After we familiar with op-amp from the two article ago. Before using op-amp in real application, we must have known about their specifications thoroughly. So, this section we are explain the various parameters of an op-amp and how to interpret the data sheet. Be aware that different manufacturers use slightly different terminology.
Op-amps can be generally grouped as follows:
| 1. | General Purpose: These are the most common type of op-amp used. They have decent specifications and are low cost. They generally have gain-bandwidth products in the 1 to 10 MHz range and are very often unity gain stable. These op-amps would be used where critical specifications are not important. |
| 2. | Wideband: These op-amps have gain-bandwidth products in the 10 MHz to over 1 GHz range. The DC specifications are often very inferior to other types, high offset voltage, high bias current, etc. Many of these op-amps are not unity gain stable and must operate with a minimum closed loop gain or they will oscillate. |
| 3. | Precision: These op-amps feature premium DC specifications such as very low offset voltage and drift as well as very low bias current and drift. These tend to be expensive but are well worth it for the intended applications. |
| 4. | Special feature: These op-amps have some special characteristic that makes them particularly suited for special applications. A short list of op-amps of this type would include micro-power, audio applications, high output drive current, zero offset voltage, zero diff voltage, etc. |
Some Example Devices
| ▪ | LM741 (basic application) |
| ▪ | LMC660 (CMOS - low power supply) |
| ▪ | LT1220/1221 (fast response) |
| ▪ | LM675 (medium power output) |
| ▪ | LM12 (high power output) |
Data Sheet Parameters
The following is a list of common data sheet parameters for op-amps.
| 1. | Open-loop gain, Av: The open-loop gain, Av, of an op-amp is typically very large and may range from around 10,000 to over 1,000,000. Av varies with temperature and power supply voltage. Av for large signals is generally less than Av for small signals. |
| 2. | Offset voltage: This is the voltage difference between the non-inverting input and the inverting input when the op-amp is in a stable condition with negative feedback and the output voltage is not saturated at either the upper or lower power supply rail. This voltage is typically in the single digit millivolts but can be over ten millivolts for very high speed op-amps. |
| 3. | Offset voltage drift: This specification is usually given as it relates to drift with temperature and has typical units of microvolts per degree C. |
| 4. | Input bias current: All op-amps must have some input bias current however small. This value is typically in the nanoampere region but is can be in the picoampere region for premium parts. Bias current is not purely static. It varies with power supply voltage, temperature, common-mode voltage, and other parameters. |
| 5. | Input offset bias current: The difference between the bias current for the noninverting input and the bias current for the inverting input is known as the offset bias current (often referred to as the offset current).Input offset bias current is varies with power supply voltage, temperature, common-mode voltage, and other parameters. |
| 6. | Input bias current drift: This specification is usually given as it relates to drift with temperature and has typical units of picoamperes per degree C. |
| 7. | Common-mode input voltage range: The common-mode input voltage must be between the specified limits in order for the op-amp to work. If the voltage is outside this range then the output of the amplifier is undefined. Some op-amps are known as rail-rail input and the common mode input voltage includes the entire power supply range. |
| 8. | Maximum input differential voltage: In normal usage with negative feedback, the input differential voltage is zero. However, when the op-amp is driven such that the output voltage cannot drive the inverting input to match the non-inverting input then a differential input voltage exists. |
| 9. | Maximum output voltage: This specification is a measure of how close the output voltage can be to the upper power supply. Typically, this is roughly 2.5 volts which means that for a +15 volt VCC, the maximum output voltage of a common op-amp is about 12.5 volts. Some op-amps have what is known as rail-rail output stages and those can put out a voltage that is within millivolts of VCC. |
| 10. | Minimum output voltage: This specification is a measure of how close the output voltage can be to the lower power supply. Typically, this is roughly 2.5 volts which means that for a -15 volt VEE, the minimum output voltage of a common op-amp is about -12.5 volts. Some op-amps have what is known as rail-rail output stages and those can put out a voltage that is within millivolts of VEE. |
| 11. | Maximum output current: There is a limit to how much current the output stage of an op-amp can source or sink. Generally, this is around 5 mA but can be higher. |
| 12. | Maximum power supply differential voltage: This is the maximum voltage that can exist between the VCC and VEE terminals. For many op-amps this is typically around 36 volts but can be higher or lower. A common power supply voltage for op-amps is +-15 volts for a total differential of 30 volts. |
| 13. | Gain-bandwidth product: This is the extrapolated unity gain frequency which by definition is the frequency at which the gain of the op-amp has dropped to 1.0. This term comes from multiplying the gain of the amplifier at DC by the frequency at which the gain has dropped by 3 dB. This is often at least several hundred kHz and could be over 1 GHz for some op-amps. |
| 14. | Slew rate: The slew rate is the maximum rate of change in the output voltage and has typical units of volts per microsecond. Low-bandwidth amplifiers may have a limit of less than a volt per microsecond. Wide bandwidth amplifiers may have a limit of over 50 volts per microsecond. |
| 15. | Large signal bandwidth or Full power bandwidth: This is the bandwidth over which the output of the amplifier can produce an undistorted full amplitude signal. This bandwidth is typically much less than the gain-bandwidth product because of slew rate limiting. |
| 16. | Minimum stable gain: This term is described in different ways depending on whose data sheet you are reading but it refers to the fact that many op-amps, especially wideband op-amps, will oscillate if the closed loop gain is less than a specified amount. Other op-amps are advertised as “unity gain stable” which means they will remain stable even for a closed loop gain of one. |
| 17. | Input resistance: This is the input resistance with respect to circuit ground and is often many megohms, especially if the input stage is based on field-effect transistors. Except for wideband amplifiers the input resistance is often treated as infinite. |
| 18. | Differential input resistance: This is the resistance between the non-inverting and inverting inputs. It is typically many megohms and in many cases can be treated as infinite. |
| 19. | Output resistance: This is the open loop output resistance and is often in the 50 to 200 ohm range at low frequencies and typically increases with frequency. |
| 20. | Common-mode rejection: CMR is a measure of how well the amplifier rejects common-mode signals at the inputs. An ideal amplifier would have infinite rejection. CMR is given at DC and has typical specs in the 60 to 100 dB range and degrades as frequency is increased. |
| 21. | Power supply rejection: PSR is a measure of how well the amplifier rejects variations on the power supply voltages. An ideal amplifier would have infinite rejection. PSR is generally provided at some low frequency has typical specs in the 60 to 100 dB range and degrades as frequency is increased. |
| 22. | Input voltage noise: This specification is very important in determining how small a signal can be processed by the amplifier. Because the manufacturer cannot know what bandwidth you will be using they provide the noise specification normalized to a one Hertz bandwidth. Thus, the units are typically nanovolts per root Hz. You multiply this value by the square root of the bandwidth you are using to obtain the rms noise voltage referred to the input of the op-amp. The noise at the output is input noise multiplied by the closed loop gain. |
| 23. | Input current noise: This specification is very important in determining how small a signal can be processed by the amplifier. The manufacturers provide this specification and you can compute this value as same as the input voltage noise. The current noise becomes a voltage noise when passing through external resistances at each amplifier input. The noise at the output is input noise voltage multiplied by the closed loop gain. |
Other notes
| Many of the newer op-amps feature a common mode input range that extends from the lower power supply to the upper power supply. These are known as rail-rail input op-amps. The designer must understand the application and choose the appropriate op-amp. The general advice is to avoid these unless the rail-rail feature is essential. Always consult application information before using the part. That an op-amp has rail-rail inputs does not infer that it also has a rail-rail output. |
| Another feature of some of the newer op-amps is an output voltage range that extends from the lower power supply to the upper power supply. These are known as rail-rail output op-amps. Be sure to study the application information for the part as some unexpected effects may be present. |
| Rail-rail output is very convenient and is often essential for very low voltage applications. That an op-amp has a rail-rail output does not infer that it also has rail-rail inputs. |
| Some op-amps feature rail-rail inputs and rail-rail output. These are sold at a premium price. They are very convenient to use and generally work well but be aware of possible limitations or compromises in specifications. The general advice is to avoid these unless the rail-rail features are essential. At low power supply voltages of 5 volts or less, rail-rail input and output is typically a must have feature. |
