how to simulate the offset of fully differential amplifier
Publishing Date (Electronic version). 25th June the OpAmp is evaluated using Cadence and Matlab simulations and it satisfies the the main error sources in a pipelined ADC are gain, offset and nonlinearity errors in the. Ideal Four Port Op-Amp with Finite Gain and Input Offset .. Setup for the simulation of output voltage swing of nulling op-amp on HSPICE. .. stabilizing vacuum tubes with a mechanical chopper that dates back to the 's . Operational Amplifier Simulations. Opamps have very high differential gain and any small offset voltage can saturate an opamp to the positive or the negative.
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Op-Amp Practical Considerations
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The problem is this: In order for this circuit to work properly, we must ground one of the input wires, thus providing a path to or from ground for both currents: Not necessarily an obvious problem, but a very real one! Another way input bias currents may cause trouble is by dropping unwanted voltages across circuit resistances. Take this circuit for example: We expect a voltage follower circuit such as the one above to reproduce the input voltage precisely at the output.
But what about the resistance in series with the input voltage source? But even then, what slight bias currents may remain can cause measurement errors to occur, so we have to find some way to mitigate them through good design. One way to do so is based on the assumption that the two input bias currents will be the same.
In reality, they are often close to being the same, the difference between them referred to as the input offset current. If they are the same, then we should be able to cancel out the effects of input resistance voltage drop by inserting an equal amount of resistance in series with the other input, like this: With the additional resistance added to the circuit, the output voltage will be closer to Vin than before, even if there is some offset between the two input currents.
In either case, the compensating resistor value is determined by calculating the parallel resistance value of R1 and R2. Why is the value equal to the parallel equivalent of R1 and R2? This gives two parallel paths for bias current through R1 and through R2, both to ground. A related problem, occasionally experienced by students just learning to build operational amplifier circuits, is caused by a lack of a common ground connection to the power supply.
This provides a complete path for the bias currents, feedback current sand for the load output current. Take this circuit illustration, for instance, showing a properly grounded power supply: The effect of doing this is profound: This effectively renders the op-amp useless: The bias currents are also stopped, because they rely on a path to the power supply and back to the input source through ground.
The following diagram shows the bias currents onlyas they go through the input terminals of the op-amp, through the base terminals of the input transistors, and eventually through the power supply terminal s and back to ground. Without a ground reference on the power supply, the bias currents will have no complete path for a circuit, and they will halt.
Since bipolar junction transistors are current-controlled devices, this renders the input stage of the op-amp useless as well, as both input transistors will be forced into cutoff by the complete lack of base current. Bias currents are small in the microamp rangebut large enough to cause problems in some applications.Op-Amp: CMRR (Common Mode Rejection Ratio) Explained (with example)
It is not enough to just have a conductive path from one input to the other. To cancel any offset voltages caused by bias current flowing through resistances, just add an equivalent resistance in series with the other op-amp input called a compensating resistor.
This corrective measure is based on the assumption that the two input bias currents will be equal. Any inequality between bias currents in an op-amp constitutes what is called an input offset current. It is essential for proper op-amp operation that there be a ground reference on some terminal of the power supply, to form complete paths for bias currents, feedback current sand load current.
Drift Being semiconductor devices, op-amps are subject to slight changes in behavior with changes in operating temperature. Any changes in op-amp performance with temperature fall under the category of op-amp drift. Drift parameters can be specified for bias currents, offset voltage, and the like.
The latter action may involve providing some form of temperature control for the inside of the equipment housing the op-amp s. This is not as strange as it may first seem. If extremely high accuracy is desired over the usual factors of cost and flexibility, this may be an option worth looking at. Op-amps, being semiconductor devices, are susceptible to variations in temperature. Any variations in amplifier performance resulting from changes in temperature is known as drift.
Drift is best minimized with environmental temperature control. Frequency Response With their incredibly high differential voltage gains, op-amps are prime candidates for a phenomenon known as feedback oscillation. An op-amp circuit can manifest this same effect, with the feedback happening electrically rather than audibly. A case example of this is seen in the op-amp, if it is connected as a voltage follower with the bare minimum of wiring connections the two inputs, output, and the power supply connections.
The output of this op-amp will self-oscillate due to its high gain, no matter what the input voltage. To combat this, a small compensation capacitor must be connected to two specially-provided terminals on the op-amp. If the op-amp is being used to amplify high-frequency signals, this compensation capacitor may not be needed, but it is absolutely essential for DC or low-frequency AC signal operation.
Some op-amps, such as the modelhave a compensation capacitor built in to minimize the need for external components. This improved simplicity is not without a cost: Op-amp manufacturers will publish the frequency response curves for their products.
The circuit designer must take this into account if good performance is to be maintained over the required range of signal frequencies.
Op-Amp Practical Considerations | Operational Amplifiers | Electronics Textbook
Due to capacitances within op-amps, their differential voltage gain tends to decrease as the input frequency increases. Frequency response curves for op-amps are available from the manufacturer. Input to Output Phase Shift In order to illustrate the phase shift from input to output of an operational amplifier op-ampthe OPA was tested in our lab.
The OPA was constructed in a typical non-inverting configuration Figure below. The input excitation at Vsrc was set to 10 mVp, and three frequencies of interest: Frequency plot To help predict the closed loop phase shift from input to output, we can use the open loop gain and phase curve.
What is actually at work here is the negative feedback from the closed loop modifies the open loop response.
Closing the loop with negative feedback establishes a closed loop pole at 22 kHz. Much like the dominant pole in the open loop phase curve, we will expect phase shift in the closed loop response.
How much phase shift will we see? Since the new pole is now at 22 kHz, this is also the -3 dB point as the pole starts to roll off the closed loop again at 20 dB per decade as stated earlier.
As with any pole in basic control theory, phase shift starts to occur one decade in frequency before the pole, and ends at 90o of phase shift one decade in frequency after the pole. So what does this predict for the closed loop response in our circuit? This will predict phase shift starting at 2. The three Figures shown below are oscilloscope captures at the frequencies of interest for our OPA circuit. Figure below is set for 2. The scope plots were captured using a LeCroy 44x Wavesurfer.