Voltage controlled diode attenuator

Abstract: In a voltage controlled diode attenuator a first diode (11) and a second diode (12) are connected in series in the same direction of conduction and a first resistor (31) having one end connected to the first diode (11) has its other end connected to the output terminal of a differential amplifier (40) having a first and a second input terminal, a predeter­ mined dc bias voltage being applied to the first (11) and the second (12) diodes connected in series via the first resistor (31), a reference voltage source (Vr) being connected to the first input terminal of the amplifier (40), negative feedback means (32) connecting the junction of the first diode (11) and the one end of the first resistor (31) to the second input terminal of the amplifier (40), a variable control voltage being applied to the second diode (12) via a further resistor (33), a high frequency signal being applied to the junction between the first (11) and the second (12) diodes and an output being derived from the one end of the first resistor (31).

Description: The present invention relates to a diode attenuator for high frequencies and, more particularly, to an improvement in a voltage controlled diode attenuator. A prior art diode attenuator of the kind described is constructed to apply a predetermined dc bias voltage to a series connection of two diodes via a choke coil, and a variable bias control voltage to one of the diodes via another choke coil. The control voltage is varied to control the attenuation of a high frequency signal which flows through the other of the two diodes. Due to the predetermined voltage applied to between the two diodes, the input impedance of the attenuator advantageously remains constant despite changes in the attenuation of the diode caused by the variable control voltage, as well known in the art. A problem encountered with such a prior art attenuator is that the choke coils included therein make it difficult to set up a wide usable frequency band attenuator or to provide the attenuator in an integrated circuit. Although the use of resistors in place of the choke coils may be contemplated for biasing purpose, it brings about another problem that a change in the control voltage causes voltage drops of the resistors to fluctuate and, thereby, prevents the voltage applied to between the two diodes from setting at a predetermined level, resulting in fluctuation of the input impedance of the attenuator. In United States Patent Specification No. 4,236,126, which issued on November 25 1980, there is described a variable pi RF attenuator which is particularly adapted for construction as a thin film device and wherein the level of a d.c. control voltage is converted into first and second d.c. current sources having magnitudes that vary in opposite directions in response to the control voltage level. It is therefore an object of the present invention to provide a voltage controlled diode attenuator which, even in an attenuator configuration with biasing resistors, maintains the input impedance constant despite the changes in attenuation. In an embodiment of the invention to be described a voltage controlled diode attenuator comprises: first and second diodes connected in series for d.c. current such that their conducting directions are the same; a first resistor whose one end is connected to the first diode; first means for applying a predetermined dc bias voltage to the first and second diodes connected in series via the first resistor, said first means comprising a differential amplifier having first and second input terminals and an output terminal connected to the other end of the first resistor, a reference voltage source connected to the first input terminal, and negative feedback means for connecting a junction of the first diode and the one end of the first resistor to the second input terminal; second means for applying a variable control voltage to the second diode via a second resistor; and third means for applying a high frequency signal to a junction of the first and second diodes and deriving it from the one end of the first resistor. Brief Description of the Drawings Fig. 1 is a circuit diagram showing an example of prior art voltage controlled diode attenuators; Fig. 2 is a circuit diagram of a voltage controlled attenuator embodying the present invention; and Fig. 3 is a circuit diagram showing another embodiment of the present invention. Detailed Description of the Invention To facilitate understanding of the present invention, a brief reference will be made to a prior art diode attenuator shown in Fig. 1, which is of the type generally referred to as an inverted-L type attenuator. A predetermined dc bias voltage +V₁ applied to a terminal 3 is routed to a series connection of diodes (e.g. PIN-diodes) 11 and 12 via a choke coil 21. Meanwhile, a variable bias control voltage +V₂ applied to a terminal 4 is fed to the diode 12 via a choke coil 22. A high frequency input signal is applied to an input terminal 2 by way of a coupling capacitor (not shown). The input signal is routed through the diode 11 to an output terminal 1 and delivered via another coupling capacitor (not shown). The resistance of the diodes 11 and 12 is determined by the dc bias voltage +V₁ and the variable bias control voltage +V₂ and attenuates the high frequency signal. A characteristic feature of the illustrated prior art attenuator is that, due to the application of the predetermined voltage +V₂ to the junction of the two diodes 11 and 12, the input impedance of the attenuator remains unchanged when the attenuation by the diodes is changed in response to the variable control voltage. However, the use of choke coils inherent in the prior art attenuator makes it difficult to offer a wide usable frequency band or to construct the attenuator in an integrated circuit. Concerning a diode attenuator of the type using choke coils, a reference may be made to Viles "Need a PIN-diode Attenuator? An improved circuit provides a constant characteristic impedance, logarithmic control and temperature compensation", ELECTRONICS DESIGN 7, pp. 100-102, March 29, 1977. Where the choke coils shown in Fig. 1 are replaced by biasing resistors, a change in the control voltage causes the voltage drops of the resistors to fluctuate so that the voltage between the diodes fails to be maintained constant. Referring to Fig. 2, an inverted-L type diode attenuator in accordance with the present invention is shown. The diode attenuator in Fig. 2 comprises diodes 11 and 12, resistors 31-33, a differential amplifier 40, and a reference voltage source 50. An output terminal 1 is connected to an anode of the diode 11 and one end of each of the resistors 31 and 32. The other end of the resistor 31 is connected to an output of the differential amplifier 40 by way of a control terminal 3. The other end of the resistor 32 is connected to a negative input of the amplifier 40. The positive input of the amplifier 40 is connected to a positive terminal of the reference voltage source 50, while a negative terminal of the reference voltage source 50 is connected to a common potential. Meanwhile, an input terminal 2 is connected to a junction of a cathode of the diode 11 and an anode of the diode 12, while the junction in turn is connected to one end of the resistor 33. The other end of the resistor 33 is connected to a control terminal 4. The cathode of the diode 12 is connected to a common potential (ground potential). In operation, when bias voltages applied from the outside to the control terminals 3 and 4 are so adjusted as to render the diode 11 conductive and the diode 12 cutoff, the attenuator sets up the minimum attenuation. It will be seen that during such adjustment of the external bias voltages the input impedance will be held substantially constant if the sum of the bias voltages applied to the diodes 11 and 12 is held constant. In the illustrative embodiment, when a reference voltage Vr is applied from the reference voltage source 50 to the amplifier 40, the output of the amplifier 40 causes a change in the bias voltage applied to the control terminal 3 with the result that the sum of the voltages applied to the diodes 11 and 12 equals the reference voltage Vr. It follows that even if the bias voltage applied to the control terminal 4 is adjusted to vary the current flowing through the diodes 11 and 12 and, thereby, the attenuation of the attenuator, the sum of the voltage drops developing in the diodes 11 and 12 remains unchanged to maintain the input impedance of the attenuator constant. Referring to Fig. 3, another embodiment of the present invention is shown which has a π-type diode attenuator construction. As shown, the attenuator additionally includes a circuit similar to the circuit made up of the two diodes 11 and 12 and resistor 33, i.e., a circuit made up of diodes 13 and 14 and a resistor 34, which is connected to the output terminal 1 side. Such a circuit arrangement accomplishes desirable impedance characteristics both in the input and the output. In summary, it will be seen that the present invention provides a voltage controlled diode attenuator which promotes the use of a wide frequency band due to the substitution of resistors for the choke coils particular to the prior art circuitry. In addition, the resistors allow the attenuator to be produced as a compact, integrated circuit. Furthermore, the attenuator of the present invention finds practically unlimited applications because at least the input impedance thereof is not affected by the adjustment of attenuation.

Claims: A voltage controlled diode attenuator including a first diode (11) and a second diode (12) connected in series in the same direction of conduction; a first resistor (31), one end of which is connected to the first diode (11); a differential amplifier (40) having a first and a second input terminal and an output terminal, the output terminal being connected to the other end of the first resistor (31); a reference voltage source (Vr) being connected to the first input terminal of the amplifier (40); means (4) far applying a variable control voltage to the second diode (12) via a further resistor (33); and means (2) for applying a high frequency signal to the junction between the first (11) and the second (12) diodes and means (1) for deriving an output from the one end of the first resistor (31); characterized in that the attenuator further includes negative feedback means (32) connecting the junction of the first diode (11) and the one end of the first resistor (31) to the second input terminal of the amplifier (40), and in that said differential amplifier (40) applies a predetermined dc bias voltage to the first (11) and the second (12) diodes connected in series via the first resistor (31). The voltage controlled diode attenuator of claim 1 wherein said means for applying said variable control voltage to said junction via a resistor (33). The voltage controlled diode attenuator of claim 1 wherein said means for applying said fixed bias comprises a resistor (31) coupled between an output (3) of said differential amplifier (40) and said opposite side of said one diode (11). The voltage controlled diode attenuator of claim 3 wherein said differential amplifier (40) has two inputs, and negative feedback means (32) being connected to one of said two inputs and a reference voltage Vr) being connected to another of said two inputs. The voltage controlled diode attenuator of claim 4 and means for applying said variable control voltage to said junction via a resistor (33). The voltage controlled diode attenuator of claim 1 wherein there are two pair of said similarly poled diodes (11-14), each (11, 12; 13, 14) of said pairs being connected in series and the resulting two series circuits being connected in parallel. The voltage controlled diode attenuator of claim 6 and means for applying said variable control voltage from a single terminal (4) to the junctions in each of said two pair of diodes via two resistors (33, 34) with said two pair. The voltage controlled diode attenuator of claim 7 wherein said means for applying said fixed bias comprises a resistor (31) coupled between an output (3) of said differential amplifier and said opposite side of said one diode in each of said two pairs of diodes. The voltage controlled diode attenuator of claim 8 wherein said differential amplifier has two inputs, said negative feedback means (32) being connected to one of said two inputs and a reference voltage (Vr) being connected to another of said two inputs.

Inventors: TANAAMI SHIGEYUKI, YAMAMOTO OSAMU, TANAAMI, SHIGEYUKI, YAMAMOTO, OSAMU
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1985
Main classification: H03H7
IPC codes: H03H7, H03H11