both rails. When the input and output voltage range cover from negative rail to positive rail, op-amps are qualified as RRIO which means rail-to-rail-input-output.

### Output impedance (Ohm)

Output impedance is most of the time specified as the open-loop output impedance, this value can be few ohms to few hundred ohms. The complete circuit output impedance (closed loop output impedance) depends on the topology, it is approximately the open loop impedance divided by the gain margin. Note that close loop output impedance tends to increase with frequency due to the reducing gain margin.

### Output Current max (mA)

Output current max is the maximum current at which the op-amp is linear, at higher current there could be open-loop gain loss and clipping.

### Slew rate (V/us)

Slew rate is limited by the op-amp internal current source feeding the internal capacitor creating the dominant pole. In a way, the slew rate is related to the gain bandwidth product. The slew rate limitation can create distortion in high amplitude and high-frequency signal. As example if the op-amp is used to create a square wave, the edge of the square wave will be limited by the op-amp maximum slew rate.

### Supply Current (uA, mA)

Supply current per op-amp can range from few uA to few hundred mA. Generally, the higher the gain bandwidth product, higher is the supply current. Very low power op-amp have low gain bandwidth product.

### Supply Voltage (V)

Supply voltage can be as low as one or two volts up to fifty volts and more for specialized op-amp.

### Supply Voltage Rejection Ratio (dB)

Supply voltage rejection ration is a very important parameter to consider because it is the factor reducing the open-loop gain seen by the noise present on the supply voltage rails. At DC, it can be expressed as the ratio of input offset voltage change on the supply voltage change. Note that the rejection degrades with frequency increase.

### Operating Temperature (C)

The operating temperature is the suggested temperature range in which the op-amp can be operated without imminent failure. Note that operating an op-amp at or near maximum operating temperature will reduce its life expectancy.

### Absolute Maximum Ratings

Operating the op-amp over these limits may cause the device to become defective. As example maximum voltage on supply rails should be respected as well as maximum junction temperature and all other limits.

**Why So Many Op-Amp?**

Each different applications required different parameter set, as example for the follower we discussed above, we may be interested in lowering the DC error between the input and output, so in this case we look for small input offset voltage op-amp or we may be interested in a sufficiently large bandwidth to minimize the error up to 1Mhz. There are op-amp with input offset voltage from below 1uV to near 10mV and bandwidth from few kilohertz to few hundred megahertz, just 5 values in each dimension and you end-up with 25 flavors and this is only for two parameters and one manufacturer, it is easy to understand that many hundred flavors of op-amp exist. Note that the parameters are somewhat dependent; I mean they cannot all be controlled independently. As example the gain bandwidth product and the current consumption are related since for higher bandwidth transistor need more bias current.

You can visit a distributor web site and select general purpose op-amp to find that the offering is over many thousands various op-amp, and manufacturers release new version every year for over fifty years!

**Which Op-Amp to Select?**

This is a much more complex question to answer and there isn’t a single good answer. It depends on the application, including its environment, and trade-off between all parameters including price and availability.

Let’s start with the quality level such as commercial, industrial, automotive, military, medical, space. We should identify to which category the product belongs and limit our search to the category identified. As a note, there is less and less choice from commercial category to space category, in which one there are very few choices, and the costs are extremely high.

Then we go deeper with the most important parameters such as supply voltage, bandwidth, input offset voltage, input bias current, slew rate is important for large output signal, and of course cost is very important in some applications. You may also need to check the input common mode range, a parameter that is often overlooked.

There are some non-technical parameters like the cost and the manufacturer (Some companies try to focus on a limited set of manufacturers for strategic reasons).

The following table can be used to mark down the desired parameter values before starting the selection, if the application requires more parameters just add them to the list.

Parameters | Value | Unit |
---|---|---|

Category | ||

Mounting Type | ||

Supply Voltage | ||

Bandwidth | ||

Input Offset Voltage | ||

Input Offset Voltage drift | ||

Input Bias Current | ||

Input offset current | ||

Slew Rate | ||

Input Common Mode Range | ||

Output Voltage Range | ||

Supply Current |