Controllable RF broadband and amplifier with a constant input impedance

This application claims the benefit, under 35 U.S.C. § 365 of International Application PCT/EP03/013741, filed Dec. 5, 2003, which was published in accordance with PCT Article 21(2) on Jul. 29, 2004 in English and which claims the benefit of German patent application No. 10300431.9, filed Jan. 9, 2003.

The invention relates to the field of controllable radio-frequency amplifiers, in particular controllable gain radio-frequency amplifiers, whose input impedance is independent of the gain. A further aspect of the invention relates to receivers for modulated radio-frequency signals.

Signals which have a wide dynamic range are processed in radio-frequency amplifiers, such as those used in radio and television receivers, or satellite receivers. Radio-frequency amplifiers of this type are normally used as input amplifiers. In order to allow both weak and strong signals to be processed by the radio-frequency amplifier, it is desirable for the gain level to be adjustable. The signal level can thus be matched to the downstream signal processing stages. When a weak signal is applied to the input amplifier, the amplifier has to operate with high gain while, at the same time, as little interference as possible may be added to the signal. Undesirable interference includes, for example, noise and distortion. When a strong signal is applied to the input of the amplifier, the amplifier has to operate with less gain, and must not be saturated by the signal. In both modes, good linearity and an input impedance which remains the same over a wide frequency range are desirable.

Amplifiers with a constant input impedance and variable gain are known from the prior art, in which a symmetrical input signal is passed to two or more differential amplifiers with different gain factors. The individual differential amplifiers can be connected depending on the desired gain, and have a high input impedance. The input impedance of the entire circuit is set by means of a low impedance, which is connected between the differential inputs and is very small in comparison to the high input impedances of the differential amplifiers. The input impedances of the amplifiers can thus be ignored. Circuits such as these frequently have an undesirable noise response, and the circuit configuration as a differential amplifier for a symmetrical input signal considerably increases the circuit complexity.

A variable gain for a radio-frequency amplifier can also be achieved by use of a variable control of the operating point of the amplifier in the linear region. However, shifting the operating point of an amplifier can result in changes in the phase response or in the bandwidth.

One object of the invention is now to propose a radio-frequency amplifier with a constant input impedance, a variable gain, high linearity and an improved noise response. One amplifier such as this is described in claim 1 . Advantageous refinements of the amplifier according to the invention are specified in the dependent claims. A further object of the invention is to propose a receiver for modulated radio-frequency signals.

The amplifier according to the invention comprises two or more elementary amplifiers which are connected in parallel and are selected as a function of the desired amplification level. The input signal is passed in parallel to all the elementary amplifiers. The input impedance of one elementary amplifier is in this case at the same time used for scaling the input signal. The individual elementary amplifiers each have essentially the same gain. The selection of one specific elementary amplifier at the same time results in selection of the input impedance which is associated with this amplifier. Those amplifiers which are not selected as well as the impedances associated with them are disconnected from the circuit via switches, such that they do not have any influence on the input impedance of the active circuit part. The switches are, for example, switching diodes, although transistor switches or the like may also be used. It is also possible to use different types of switch in different circuit parts at the same time. The configuration of the radio-frequency amplifier according to the invention makes it possible to select freely variable and reproducible gain steps with little complexity, while at the same time ensuring an input impedance which is essentially independent of the gain. Furthermore, the circuit is highly suitable for integration in an integrated circuit.

The field of application of the invention is not restricted to the radio, television and satellite receivers mentioned initially. The radio-frequency amplifier according to the invention may be used in all appropriate applications which require constant input impedances and variable gains.

In a second aspect of the invention, a receiver is proposed for receiving broadcast radio signals using a radio-frequency amplifier according to the invention. In this case, broadcast radio receivers are receivers for analog or digital audio and/or video transmissions, such as receivers for VHF radio, digital audio broadcast (DAB) or digital television (DVB). Although a receiver according to the invention can be used particularly advantageously for receiving wireless transmissions, the receiver is also suitable for cable transmissions.

In a third aspect of the invention, a receiver is proposed for receiving data signals using a radio-frequency amplifier according to the invention. A receiver such as this is used, inter alia, in wireless or cable communications networks. In this case, communications networks comprise both cable data networks, such as Local Area Networks (LAN) or Controller Area Networks (CAN), and wireless data networks, such as the Universal Mobile Telecommunications System (UMTS), the Global System for Mobile Communication (GSM) or the Digital Enhanced Cordless Telephone (DECT). The data content, the frequency range used and the type of modulation are in this case irrelevant. It is thus possible to receive computer data or else speech data, and the circuit arrangement is not restricted to specific frequency ranges.

The radio-frequency amplifier according to the invention will now be described in more detail with reference to the drawing, in which:

FIG. 1 shows a schematic illustration of a radio-frequency amplifier according to the invention having two amplification stages;

FIG. 2 shows the circuit of the radio-frequency amplifier according to the invention as shown in FIG. 1 , with a first selected gain level;

FIG. 3 shows the radio-frequency amplifier according to the invention as shown in FIG. 1 , with a second selected gain level.

In the figures, identical reference symbols denote the same or similar elements.

In the schematic illustration of a radio-frequency amplifier according to the invention as shown in FIG. 1 , an input signal, which is not shown, is passed to an input IN. The input IN is connected to a first and to a second parallel-connected amplifier branch. The first amplifier branch has a resistor R 1 , whose first connection is connected to the input IN, and whose second connection is connected to the emitter electrode of a transistor T 1 . A current source J 1 which can be switched on via a switch S 1 is also connected to the emitter electrode of the transistor T 1 . The base connection of the transistor T 1 is connected to a switch S 2 , by means of which the base can be connected either to ground or to a DC voltage source U 1 . The collector of the transistor T 1 forms the output of the circuit, and is connected to the output OUT. The second amplifier branch is also connected to the input IN. The second amplifier branch has a resistor R 2 , whose first connection is connected to the input IN of the amplifier. The resistor can be connected to ground via a switch S 3 which is connected to a second connection of the resistor R 2 . The first connection of the resistor R 2 and the input IN are also connected to a resistor R 3 via which the emitter electrode of a transistor T 2 is connected to the input IN. The emitter electrode of the transistor T 2 is also connected to a current source J 2 , which is connected in a switchable manner to ground via a switch S 4 . The base connection of the transistor T 2 is connected via a switch S 5 to ground or to a DC voltage source U 2 . The collector connection of the transistor T 2 is connected to the collector connection of the transistor T 1 and, together with it, forms the output OUT.

The first amplifier branch is activated when the radio-frequency amplifier as shown in FIG. 1 is in the configuration shown in FIG. 2 . For this purpose, the switch S 1 is closed, and the current source J 1 is thus activated. The switch S 2 connects the base of the transistor T 1 to the DC voltage source U 1 . A signal which is applied to the input IN, but is not illustrated, is now passed via the resistor R 1 to the emitter connection of the transistor T 1 . The current source J 1 and the potential U 1 at the base of the transistor T 1 set the operating point of the transistor. The signal is passed from the collector connection of the transistor T 1 , which is in this mode operated as a common-base circuit, to the output OUT. Since the switch S 3 is open, only a first connection of the resistor R 2 is connected to the circuit, and it therefore has no effect. The open switch S 4 deactivates the current source J 2 , and the switch S 5 connects the base of the transistor T 2 to ground potential. Only one connection of the resistor R 3 is thus likewise effectively connected to the circuit, and it has no influence on the input impedance of the active circuit. The current source J 1 has a high impedance for high frequencies, and the voltage source U 1 represents a short circuit for high frequencies. Owing to the low differential resistance of the base/emitter junction of the transistor T 1 , the resistor R 1 is connected to ground for high frequencies, via the base/emitter junction and the voltage source U 1 . The input impedance of the circuit is thus essentially governed by R 1 , and the signal is passed in its entirety to the emitter of the transistor T 1 .

In the illustration of the radio-frequency amplifier according to the invention as shown in FIG. 3 , the second amplifier branch is activated. For this purpose, the switch S 3 is closed, and connects the input IN via the resistor R 2 to ground. The input IN is also connected to the resistor R 3 , which connects the input IN to the emitter connection of the transistor T 2 . The current source J 2 which is connected to the emitter connection of the transistor T 2 is activated by the switch S 4 , which is now closed. The switch S 5 connects the base connection of the transistor T 2 to the DC voltage source U 2 . The switch S 1 is open, and the switch S 2 connects the base connection of the transistor T 1 to ground. Only one connection of the resistor R 1 , which is likewise connected to the input IN, is thus effectively connected to the circuit, and has no influence on the input impedance. The operating point of the amplifier T 2 is set by the DC voltage potential from the DC voltage source U 2 at the base connection of the transistor T 2 and the current source J 2 at the emitter connection of the transistor T 2 . The input impedance of the amplifier circuit is formed by the resistors R 2 and R 3 connected in parallel. In this case, as has been described above, the resistor R 2 is connected directly to ground, while the resistor R 3 is connected to ground via the base/emitter junction of the transistor T 2 , via the switch S 5 and via the DC voltage source U 2 . The base/emitter junction of the transistor T 2 in this case has a very low differential resistance in comparison to the resistance values of the resistors R 2 and R 3 , and is thus negligible. The DC voltage source U 2 represents a short circuit to ground for high frequencies, and the current source J 2 has a high impedance for high frequencies. The DC voltage source U 2 and the current source J 2 can thus likewise be ignored. The circuit arrangement of the resistors R 2 and R 3 results in a part of the signal which is applied to the input IN being tapped off directly to ground, and only a part of the signal being passed via the resistor R 3 to the transistor T 2 . The magnitude of the part of the signal which is passed to the transistor T 2 is governed by the ratio of the resistors R 2 and R 3 . The input signal that has been scaled in this way is amplified by the transistor T 2 and is produced at the output OUT. The two resistors R 2 and R 3 can be selected such that the parallel circuit formed by these resistors has the same value as the resistor R 1 which governs the input impedance of the circuit when the first amplifier branch is activated. If, by way of example, the input signal is intended to be halved before amplification, the resistors R 2 and R 3 may each be twice as large as R 1 from the description relating to FIG. 2 . The input impedance is then calculated as follows:

;

Z

IN

=

The example described further above with reference to FIG. 2 to this extent represents a special case of the situation described with reference to FIG. 3 , in that there is no resistor corresponding to the resistor R 2 from FIG. 3 . In order to obtain an elementary amplifier according to the invention in the general configuration as described in FIG. 3 , it would thus be necessary to insert a resistor of infinitely high resistance into the circuit such that it, together with the resistor R 1 , forms a voltage divider for the signal, and is connected in parallel with the resistor R 1 with regard to the calculation of the input impedance.

Owing to the wide range of possible ways of scaling a signal by means of two resistors while at the same time setting an essentially constant input impedance, the circuit can easily be extended by any desired gain factors. One advantageous feature in this case is that only the selected part of the circuit in each case contributes to the amplification and to the input impedance. The simple configuration of the circuit and the use of semiconductor switches particularly advantageously allow the radio-frequency amplifier according to the invention to be integrated in integrated circuits.