Differential amplifier

[0002] This application is based on German priority application 19930171.9, which is hereby incorporated in its entirety by reference.

[0003] The present invention relates to a differential amplifier, and more particularly to a selective, switchable low-noise differential amplifier in the form of an integrated circuit (IC).

[0004] The invention relates to a differential amplifier as an IC with a push-pull input, a push-pull output, two transistor amplifier stages and two cascode stages.

[0005] A similar differential amplifier is disclosed for example in “Halbleiter-Schaltungstechnik” [Semiconductor circuitry], page 392, Fig.4.74, Tietze/Schenk, 11^thEdition, Springer-Verlag. This known differential amplifier is a cascode differential amplifier.

[0006] In mobile radio, simple amplifier stages generally comprising low-noise individual transistors have been used heretofore in the prior art, where the necessary image frequency suppression is effected by a passive filter upstream of the amplifier and by a further passive filter upstream of the mixer that is connected downstream. The required frequency selection has thus taken place outside the amplifier, which is embodied in a discrete or integrated manner.

[0007] Although previous integrated known solutions for integrated circuits were constructed in a switchable and low-noise manner, they were nonetheless constructed in an unbalanced manner. The image frequency suppression was effected by a passive filter upstream of the LNA and by a passive filter upstream of the mixer, that is to say the frequency selection took place outside the IC.

[0008] Since mobile radio equipment, and hence the modules in this equipment, too, are subject to ever greater miniaturization, efforts are made to construct the modules in an embodiment which is as compact as possible.

[0009] U.S. Pat. No. 5,352,987 shows an analog multiplexer having a differential amplifier with two transistor amplifier stages and also respective cascode stages which are coupled thereto and to which a push-pull output is connected. A multi-emitter transistor is connected to the amplifier stages and serves to effect changeover between a plurality of such differential amplifier stages.

[0010] The object of the invention, therefore, is to provide a large-scale integrated amplifier which is suitable for amplifying a specific frequency with as little noise as possible.

[0011] Accordingly, the inventor proposes improving the known differential amplifier as an IC with a push-pull input (RF_in, RFX_in), a push-pull output (RF_out, RFX_out), two transistor amplifier stages and two cascode stages such that the cascode stages can be switched by a changeover switch (DC gain switch).

[0012] An advantageous refinement of the differential amplifier according to the invention provides for there to be a selection circuit in the IC, which selection circuit restricts the gain to a specific frequency.

[0013] The differential amplifier is advantageously constructed in such a way that the two cascode stages are constructed in a balanced manner.

[0014] The selection circuit of the resonant circuit may preferably have at least one capacitor and at least one coil.

[0015] The preferred design of the at least one coil is for it to be integrated as a planar loop of at least one interconnect in the IC.

[0016] A further preferred embodiment of the cascode differential amplifier according to the invention consists in the bias supply of the transistors of at least one amplifier stage having an integrated bandgap circuit and, if appropriate, an operational amplifier connected downstream. The bandgap circuit is a term for an integrated voltage supply which provides a reference voltage that is independent of the temperature and the supply voltage.

[0017] In addition or as an alternative, the bias supply of the transistors of at least one cascode stage may also comprise a corresponding, integrated bandgap circuit and, if appropriate, an operational amplifier connected downstream.

[0018] A further refinement of the differential amplifier according to the invention consists in negative feedback, preferably parallel negative feedback, with at least one resistor and/or at least one capacitance element in each case, being provided in at least one amplifier stage, preferably in both amplifier stages.

[0019] In addition, provision may be made for conducting away the signal at the push-pull output (RF_out, RF_out) via a capacitance element and a resistor.

[0020] The transistors of the differential amplifier may be configured either as npn or pnp bipolar transistors or as field effect transistors (FETs).

[0021] The circuit arrangement of the integrated module according to the invention for the first time provides an integrated, selective, gain-switchable and low-noise cascode differential amplifier, which may be used for example as a preamplifier of reception signals in mobile radio equipment and assists the image frequency suppression for constant input impedance and output impedance.

[0022] The features of the invention mentioned herein can be used not only in the combination specified in each instance but also in other combinations or by themselves, without departing from the scope of the invention.

[0023] Embodiments of the invention will now be described by way of example with reference to the drawing in which:

[0024] FIG. 1 shows an integrated circuit differential amplifier according to the invention.

[0025] FIG. 1 shows a preferred embodiment of a balanced circuit, according to the invention, of an electronic integrated circuit with a push-pull input RF_inand RFX_in, a push-pull output RF_outand RFX_out, two transistor amplifier stages 3 and 4, and two cascode stages 5, 6. The cascode stages 5 and 6 are embodied such that they can be changed over via a DC gain switch. Furthermore, according to the invention, a selection circuit 1 is provided in the IC, which selection circuit filters the amplified signal to a specific frequency band.

[0026] In this circuit, in detail, the transistors T₁to T₄form the amplifier stages 3 and 4, the resistor R₃regulating the basic DC current of the circuit by means of the bias voltage—not illustrated for reasons of clarity—at the bases of T₁to T₄. The transistor T₅forms the collector load of T₁and the transistors T₆(1 . . . n) and T₇form the collector load of the transistor T₂(1 . . . m). Similarly, the transistor T₁₀is responsible for T₄and the transistors T₉(1 . . . n) and T₈are responsible for T₃(1 . . . m).

[0027] The collectors of the transistors T₅, T₆(1 . . . n) and T₁₀, T₉(1 . . . n) are connected to the supply voltage V_ccvia the selection circuit 1. The selection circuit 1 includes a resonant circuit with two coils L₁, L₂and one capacitance element C₃. The collectors of the transistors T₇and T₈are connected to the supply voltage V_ccvia the resistor R₅.

[0028] The bias supply of the bases of the transistors T₅, T₆(1 . . . n) and T₁₀, T₉(1 . . . n) is likewise not illustrated, for the sake of better clarity, and is realized by means of an integrated bandgap circuit with an operational amplifier connected downstream, or just a bandgap circuit.

[0029] The base voltage of the transistors T₇and T₈can be varied via the three-conductor bus shown. If this base voltage is so small that the transistors T₇and T₈are in the off state, the entire RF signal is passed, having been amplified, to the output. If the base voltage at the transistors T₇and T₈becomes so large that the latter turn on, only part of the RF input signal is amplified and passed to the output through the transistors T₅and T₁₀. In other words, an (n+1)-th of the RF input signal is passed to the output, where n corresponds to the number of parallel-connected transistors T₆(1 . . . n) or T₉(1 . . . n).

[0030] The capacitor C₃forms a resonant circuit with L₁and L₂, which resonant circuit, by way of its quality factor, preselects the amplified input signals arriving from the inputs RF_inand RFX_in. The output signal is tapped off at the outputs RF_outand RFX_out.

[0031] The series circuit formed by R₁and C₁, and also R₂and C₂, serves as parallel negative feedback for decreasing the input impedance and thus for increasing the linearity of the amplifier. It taps off the assigned signals in each case at the collector of the transistor T₂(1 . . . m) and at the collector of the transistor T₃(1 . . . m), respectively. As an alternative, purely resistive or purely capacitive negative feedback, or a tap at the collector of T₁and T₄, respectively, is also possible without significant functional limitation.

[0032] Since the transistors T₂(1 . . . m) and T₃(1 . . . m), respectively, always carry current, a change in gain will not have the effect of influencing the negative feedback. An increase in the base voltage of the transistors T₇and T₈to an extent such that the latter attain the on state leads to the DC and AC signals being conducted away via the transistors T₇, T₈and R₅to V_cc.

[0033] In order to avoid reflections of the amplified and conducted-away AC signal in the circuit and fluctuations in the supply, provision is additionally made for the RF signal to be conducted away via the capacitance element C₄and the resistor R₄to ground. The number n specifies the number of transistors connected in parallel as T₆and T₉, and the number m specifies the number of transistors connected in parallel as T₂and T₃, which may vary between 1 and any desired number in accordance with the requirements made of the differential amplifier.

[0034] These parallel transistors define the current switching ratio I₁/I₂and I₁′/I₂′, respectively, for the maximum and minimum gain. The maximum possible change in gain is calculated as Δgain=20log((N+1)/1). For n=9, therefore, Δgain=20 dB.

[0035] The quasi-parallel-connected transistors T₁and T₂(1 . . . m) and respectively T₃(1 . . . m) and T₄thus carry current in both gain states, in other words gain switching advantageously leads to no change in the input impedance. The output impedance is determined by the selection circuit 1, comprising a parallel resonant circuit formed by the capacitance element C₃and the coils L₁/L₂, or by the quality factor thereof This quality factor is very low, therefore, a change in the gain also brings about no significant change in the output impedance.

[0036] The amplifier stages have respective current paths connected in parallel. In the amplifier stage shown on the left in the figure, the current paths are formed by the transistors T₅, T₁and T₆, T₂, whose collector-emitter paths are connected in series. The transistors T₁, T₂and T₅, T₆are each driven in parallel, owing to the fact that their base terminals are connected to one another. The cascode stage of one of the current paths, namely the cascode stage comprising the transistor T₆, is designed as a current switch and comprises two emitter-coupled transistors T₆, T₇. The amplifier stage shown on the right in FIG. 1 has a balanced structure with respect thereto. The cascode stages of the two amplifier branches are coupled by the changeover switch in such a way that the transistors T₇, T₈are connected to one another by their bases and collectors, the control signal DC-Gain-Switch being applied to the coupled base terminals.

[0037] The capacitance element C₅, which is arranged between the two transistors T₇and T₈, in this case serves to block interfering frequencies. The resistors R₆to R₉serve for decoupling the amplifier from the applied bias voltages.

[0038] Overall, the circuit described above achieves a significant miniaturization of a selective, switchable and low-noise differential amplifier, preferably for the preamplification of reception signals in mobile radio, which acts to assist the suppression of image frequencies and has a constant input impedance and output impedance.

[0039] The German Application No. 19930171.9 is relied upon and hereby incorporated by reference in its entirety.

[0040] Various preferred embodiments of the invention have now been described. While these embodiments have been set forth by way of example, various other embodiments and modifications will be apparent to those skilled in the art. Accordingly, it should be understood that the invention is not limited to such embodiments, but encompasses all that which is described in the following claims.