Electrical Engineering Dictionary
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All-Pass Filter
An All-Pass filters passes all frequencies presented to the circuits input, but adds a phase shift. So an All-Pass filter is not a filter at all, but more of a phase shifter. the two general types would be a phase-lead circuit and a phase-lag circuit.
Op Amp Active Filter.
These two circuit represent an operation amplifier being used as an All-Pass Filter [Constant Amplitude Phase Circuit]. Both circuits are setup as a Non-Inverting Amplifier. However with the resistor values shown the gain of the amplifiers are one [+1]. The gain of a Non-Inverting Op Amp is:
Vo = ([R2 + R3] / R2) x Vin.
However in this case the resistor are set equal, or R1 = R2 = R3.
Any resistor value will due; however lower values tend to draw more current, while very high values tend to induce noise [in sensitive applications].
Common values could range between; 1k, 2.2k, 3.3k, 4.7k and 10k ohms
The phase shift changes from 0 degrees at DC to 90 degrees at;
90 degree phase shift; 1 / [2 x 3.14 x R x C1]
and then on up to 180 degree phase shift at higher frequencies.
This first All-Pass Filter circuit is configured to use a single power supply [Vcc] and ground.
All-Pass Filter
In the second All-Pass Filter circuit example the Op Amp uses a dual supply, or both a positive and negative voltage. In this case R1 is taken to half of Vcc. Otherwise the circuit is identical to the first circuit shown above.
All-Pass Filter
The 741 Op Amp, being used as the pin out example has a power dissipation of 500mW, an input voltage of +/-15v and a supply voltage of +/-18 volts. However the rating depends on the actual IC used, so check the data sheet. The 741C is a direct, plug-in replacement for the 709C, LM201, MC1439 and 748 in most applications. Of course there are a great many operational amplifies released after the LM741 that could be considered a replacement for that device as well. Although because the 741 is such a general purpose device, companies done normally list parts as a replacement, because there would be an endless number [lower power, high voltage, higher frequency response and so on].
The 741 can also be compensated using the off-set null pins to center the output at zero volts. Note that dual packages do not have the frequency compensation pins. The 741 by definition only has one amplifier in the package. |
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Note that in some of these circuits the Op Amp has the inverting input at the top of the component diagram while in other cases the inverting input is below the non-inverting input. Its standard practice to rearrange the input or output pin locations in a schematic diagram to make the schematic easier to read or less complicated. Most CAD tools allow different versions of a part with different pin locations.
Continue reading about operational amplifier filters:
Active Low Pass Filter Design
Active High Pass Filter Design
Band-pass Filter
Active Notch Filter Design
