Op-Amp Basics Part II (Parameters)
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Introduction of the basic parameters of op-amp
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Op-Amp Basics Part II (Parameters)
- 16. Gain and Phase Margin
Hinweis der Redaktion
- Welcome to this module on the basic parameters of op-amp from National Semiconductor.
- The first parameter is the input offset voltage of the amplifier. Ideally the output of the amplifier should sit at mid-supply when the two input voltages have the same voltage or they’re equal. However, a voltage will appear on the output under this condition. To prevent this from happening a very small amount of voltage difference or “offset” is applied to the inputs of the amplifier and this will set the output back to mid-supply. This amount of offset voltage is simply referred to as input offset voltage.
- Input bias current. Again, this is a parameter that would ideally be zero. A positive input bias current refers to small current that’s seen on the non-inverting input of the amplifier and similarly, negative input bias current is the small amount of current that’s seen on the inverting pin of the amplifier. Input bias current refers to the average of these two values. Simply put, input bias current is the average of the two input currents of the amplifier.
- Input offset current refers to the difference between the bias currents of the amplifier. Again, ideally the two currents should be equal to obtain a zero output voltage. However, there has to be a difference between the two bias currents to set the output to zero. This difference is referred to as input offset current.
- Output impedance of an amplifier: Amplifiers should have very low output impedance and usually it’s assumed to be zero and in this way op-amps will behave like a voltage source and it will be capable of driving a very wide range of loads.
- The next parameter is slew rate. Slew rate refers to the maximum rate of change of the output of the amplifier per unit time. Usually it’s expressed in volts per microsecond and it tells you how fast your output of the op-amp can follow its input. In the graph you can see the output signal is “slewing.” The blue signal is applied to the input of the amplifier and the red signal or the output is trying to follow the input as fast as it can but it slews on the rising edge and the falling edge.
- There are two major sources of noise and we’re going to touch up on them very lightly since noise is a very broad topic and covering noise in detail is outside the scope of this presentation. Internal noise of an amplifier, as the name carries it, is caused by internal components of the amplifier, bias current, and/or drift. Noise or unwanted portion of the signal is usually amplified along with the wanted portion of the signal. Noise is always gained in the non-inverting configuration or (1 + RF/RG). There are certain ways that we minimize the amount of noise. One is by keeping the feedback resistors as low as we possibly can and also we can use bypass capacitors in parallel with the feedback resistor at higher frequencies.
- Different electrical devices and components, such as power supply or resistors can be the cause of external noise in amplifiers. Engineers can use proper circuit construction to minimize the effects of external noise in our circuits. One way to achieve this is to use shielding around power supply or reducing the resistor values whenever it’s possible.
- Common mode rejection is a feature of differential amplifiers. Op-amps are amplifiers with differential input; so common mode rejection applies to operational amplifiers. Common mode signal is when both of the inputs of the amplifier have the same voltage or they have a “common voltage” across them. Under this condition the output of the amplifier should be zero or the amplifier should reject the signal and not amplify it.
- Common mode rejection ratio or CMRR is the ratio of differential gain of the amplifier to the common mode gain of the amplifier when there is no differential signal on the input. As the graphics in the current slide show, we see that the differential gain of the amplifier is in response to the differential input signal between the two inputs of the amplifier and the common mode gain of the amplifier is the gain of the common mode signal.
- Common mode rejection ratio decreases with frequency. Common mode rejection ratio is the ability of an op-amp to reject the common mode signal while it’s still amplifying its differential signal. CMRR is usually expressed in dB and the two equations that we have here are two different ways to calculate common mode rejection ratio; 20 log of the ratio of differential gain to common mode gain and/or 20 log of the ratio of the change in offset voltage to the change in common mode voltage.
- Common mode voltage range of the signal: Common mode voltage range of the signal refers to the input voltage range for which the differential input pair is still within its linear operating region. The upper and lower limits are determined by different transistors and internal circuitry of the op-amp.
- The next parameter is power supply rejection ratio. Power supply rejection ratio or PSRR is the ratio of differential gain to small gain of the power supply. Another way to look at it is ratio of change of the power supply voltage to the change in offset error. When looking at a product’s datasheet, you will notice that PSRR and CMRR are specified and guaranteed for DC conditions or for zero frequency. There might be additional supplemental AC curves for these parameters that show the behavior of PSRR and CMRR over frequency.
- Other parameters are the gain and phase margin of amplifiers. Gain margin of an amplifier is the gain of an amplifier at the point where there’s already been an 180 o phase shift and if the gain at this point, at 180 o phase shift, is more than unity then the amplifier is going to be unstable. That’s because once there’s been an 180 o phase shift feedback is actually going to be positive. In terms of dB that means negative gain is going to be stable at 180 o degree phase shift. Phase margin is the difference between the phase when gain is unity, or zero dB, and 180 o . If at zero dB the phase lag is greater than 180 o the amplifier’s unstable for the reasons that we just mentioned because that would imply that the gain margin is positive.
- In this graphic phase margin and gain margin of an amplifier is clearly stable. The gain margin has a negative value and it is measured when there’s been 180o phase shift in the amplifier. The phase margin happens at 0dB and the phase lag is less than 180 o .
- The next two parameters are actually datasheet limits that play important roles. The first one is absolute maximum ratings of an amplifier. As the term carries it, the maximum here refers to the maximum limits that an op-amp will actually be able to operate safely. If the op-amp is forced to operate under conditions that exceed the maximum ratings, there will be permanent damage to the internal circuitry of the op-amp. Absolute maximum ratings are usually mentioned in the data sheets.
- Next is operating ratings of the amplifier. These are conditions under which the amplifier is functional. However, specific performance guarantees do not apply to these conditions. An example could be an op-amp for which the guaranteed table specifications are when there is ± 2.5 volts supply, but operating rating of the amplifier exceeds these values.
- Thank you for taking the time to view this presentation. If you would like to learn more or go on to purchase some of these devices, you can either click on the link embedded in this presentation, or simple call our sales hotline. For more technical information you can either visit the National Semiconductor site – link shown – or if you would prefer to speak to someone live, please call our hotline number shown, or even use our ‘live chat’ online facility.