Integrated low-pass filter

The integrated low-pass filter has a resistor connected, on the one hand, to an input terminal and, on the other hand, to a first capacitor and to the control terminal of a transistor. The output circuit of the transistor is connected, on the one hand, to the first capacitor and to an output terminal of the circuit. Furthermore, the output circuit of the transistor is also connected, via a current source and a second capacitor, to a first reference potential. On the other end, the output circuit of the transistor is connected to a second reference potential.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention is in the field of electronic filters. More specifically, the
 invention relates to an integrated low-pass filter for damping relatively
 high-frequency components of a signal (e.g. harmonics).
 Multipolar single-stage filters are difficult to implement in integrated
 bipolar high-frequency circuits. One essential goal is a steep edge
 dropoff in the amplitude response of the filter toward high frequencies,
 but without having to have recourse to multistage filter arrays.
 In the past, the art has either dispensed with explicit filter action
 entirely, or else used unipolar filters in the form of RC circuits.
 2. Summary of the Invention
 It is accordingly an object of the invention to provide an intergrated
 low-pass filter, which overcomes the above-mentioned disadvantages of the
 heretofore-known devices and methods of this general type and that has a
 high selectivity in the depletion range, and an optimized characteristic
 curve with minimal damping in the conducting range, and that can be
 integrated in an integrated circuit.
 With the foregoing and other objects in view there is provided, in
 accordance with the invention, an integrated low-pass filter, comprising:
 an input terminal, a transistor having a control terminal and an output
 circuit, and a resistor connected between the input terminal and the
 control terminal of the transistor;
 a first capacitor connected between the resistor and the control terminal
 of the transistor, on the one hand, and to the output circuit of the
 transistor, on the other hand;
 an output terminal connected to the output circuit of the transistor;
 a second capacitor connected between the output terminal and a first
 reference potential; and a current source connected (in parallel with the
 second capacitor) between the output terminal and the first reference
 potential; and
 the output circuit of the transistor being connected between a second
 reference potential and, via the current source and the second capacitor
 the first reference potential.
 In accordance with an added feature of the invention, the current source is
 an adjustable current source. This allows adjusting the operating point of
 the filter. In addition, the filter can be used as a dual band filter;
 that is, a switchover between two different frequency bands with
 correspondingly different filtering is possible. Because of the adjustable
 current source, the limit frequency (corner frequency) of the low-pass
 filter is variable, thus making it possible to process a plurality of
 different fundamental frequencies with a desired constant harmonic
 damping.
 In accordance with an additional feature of the invention, the resistor is
 a parasitic component of the transistor. This allows a considerable
 savings in the required chip surface area. Similarly, in accordance with
 another feature of the invention, the first capacitor is a parasitic
 component of the transistor.
 In accordance with a further feature of the invention, the second capacitor
 is a parasitic component of the transistor.
 In accordance with again an added feature of the invention, the transistor
 is an npn transistor. Similarly, the transistor may be a pnp transistor
 that is correspondingly integrated. Alternatively, the transistor is a
 field effect transistor.
 In accordance with a concomitant feature of the invention, there is
 provided a low pass filter assembly which is formed with a plurality of
 low-pass filters. The individual filters are connected in cascade fashion,
 and a buffer amplifier is connected in between and separates the
 respective low-pass filters.
 The invention has the advantage that better selectivity can be attained
 than when there are two or more cascaded RC members. In addition, unlike
 with RC members, the useful signal is damped only minimally.
 Another advantage of the invention is that especially at intermediate
 frequencies (100 to 200 MHz), the circuit requires less chip area than
 conventional embodiments, because when inductances are used they have to
 be made very large.
 Other features which are considered as characteristic for the invention are
 set forth in the appended claims.
 Although the invention is illustrated and described herein as embodied in
 an integrated low-pass filter, it is nevertheless not intended to be
 limited to the details shown, since various modifications and structural
 changes may be made therein without departing from the spirit of the
 invention and within the scope and range of equivalents of the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to the figures of the drawing in detail and first,
 particularly, to FIG. 1 thereof, there is seen a low-pass filter with a
 transistor T1. A collector of the transistor T1 is connected to a
 reference potential VCC, a base of the transistor T1 is connected to one
 terminal of a first capacitor C1 and a first resistor R1. An emitter of
 the transistor T1 is connected to the other terminal of the first
 capacitor C1, an output terminal V.sub.out, and, via a first current
 source IC and a second capacitor C2, a reference potential M. An input
 terminal V.sub.in of the circuit is connected to a second terminal of the
 resistor R1.
 It is possible with the circuit shown in FIG. 1 to make a low-pass filter
 with an LC characteristic. The low-pass filter of the present circuit has
 a steeper edge dropoff in the amplitude response toward higher frequencies
 than a conventional RC low-pass filter.
 A source impedance Ze offered by the emitter of the transistor T1 is
 defined by
 ##EQU1##
 wherein:
 .beta.=differential current amplification (dIC:dIB with UCE=constant)
 .beta..sub.o =direct current amplification
 g.sub.m =direct current admittance of the transistor T1
 V.sub.T =temperature voltage.
 The impedance Z.sub.e has the imaginary component .omega.R.sub.1 C.sub.1
 V.sub.T /I.sub.c.
 This can be imagined as being generated by an inductance
 ##EQU2##
 By the embodiment of the invention, a steeper edge dropoff of the amplitude
 response of the single-stage filter toward higher frequencies is generated
 than would be possible with a conventional RC low-pass filter.
 Advantageously, the filter circuit can be used as a current-controlled
 filter. The filter limit frequency can be adjusted steadily or discretely
 by means of a steady or discrete variation in the bias current that is
 generated by the current source IC.
 Referring now to FIG. 2, the filter circuit shown therein differs from that
 of FIG. 1 in the use of a pnp transistor, instead of an npn transistor as
 shown in FIG. 1. The base of the transistor T2 of FIG. 2 is connected to
 both the first terminal of the capacitor C1 and the first terminal of the
 resistor R1. The second terminal of the resistor R1 is connected to the
 input terminal V.sub.in. The collector of the transistor T2 is located at
 the reference potential M. The emitter of the transistor T2 is connected
 to the second terminal of the first capacitor C1, the output terminal
 V.sub.out, and via the current source IC and the second capacitor C2 to
 the reference potential VCC.
 The circuit shown in FIG. 2 is equivalent in function to the circuit shown
 in FIG. 1.
 It will be understood by those skilled in the art that corresponding field
 effect transistors may also be provided instead of the npn transistor T1
 or the pnp transistor T2.
 In general, it is true that the greater the current generated by the
 current source IC, the higher the filter limit frequency becomes, and the
 smaller the imaginary component of the source impedance Ze becomes.
 The resistor R1 and the two capacitors C1 and C2 serve to dimension the
 circuit. Depending on the desired frequency characteristic, the resistor
 R1 and the two capacitors C1 and C2 can optionally be realized by means of
 the parasitic effects of the transistor.
 The exemplary embodiments shown in FIGS. 1 and 2 can also be cascaded as
 shown in FIGS. 3 and 4. To that end, a respective buffer amplifier with
 high input impedance should be connected between the individual filter
 stages, in order to prevent the first filter stage from being loaded by
 the following filter stage.
 The circuit configuration of the invention can be used especially
 advantageously at frequencies between 100 and 200 MHz, because--in
 comparative terms--the inductive component for a corresponding LC circuit
 would be very large. Thus surface area on the integrated circuit can be
 saved by means of the circuit configuration of the invention.
 Damping of 40 dB per decade can be attained with the circuit of the
 invention.