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US 5,489,873 AGranted
Active low-pass filter
AssigneeMotorola, Inc.
InventorsTakatsugu Kamata, Jun Takahashi
Life of this patent
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AssigneeMotorola, Inc.
InventorsTakatsugu Kamata, Jun Takahashi
Estimating the term…
An active low-pass filter suitable as a 90.degree. phase shifter circuit is provided which requires few external components when formed as an IC. The current resulting from the input signal outputted from a first impedance element is amplified by a first current amplifier, and the resulting output current is converted by a first converting means to a voltage, which is supplied via a second impedance element to a second current amplifier; the output current of the second current amplifier is converted by a second converting means to a voltage, which is outputted as a filter output, while the current corresponding to the output voltage of the second converting means is negatively fed back into the inputs of the first and second current amplifiers; furthermore, at least the current amplification factor of the first current amplifier is made controllable, thereby permitting the cut-off frequency to be varied arbitrarily.
An active low-pass filter, comprising:
Dependencies are inferred from the claim text.
Drawings are temporarily unavailable. Open the PDF at USPTO
By detecting phase difference between two signals obtained from input signal: by means of active elements with more than two electrodes to which…
Using a plurality of operational amplifiers: two integrator-loop-filters
Networks for phase shifting: two-port phase shifters providing a predetermined phase shift, e.g. "all-pass" filters
Demodulation of angle-, frequency- or phase- modulated oscillations: by detecting phase difference between two signals obtained from input signal
Term ended 27 Feb 2015 (estimated)
USPTO file history isn’t available for this patent, so only indexed dates are shown.
The present invention relates to an active low-pass filter suitable as a 90.degree. phase shifter circuit.
A 90.degree. phase shifter circuit is used in, for example, a quadrature FM demodulator circuit to generate a signal 90.degree. different in phase from an input intermediate frequency (IF) signal. The 90.degree. phase shifter circuit of the FM demodulator circuit is formed of a BPF (band-pass filter) using LCR (coil, capacitor and resistor) as shown in FIG. 1, or a bi-quad LPF (low-pass filter) using an op-amp as shown in FIG. 2. With the BPF using the LCR, to vary the center frequency so as to provide a 90.degree. phase difference, the coil is adjusted, or a varicap diode is used to vary it to a capacitance value corresponding to its terminal voltage. With the by-quad LPF, a cut-off frequency or value of Q may be obtained by varying the constants of external elements connected to the op-amp.
However, even when either the BPF or bi-quad LPF is used as a 90.degree. phase shifter circuit, there is a problem that external components such as coils, capacitors, and resistors are required to form it as an IC (integrated circuit), so that its size and weight cannot be reduced, or the need for adjustment cannot be eliminated.
FIG. 1 is a circuit diagram depicting a band-pass filter conventionally used as a 90.degree. phase shifter circuit.
FIG. 2 is a circuit diagram depicting a low-pass filter conventionally used as a 90.degree. phase shifter circuit.
FIG. 3 is a circuit diagram depicting an embodiment of the present invention.
Now, an embodiment of the present invention will be described in detail hereinbelow with reference to the drawings.
FIG. 3 depicts a by-quad active low-pass filter according to the present invention. With this active low-pass filter, three current amplifiers 1, 2 and 3 are provided. Each of these current amplifiers 1, 2, and 3 is comprised of a differential amplifier. The current amplifier 1, which corresponds to a first impedance element, provides a constant current amplification on a input signal to supply its output signal to the current amplifier 2. To the current amplifier 2 is connected an amplification factor control circuit 4 having a variable current source, wherein the current amplification factor of the current amplifier 2 is varied according to the output current of the amplification factor control circuit 4. To the differential output of the current amplifier 2 is connected a voltage converting circuit 8, which is comprised of capacitors 5a and 5b, buffers 6a and 6b, and resistors 7a and 7b. The resistors 7a and 7b correspond to a second impedance element. The output of the voltage converting circuit 8 is connected to the current amplifier 3. That is, the voltage on the capacitors 5a and 5b derived by drawing the output current from the current amplifier 2 into the capacitors 5a and 5b is supplied to the current amplifier 3 via the buffers 6a and 6b and resistors 7a and 7b. The current amplifier 3 amplifies by a constant amplification factor the current drawn due to the output voltage of the voltage converting circuit 8.
To the differential output of the current amplifier 3 is connected a voltage converting circuit 11 which is comprised of capacitors 9a and 9b and buffers 10a and 10b. The output signal of the voltage converting circuit 11 is supplied to the output terminal. That is, the voltage on the capacitors 9a and 9b derived by drawing the output current from the current amplifier 3 to the capacitors 9a and 9b is outputted via the buffers 10a and 10b. Resistors 12a, 12b, 13a and 13b are provided in order to negatively feed back the output of the voltage converting circuit 11 into the inputs of the current amplifiers 2 and 3. The output voltage of the buffer 10 is supplied to one input terminal of the current amplifier 2 via the resistor 12a, and also supplied to one input terminal of the current amplifier 3 via the resistor 13a. Similarly, the output voltage of the buffer 10a is supplied to the other input terminal of the current amplifier 2 via the resistor 12b, and also supplied to the other input terminal of the current amplifier 3 via the resistor 13b.