Non investing summing operational amplifier as a comparator
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Some designers might be tempted to use or substitute readily available op amps as voltage comparators in their projects. There are very important differences however. Comparators are designed to work without negative feedback or open-loop, they are generally designed to drive digital logic circuits from their outputs, and they are designed to work at high speed with minimal instability.
Op amps are not generally designed for use as comparators, their input structures may saturate if over-driven which may cause it to respond comparatively slowly. Many have input stages which behave in unexpected ways when driven with large differential voltages or beyond the specified common mode range.
In fact, in many cases, the differential input voltage range of an op amp is limited or clamped to prevent damage to the input stage devices. Friend or Foe? Warning: Using op-amps with built-in input clamps as a voltage comparator may damage the IC!
Yet many designers still try to use op amps as comparators. While this may work in some cases at low speeds and low resolutions, many times the results are not satisfactory. Not all of the issues involved with using an op amp as a comparator can be resolved by reference to the op amp datasheet, since op amps are not intended for use as comparators.
The most common issues are speed as we have already mentioned , the effects of input structures protection diodes, phase inversion in FET amplifiers such as the ADTL, and many others , output structures which are not intended to drive logic, hysteresis and stability, and common-mode effects. The circuit compares the voltages at the two inputs and generates an output based on the relative values. Figure 1 Op-Amp as Comparator Hardware Setup: Comparators are used in different ways, and in future sections we will see them in action in several labs.
Here we will use the comparator in a common configuration that generates a square wave with a variable pulse width: Start by shutting off the power supplies and assemble the circuit. As with the summing amplifier circuit earlier, use the second waveform generator output for the DC source Vref , and turn the amplitude to zero and the output offset all the way down so that you can adjust up from zero during the experiment.
Again configure the waveform generator Vin for a 2V amplitude peak-to-peak sine wave at 1 kHz. With the power supply on and Vref at zero volts, export the output waveform. Now slowly increase Vref and observe what happens. Keep increasing Vref until it exceeds 2V and observe what happens.
Can you explain this? Repeat the above for a triangular input waveform and record your observations for your lab report. Figure 2. Comparator Breadboard Circuit Procedure: Use the first waveform generator as source Vin to provide a 2V amplitude peak-to-peak, 1 kHz sine wave excitation to the circuit. Configure the scope so that the input signal is displayed on channel 1 and the output signal is displayed on channel 2.
A plot example is presented in Figure 3. Figure 3. Comparator Waveforms Hysteresis Comparator Hysteresis is the dependence of a system's current state on previous values of quantities determining it. The output value is not a strict function of the corresponding input, but also incorporates some lag, delay, or history dependence.
In particular, the response for a decrease in the input variable is different from the response for an increase in the input variable. The threshold values should depend on the output value which is fed back to the input and contributes to the threshold values positive feedback.
Via a resistive divider, a fraction of the output voltage is fed back to the non-inverting input. When analyzing hysteresis comparators, we have to take into consideration the moving direction of the hysteresis and the fact that at a certain moment only one threshold is active. The input signal triggers the switching of the output, switching process being sustained by the positive feedback.
Non-inverting hysteresis Comparator Consider the circuit presented in Figure 4. Inverting Summing Amplifier The most commonly used Summing Amplifier is an extended version of the Inverting Amplifier configuration i. Due to this configuration, the output of Voltage Adder circuit is out of phase by o with respect to the input. A general design of the Summing Amplifier is shown in the following circuit.
If more input voltages are connected to the inverting input terminal as shown, the resulting output will be the sum of all the input voltages applied, but inverted. Before analyzing the above circuit, let us discuss about an important point in this setup: The concept of Virtual Ground. As the Non-Inverting Input of the above circuit is connected to ground, the Inverting Input terminal of the Op Amp is at virtual ground. As a result, the inverting input node becomes an ideal node for summing the input currents.
The circuit diagram of a summing amplifier is as shown in the figure above. Instead of using a single input resistor, all the input sources have their own input drive resistors. A circuit like this amplifies each input signal. The gain for each input is given by the ratio of the feedback resistor Rf to the input resistance in the respective branch. It is already been said that a summing amplifier is basically an Inverting Amplifier with more than one voltage at the inverting input terminal.
The output voltage for each channel can be calculated individually and the final output voltage will be the sum of all the individual outputs. To calculate the output voltage of a particular channel, we have to ground all the remaining channels and use the basic inverting amplifier output voltage formula for each channel.
The output signal is the algebraic sum of individual outputs or in other words it is the sum of all the inputs multiplied by their respective gains. But if all the input resistances are chosen to be of equal magnitude, then the Summing Amplifier is said to be having an equal-weighted configuration, where the gain for each input channel is same.
Sometimes, it is necessary to just add the input voltages without amplifying them. In such situations, the value of input resistance R1, R2, R3 etc. As a result, the gain of the amplifier will be unity. Hence, the output voltage will be an addition of the input voltages.
However, it must be noted that all of the input currents are added and then fed back through the resistor Rf, so we should be aware of the power rating of the resistors. Here, the input voltages are applied to the non-inverting input terminal of the Op Amp and a part of the output is fed back to the inverting input terminal, through voltage-divider-bias feedback. The circuit of a Non-Inverting Summing Amplifier is shown in the following image.
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