Attenuator with a control circuit

An attenuator system comprises an attenuator and a control circuit for controlling the attenuation of the attenuator. In one embodiment, the attenuator comprises two diodes or two diode connected transistors, and the control circuit comprises two transistors as the only active devices. In another embodiment, the control circuit comprises another transistor in a shut down circuit.

TECHNICAL FIELD

The present invention generally relates to radio frequency (RF) attenuators, and more particularly, to controllable RF attenuators.

BACKGROUND

RF systems may use an attenuator for controlling an input RF signal applied to a power amplifier. Various types of attenuators may be used including T-type attenuators. Some T-type attenuators use field effect transistors (FETS) to shunt the T-arranged resistors. (See e.g., U.S. Pat. No. 5,666,089). These attenuators use a FET process and need control signals having opposite voltages. Other T-type attenuators use diode and capacitor circuits that require more than five transistors to drive the diodes. These attenuators also require a DC supply greater than 3.6 volts for the diodes to turn on.

SUMMARY

An attenuator system comprises an attenuator and a control circuit. The attenuator includes a first input terminal for receiving a radio frequency power signal, includes second and third input terminals for receiving first and second control signals, respectively, and includes a first output terminal for providing a power signal in response to the radio frequency power signal and the first and second control signals. The control circuit includes a fourth input terminal for receiving a control voltage, includes a fifth input terminal for receiving a supply voltage, and includes second and third output terminals for providing the first and second control signals.

In one aspect, the attenuator controls a power level of the power signal in response to the first and second control signals. In another aspect, the attenuator comprises two diodes or two diode connected transistors, and the control circuit comprises two transistors. In yet another aspect, the control circuit comprises another transistor in a shut down circuit.

DETAILED DESCRIPTION

FIG. 1is a block diagram illustrating an attenuator system100. The attenuator system100comprises a control circuit101and an attenuator102. The attenuator102may be, for example, a T-type attenuator. The attenuator102provides an RF power output signal104in response to an RF power input signal103and control signals105and106. The control circuit101provides the control signals105and106in response to a control voltage107and a DC supply voltage (VCC)108for adjusting the attenuation of the attenuator102to control the power level of the RF power output signal104. The control signals105and106control the attenuation of the attenuator102in response to the control voltage107. In one embodiment, the control voltage107sets the attenuation of the attenuator102. In one embodiment, the control voltage107is about 1.8 volts for a first attenuation level and about 0.3 volts for a second attenuation level. In one embodiment, the control voltage107is received from an external system (not shown) and may be digitally controlled. Although two control signals105and106are shown, one or more control signals may be used to control the attenuator102.

In one embodiment, the attenuator system100may be used in a power amplifier for a wireless communications system, such as WiMax (worldwide interoperability for microwave and access) or an IEEE 802.16e Standard system. In an IEEE 802.16e Standard system, the attenuator of the attenuator system100is controllable in 20 dB steps.

FIG. 2is a schematic diagram illustrating an attenuator system200, which may be one embodiment of the attenuator system100(FIG. 1). The control circuit201comprises a plurality of resistors214,215,216,217,218, and219and a plurality of transistors221and222. In one embodiment, the bipolar transistors221and222are heterojunction bipolar transistors (HBTs). The control voltage107controls the bias of the transistor221, which in turn controls the bias of the transistor222for controlling the currents through the resistors216and219. These currents corresponds to the control signals105and106(FIG. 1). The attenuator202comprises a plurality of resistors211,212and213, a plurality of diodes231and232, and a capacitor241. The resistor216provides the control signal105(FIG. 1) to the anode of the diode231and the capacitor241. The resistor219provides the control signal106(FIG. 1) to the anode of the diode232and a common node formed between the resistors211and213.

The operation of the attenuator system200is next described. Setting the control voltage107to a low state (e.g., 0.7 volts) sets the attenuator202to a pass-through state. In this state, the transistor221is off. The collector of the transistor221is pulled up by the supply voltage108, and thereby turns on the transistor222. The collector of the transistor222is a low level of around 0.2 volts. Further, the diode231is turned on, and the diode232is turned off. The DC current flows from the DC supply108through the resistors215and216, the diode231, the resistors211and219, and the transistor222to ground. The turned-on diode231has lower impedance to RF power than that of the turned-off diode232and than resistors211and213. Therefore, RF power flowing into the attenuator202from the RF power input signal103passes through the diode231and the capacitor241and the RF power flows out of the attenuator202with very low attenuation on the RF power output104.

Setting the control voltage107to a high state (e.g., 1.8 volts) sets the attenuator202to an attenuation state. In this state, the transistor221is on. The collector of the transistor221is pulled down to ground, and thereby turns off the transistor222. Further, the diode231is turned off, and the diode232is turned on. Therefore, DC current flows from the DC supply108through the resistors218and219, the diode232, and the resistor212to ground. Because the diode232is on, RF power flowing into the attenuator202from the RF power input signal103is divided so that some RF power flows through the diode232reducing the RF power that flows out of the attenuator202on the RF power output104.

In an illustrative embodiment, the capacitor241has a capacitance of 5-8 picofarads. The resistors211and213each have a resistance of 60 ohms. The resistor212has a resistance of 0-1 ohms depending on the characteristics of the diode232. The resistors214,215and216have resistances of 4000, 1000 and 800 respectively. The resistors217,218, and219have resistances of 3000, 2000, and 200 ohms, respectively. In this illustrative embodiment, the diodes231and232turn on with a voltage over 1.2 volts for Gallium Arsenide diodes. The supply voltage108is about 3.3 volts.

FIG. 3is a schematic diagram illustrating an attenuator system300, which may be one embodiment of the attenuator system100(FIG. 1). The attenuator system300is similar to the attenuator200(FIG. 2), but includes a heterojunction bipolar transistor331instead of the diode231, and also includes a heterojunction bipolar transistor332instead of the diode232. The transistors331and332are diode connected. In one embodiment, the collector junctions of the transistors331and332are used as diodes because the breakdown voltage of the collector is higher than that of the emitter of a heterojunction bipolar transistor. In one embodiment, the transistors221,222,331, and332are formed by the same semiconductor processes.

FIG. 4is a schematic diagram illustrating an attenuator system400, which may be one embodiment of the attenuator system100(FIG. 1). The attenuator system400includes a shut-down circuit.

The attenuator system400is similar to the attenuator system300(FIG. 3), but further includes a shut-down circuit comprising a transistor425and a resistor420. In response to a shutdown voltage421, the transistor425pulls down the emitters of the transistors221and222to ground. The shut-down voltage421may be provided by a power amplifier (such as the amplifier inFIGS. 5 and 6), and may be a bias voltage therefrom.

The operation of the attenuator system400is next described. If the shut-down voltage421is greater than a predetermined threshold (e.g., 2 volts), the transistor425is on (the Vce of the transistor425may be for example 0.2 volts), regardless of whether either the transistor221or the transistor222is on. If the shut-down voltage421is low (e.g., 0.3 volts), the transistors221,222and425are off. Furthermore, the diode connected transistor331is off, and the diode connected transistor332is on.

FIG. 5is a schematic diagram illustrating an amplifier system500that includes a power amplifier. The amplifier system500comprises a step attenuator system400and a power amplifier501. The amplifier501includes an input terminal503coupled to the output terminal104of the step attenuator400and an output terminal504for providing an RF power output signal in response to a signal applied to the input terminal503. In one embodiment, the output impedance of the step attenuator400and the input impedance of the amplifier501are matched. In one embodiment, the step attenuator400has an output impedance of 50 ohms, and the amplifier501has an input impedance of 50 ohms. In alternative embodiments, the step attenuator system400maybe the attenuator systems100(FIG. 1),200(FIG. 2), or300(FIG. 3).

FIG. 6is a schematic diagram illustrating a power amplifier600, which is one embodiment of the power amplifier501. The amplifier600comprises a plurality of capacitors610and611, a resistor620, an inductor630, and a transistor640. The capacitor610functions as a DC block and couples the input terminal503to the base of the transistor630. The resistor620couples the bias voltage terminal502to the base of the transistor630. The inductor640couples the DC supply voltage109to the collector of the transistor630. The capacitor611couples the collector of the transistor630to the RF power output terminal504.

FIG. 7is a top plan view illustrating one embodiment of an integrated circuit layout of the attenuator system200(FIG. 2) except adding an input capacitor between the RF power input terminal103and the resistor211and adding a output capacitor between the resistor213and the RF power output104as DC blocks for measurement. In addition, the resistor212is omitted. All of the terminals of the RF input103, control voltage107, DC supply108ant RF power output104are laid out as bonding pads of 100×100 square microns. A via701of 50 microns in diameter is disposed through top metal layers to a back side (not shown) of the circuit chip. The capacitors have a capacitance density of around 560 picofarads/square millimeter. The resistors have a sheet resistance of around 50 ohm/square. The transistors221and217have an emitter area of around42square microns. The diodes are formed by the junctions of collectors of the HBT.

Although the attenuator systems herein have been described using HBT, other type s of transistors may be used. For example, Si (Silicon) or Si Ge (Silicon Germanium) bipolar junction transistors (either npn or pnp) may be used to form the transistors and diodes.

In the foregoing description, various methods and apparatus, and specific embodiments are described. However, it should be obvious to one conversant in the art, various alternatives, modifications, and changes may be possible without departing from the spirit and the scope of the invention which is defined by the metes and bounds of the appended claims.