Power amplifier with bias control

[0001] The present invention relates to a power amplifier. More in particular, the present invention relates to a power amplifier which may for example be utilized in high-frequency applications, such as mobile telecommunications equipment.

[0002] Power amplifiers are well known. To achieve a suitable amplification of an input signal at the various stages the amplifier may have, a gain control signal typically determines the operating point and thereby the gain of each stage. To achieve a desired overall gain characteristic, the so-called power control curve, a careful tuning of the operating points of the various stages is required.

[0003] An multi-stage radio frequency power amplifier is disclosed in U.S. Pat. No. 6,259,901. The first and second stages of this known multi-stage power amplifier are differential amplifiers, each coupled to ground via a bipolar transistor acting as a current source and to which a bias current is fed. The bias currents of the respective stages are supplied by bias circuits which receive a gain control signal and a single reference voltage. Details of the bias current circuits are not disclosed.

[0004] This known power amplifier presupposes a balanced input signal, for which a differential amplifier is provided. However, in many applications a balanced input signal is not available or not desirable. In addition, for many applications it is necessary to have a well-defined, substantially straight power control curve. Such a power control curve is not guaranteed by the power amplifier known from the above US patent.

[0005] It is an object of the present invention to overcome these and other problems of the Prior Art and to provide a power amplifier for single-sided (unbalanced) input signals which has a well-defined power control curve.

[0006] It is another object of the present invention to provide a power amplifier the gain of which is independent of the signal to be amplified.

[0007] It is a further object of the present invention to provide a power amplifier which is suitable for radio frequency applications.

[0008] Accordingly, the present invention provides a power amplifier comprising:

• [0009] a first stage for amplifying an input signal, and
• [0010] a first bias circuit for providing a bias current to the first stage,
• [0011] the first bias circuit comprising a controlled current source, and the first bias circuit being arranged for feeding its bias current to a control electrode of a signal amplification transistor of the first stage.

[0012] By feeding the bias current to a signal amplification transistor of the first stage, the operating point of the transistor and hence its gain can be carefully controlled. The control electrode preferably is the base of a bipolar transistor. The first stage preferably only has a single transistor but if it is provided with two or more transistors, the bias current is preferably fed to the first transistor in the signal path but may be fed to the other transistors as well.

[0013] In a preferred embodiment, a bias circuit comprises a non-linear voltage/current converter, preferably coupled with a current mirror. That is, the relationship between the gain control signal and the bias current is preferably non-linear. In this way, a very advantageous gain characteristic can be obtained. However, approximately linear voltage/current converters can also be utilized.

[0014] The non-linear voltage/current converter preferably comprises at least one differential stage, each stage coupled to a reference voltage. The differential amplifier may consist of only two transistors and a resistor, thus providing a very compact yet efficient circuit. Any non-linear characteristics of the voltage/current converter can be achieved by utilizing the non-linear properties of the transistors. If two differential stages are used within a single converter, each stage is preferably coupled to a distinct respective reference voltage.

[0015] In a preferred embodiment of the power amplifier of the present invention, at least one bias circuit comprises at least two distinct voltage/current converters for converting two distinct gain control voltages. This allows a different bias to be provided for different modes of operation, such as, in the case of mobile telephone sets, GSM, Edge and UMTS. Each of the at least two converters may convert the respective gain control voltage into a current, as described above. In a particularly advantageous embodiment of the present invention, however, the first bias circuit further comprises bias voltage means for additionally providing a bias voltage to the first stage. That is, the first bias circuit may produce a bias current in response to a first gain control signal and a bias voltage in response to a second gain control signal. In this way, the power amplifier of the present invention can be easily adapted to various different applications having distinct requirements.

[0016] In a further advantageous embodiment, in the first bias circuit an additional transistor is coupled between the at least one voltage/current converter and the controlled current source so as to compensate for the DC current gain of the signal amplification transistor.

[0017] To achieve a higher overall gain the power amplifier of the present invention may further comprise a second stage for amplifying a signal output by the first stage and a second bias circuit for providing a bias current to the second stage, and optionally a third stage for amplifying a signal output by the second stage, and an associated third bias circuit for providing a bias current to the third stage.

[0018] The power amplifier according to the present invention may be arranged for amplifying high frequency signals, such as signals used for mobile telephony.

[0019] The present invention further provides a device provided with a power amplifier as defined above. The device preferably is a mobile telecommunications device, more preferably a GSM telephone set.

[0027] The power amplifier 100 shown merely by way of non-limiting example in FIG. 1 comprises a first stage 1, a second stage 2 and a third stage 3. Each stage has an associated bias circuit 4, 5, 6. The embodiment shown is designed for RF (radio frequency) applications and contains various components, in particular inductors, especially for this purpose. The invention is, however, not limited to RF applications and those skilled in the art will appreciate that such dedicated components may be omitted in other applications.

[0028] In the embodiment shown, the first stage 1 comprises a first bipolar transistor T₁ of the NPN-type. The input signal V_in, in the present example an RF signal, is fed via a coupling capacitor C₁ to the base of transistor T₁. The emitter of transistor T₁ is connected to ground, while its collector is connected via an inductor L₂ to a positive supply voltage V_s and via a further coupling capacitor C₂ to the second stage 2. The first stage 1 further comprises a filter circuit consisting of a first inductor L₁, a first resistance R₁ connected in parallel to L₁, and a capacitor C₅. As this filter circuit is designed for decoupling RF signals, its components are not essential to the present invention.

[0029] A first bias circuit 4 is connected to the base of the first transistor T₁ to supply a (first) bias current I_b1. In the embodiment shown in FIG. 1, the first bias circuit 4 comprises a controlled current mirror 40, constituted by transistors T₆ and T₇, and two voltage/current converters 41 and 42. These converters 41, 42 may receive a gain control signal V_c1, V_c2 respectively. Depending on the particular mode of operation of the power amplifier 100, for example GSM or UMTS, either the first gain control signal V_c1 or the second gain control signal V_c2 is present. The converter 41 or 42 converts its respective gain control signal into a current which is fed from transistor T₆ of current mirror 40. As a result, a current I_b1 having a proportional magnitude is fed from transistor T₇ to transistor T₁. This bias current I_b1, which typically is a DC current, will not be affected by inductor L₁ and will flow effectively directly into the base of transistor T₁. As will be clear from the above, the bias current I_b1 is completely independent from the RF input signal V_in. As a result, the operating point and gain of the signal amplifying transistor T₁ are independent from the input signal.

[0030] In Prior Art arrangements a bias voltage is applied to the first signal amplifying transistor of the first stage. The present inventors have found that a bias current instead of a bias voltage offers a much better defined operating point of the first stage of the amplifier.

[0031] The bias circuits 4, 5 and 6 of the present invention are particularly suitable for providing a well-defined bias current. The converters 41 and 42 and their counterparts 51, 52 and 61, 62 will be discussed later in more detail with reference to FIG. 4. It is noted that in each bias circuit 4, 5, and 6 a single converter (41 or 42 in the case of bias circuit 4) would in principle suffice. According to further aspect of the present invention, however, multiple converters are provided per bias circuit so as to allow multiple gain control signals to be used independently.

[0032] The second stage 5 differs from the first stage 1 in that it comprises a current mirror consisting of the transistors T₂ and T₃, the bases of which are coupled by an inductor L₃. The RF signal amplified by the first stage 1 is fed, via coupling capacitor C₂, to the base of the third transistor T₃. Like transistor T₁, the collector of transistor T₃ is connected via an inductor L₄ to a positive supply voltage V_s and via a further coupling capacitor C₃ to the third stage 3. The collector of transistor T₂ is coupled to second bias circuit 5 which is, in the embodiment shown, identical to the first bias circuit 4 and the third bias circuit 6. As in the first bias circuit 4, a first bias transistor T₈ and a second bias transistor T₉ form a current mirror 50, transistor T₈ being connected to converters 51 and 52 which convert gain control voltage signals into suitable currents. In contrast to the first stage, the collector of second bias transistor T₉ is not directly connected to signal amplifying transistor T₃ but to the collector of transistor T₂, which together with transistor T₃ constitutes a current mirror. A transistor T₁₂, the collector of which is connected to a supply voltage V_s, provides a suitable base current for transistors T₂ and T₃ (base current compensation). As a result, the bias of the signal amplifying transistor T₃ in the second stage 2 is a voltage bias rather than a current bias as in the first stage 1. A current bias would be possible in this stage but is avoided in the higher stages to prevent any breakdown caused by the well-known avalanche effect.

[0033] The third stage 3 is substantially identical to the second stage 2 and comprises transistors T₄ and T₅, the bases of which are coupled by an inductor L₅, and the collector of transistor T₄ being coupled to the third bias circuit 6 and to the base of a base current compensation transistor T₁₃. The collector of transistor T₅ is connected via an inductor L₆ to a positive supply voltage V_s. At the collector of transistor T₅ the output signal V_out of the power amplifier 100 is produced, in the embodiment shown via an impedance matching circuit 7.

[0034] Although the power amplifier 100 of FIG. 1 is shown to have a double set of converters (41 & 42; 51 & 52; 61 & 62) so to be able to receive two gain control signals corresponding with two distinct modes of operation, this is by no means necessary and in many embodiments only a single set of converters will be provided, that is, only one converter per bias circuit. It is, on the other hand, also possible to provide three or more converters per bias circuit so as to be able to receive three or more gain control signals, possible corresponding to three or more distinct modes of operation.

[0035] In FIG. 1, each bias circuit is shown to have a single current mirror circuit (40; T₆ & T₇ in bias circuit 4). In the alternative embodiment shown in FIG. 2, however, the first bias circuit 4 comprises two current mirrors 40 and 40′. Current mirror 40, consisting of transistors T₆ and T₇ and coupled to converter 41 to provide a bias current in response to gain control signal V_c1, is connected as in the first stage 1 of FIG. 1, supplying a bias current to the base of transistor T1. Additional current mirror 40′, consisting of transistor T₆′ and T₇′ and coupled with converter 42, is arranged to provide a bias voltage, rather than a bias current, as in the second stage 2 and the third stage 3 in FIG. 1. To this end, an additional transistor T₁₄ is provided which, together with transistor T₁, constitutes a current mirror circuit for DC signals. As in FIG. 1, a transistor T₁₆ provides a suitable base current for transistor T₁.

[0036] The arrangement of FIG. 2 allows current and voltage biasing to be used alternatively. In other words, in response to gain control voltages V_c1 and V_c2 a suitable biasing mode can be selected. It will be understood that additional converters can be provided to allow additional gain control signals to be received and to be converted into suitable bias currents or bias voltages. It can also be envisaged that the arrangement of FIG. 2 is used to simultaneously provide a bias current and a bias voltage.

[0037] Another alternative embodiment of the first bias circuit 4 is shown is FIG. 3, where a compensation transistor T₁₅ is arranged between the converter 41 and the current mirror 40. This compensation transistor T₁₅ serves to compensate for any variations in the current gain factor beta of signal amplifying transistor T₁. The compensation transistor T₁₅ is preferably matched to transistor T₁ so that both transistors have virtually identical current gain factors beta. Transistor T₁₅ effectively divides the (bias) current in current mirror 40 by beta, while transistor T₁ multiplies this current by beta, thus effectively canceling out beta. As a result, the bias current is independent from beta. It will be understood that this arrangement can be applied in the circuits of both FIG. 1 and FIG. 2.

[0038] In a further advantageous embodiment (not shown), a single voltage/current converter supplies bias currents to two or more parallel first stages 1, or parallel first signal amplifying transistors T₁, T₁′ in a single first stage 1.

[0039] It will be understood that the features of the above embodiments may be combined as desired. For example, a first stage could have both a beta compensation transistor and a combined voltage and current bias circuit.

[0040] A particularly advantageous embodiment of a voltage/current converter is shown in FIG. 4. The converter of FIG. 4, which may constitute any or all of converters 41, 42, 51, 52, 61 and 62 of FIG. 1, essentially consists of a differential stage circuit comprised of transistors T₂₁ and T₂₂, which in the embodiment shown are bipolar NPN-type transistors, the emitters of which are connected via a resistor R₂₁. The base of transistor T₂₁ receives a control voltage V_c via a resistor R₂₂, while transistor T₂₂ receives a reference voltage V_r1 via a resistor R₂₃. The collector of transistor T₂₂ is connected to a supply voltage V_s, while the collector of transistor T₂₁ is connected to one of the current mirrors 40, 50, 60 of FIG. 1. Current sources S₁ and S₂ are connected to the emitters of T₂₁ and T₂₂ respectively. In a preferred embodiment the currents of current sources S₁ and S₂ have substantially equal magnitudes. The output current I_out is constituted by the current flowing into the collector of T₂₁.

[0041] An alternative embodiment of a voltage/current converter according to the present invention is shown in FIG. 5. Part of this converter is identical to the one of FIG. 4 and constitutes a first differential stage circuit with transistors T₂₁ and T₂₂, resistors R₂₁, R₂₂ and R₂₃, and current sources S₁ and S₂ connected as before. A second differential stage circuit, consisting of transistors T₂₃ and T₂₄, resistors R₂₄ and R₂₅ and current sources S₃ and S₄ is connected in parallel to the first circuit. Transistors T₂₂ and T₂₄ receive a reference voltage V_r1 and V_r2 via resistors R₂₃ and R₂₅ respectively, while transistors T₂₁ and T₂₃ both receive the control voltage V_c via a resistor R₂₂. In the preferred embodiment, the reference voltages V_r1 and V_r2 are not equal, resulting in different “opening points” per circuit, that is, different values of the control voltage V_c at which the particular circuit will start to conduct. The currents I_out1 flowing into the collector of transistor T₂₁ and I_out2 flowing into the collector of transistor T₂₃ together form output current I_out.

[0042] The circuit of FIG. 5 utilizes the two differential stages to produce a non-linear relationship between the input voltage V_c and the output current I_out. The inventors have found that this non-linear relationship is particularly suitable for producing a bias current as in the circuits of FIGS. 1-3. In addition, the circuit of FIG. 5 is relatively simple and economical.

[0043] The non-linear relationship between the control voltage V_c and the output current I_out in the circuit of FIG. 5 is schematically depicted in FIG. 6. Below a certain threshold voltage, the output current I_out will be virtually zero, this area is indicated I in FIG. 6. In a second area II, only the first differential stage (transistors T₂₁ and T₂₂) are conducting and the output current I_out (=I_out1) rises approximately proportionally with the control voltage V_c. Then another threshold voltage is reached and in an area III both differential stages are conducting, resulting in a total output current I_out (=I_out1+I_out2) which rises more than proportionally with the control voltage V_c. It can thus be seen that the converter as a whole exhibits a non-linear behavior which, as the inventors have found, is very suitable for providing bias currents in power amplifiers. The regions I, II and III, as well as the slope of the curve are well defined by the resistors. Due to the balanced structure the output current is temperature and supply voltage independent and virtually insensitive to process spread. In addition, the circuit of FIG. 5 is relatively simple and economical.

[0044] In the circuits described above, in particular those depicted in FIGS. 1-3, additional components may be provided in practical embodiments for the purposes of, for example, filtering or setting the operating point of the transistors of the various current mirrors. In the circuit of FIG. 3, for example, two resistors could be provided between the bases of transistors T₆ and T₇, the junction point of the transistors being connected to the collector of T₆. A capacitor could be connected between the base of transistor T₇ and the supply for noise suppression purposes. Another capacitor could be connected between the collector of transistor T₇ and ground, and a resistor could be connected between the collector of T₇ and the first stage 1. It will be understood that such components are not essential to the present invention and can be added or omitted as may be required in a specific embodiment.

[0045] The power amplifier of the present invention may suitably partitioned into sections embodied in different technologies. In the embodiment of FIG. 1, for example, the third stage 3 and possibly the second stage 2 can advantageously be implemented in GaAs (gallium arsenide), the remainder of the power amplifier being implemented in Si (silicon), thus exploiting the advantageous high frequency properties of circuits implemented in GaAs.

[0046] The present invention is based upon the insight that a well-defined bias current fed to a signal amplifying transistor allows an amplifier stage to have a well-defined gain. The present invention utilizes the further insight that non-linear bias circuits may produce very desirable overall amplification characteristics.

[0047] It is noted that any terms used in this document should not be construed so as to limit the scope of the present invention. In particular, the words “comprise(s)” and “comprising” are not meant to exclude any elements not specifically stated. Single (circuit) elements may be substituted with multiple (circuit) elements or with their equivalents.

[0048] Although various aspects of the present invention have been explained above with reference to multi-stage power amplifiers, it will be understood that the teachings of the present invention are not so limited. Accordingly, providing a suitable bias current into the base of a signal amplifying transistor in the manners indicated above is also advantageous in, for example, single-stage amplifiers and non-RF amplifiers. Similarly, the “multi-mode” arrangement discussed above which allows multiple distinct bias signals to be supplied to bias circuits in dependence on one or more gain control signals can equally well be applied to single-stage amplifiers. In addition, this “multi-mode” arrangement may also be utilized independently, that is, without the bias current measures discussed above.

[0049] It will therefore be understood by those skilled in the art that the present invention is not limited to the embodiments illustrated above and that many modifications and additions may be made without departing from the scope of the invention as defined in the appending claims.