martes, 16 de marzo de 2010

Op Amps as Comparators

Op Amps as Comparators:
An op amp can be used to compare 2 different voltages. If you apply a reference voltage to one of the inputs and then use the other input to monitor a voltage from some point in a circuit, the output of the op amp will go from high to low (or vice-versa) as the monitored voltage crosses the reference voltage.
As you can see in the following demo, The yellow line shows the voltage on the negative input. The voltage follows a curve because the capacitor is charging. The capacitor charges faster at first then it slows as it approaches full charge. The blue line indicates the voltage on the positive input. Since the 2 voltage divider resistors are of equal value, the voltage on the positive input is exactly half of the power supply voltage. You can also see that the voltage on the output of the op amp is high (close to the power supply voltage). If you push the button, the capacitor will start to charge. The little lines in the window are voltage indicators. Think of them as a portion of the trace on an oscilloscope. As the capacitor charges (and the voltage starts to rise), the line goes up (it follows the voltage). As you can see, the voltage on the output does not change until the voltage on the negative input is higher than the voltage on the positive input. Remember that the previous circuits had a feedback signal return path between the output of the op amp to the negative input. Since there is no feedback, the gain is essentially (ideally) infinite. This will make the output swing from its maximum positive output voltage to its maximum negative output voltage. If there were a feedback resistor, the output voltage would not swing as far. With a feedback resistor, you could get the op amp's output voltage to be an inverted version of the voltage on the negative input. Remember that the circuit is a comparator. It's comparing the voltage on the 2 inputs. When the voltage on the negative input is below the reference voltage (on the positive input), the output is high. As soon as the voltage on the negative input goes above the voltage on the positive input, the output goes low. If you look at the white line sweeping from left to right you can see that the green line instantly transitions from high to low at the point where the blue and yellow lines intersect.

Maximum and Minimum Input/Output Values:
Some op amps can not accept inputs that equal the power supply voltage (or ground in the case of a single ended† supply). If the input is beyond the safe input values, the input may lead to unexpected output values. For instance, if the negative input in the previous circuit were grounded, some op amps would erroneously give a low output instead of a high output. As soon as the input voltage moved slightly above ground, the op amp would again operate as you'd expect. If you need an op amp to accept inputs close to ground, you need to get an op amp suited for the task.
†A single ended supply is one that uses only ground and EITHER a positive OR negative voltage. A 'split' supply has a positive voltage AND a negative voltage (below ground). If you see a power supply described as a 12 volt supply, it likely means that it is simply a single ended supply. If the supply voltage is expressed as ±15 volts, it's a split supply. In audio, split supplies are most common. For digital equipment, the comparators would likely be powered by a single ended 5 volt supply.

Op Amps as Regulators:

If you need a high quality linear regulator, an op amp can save a lot of effort. In the following demo, you can see that there is a simple zener shunt regulator connected to the positive input of the op amp. This becomes the reference voltage. If the zener is a 6.2 volt device, the reference will be 6.2 volts. Actually the reference voltage will likely be a little more or less than 6.2 volts (due to tolerances and the actual current flowing through the diode). If the voltage is precisely 6.2 volts on the positive input, the output of the regulator (the emitter of the current boost NPN bipolar transistor) will be precisely 6.2 volts. The feedback line from the emitter to the negative input of the op amp allows the op amp to monitor the output and compensate for changing load current. If the load resistor decreases in resistance, the output current increases (because we have a regulated voltage source). Without the feedback, the output from the regulator would likely drop a little. In most cases that would be fine. In some circuits, however, the change in voltage would be unacceptable.
When you push the button in the following demo, the resistance will decrease. You will notice that the regulator output current through the resistor increases in proportion to the fall in resistance. You will also notice that the output voltage is rock solid. If you look carefully, you can see that the output voltage from the op amp increases slightly to increase the current through the base of the transistor (which is needed to maintain the proper output voltage).

Bridging Module:

This circuit was designed to invert an audio signal. Most car audio amplifiers have one 'normal' channel and one 'inverted' channel. The inverted channel is needed so the amplifier can be bridged. Some of the older Orion amplifiers did not have an inverted channel and therefore needed a 'bridging module' to bridge the amplifier. To use this circuit, you would connect the left signal directly to the amplifier and the right channel to the input of this circuit. The output of this circuit would be connected to the right channel of the amplifier. You would then use the 2 positive speaker output wires for bridging. The left channel will be the positive output. This circuit will probably need to be powered by a constant source to prevent a turn on pop. If you don't want to connect it to a constant power source, connect it to a source that's controlled by the ignition switch. Do not connect it to the remote out of the radio.


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