The present invention relates to low noise amplifiers.
Low noise amplifiers are frequently used in various electronic devices. A fundamental characteristic of such amplifiers is linearity.
A low noise amplifier can be considered as a non-linear system whose input-output characteristic can be modeled by a series of powers truncated at the third order. If the input signal Vi is constituted by two tones at respective frequencies ω1 and ω2, i.e., Vi=A1×cos(ω1×t)+A2×cos(ω2×t) with ω1 almost the same as ω2, with ω1=ω and ω2−ω1<<ω and A1=A2, we have linearity that can be characterised by a parameter of intermodulation of the third order IM3 in relation to a certain level of power of the two input tones or by the intercept of the third order IP3. In fact we have
where with P1 the power of the tone at ω1 is indicated while with P3 the power of the spurious tones at 2ω2−ω1 or at 2ω1−ω2 is indicated. As in a real amplifier the two spurious tones do not necessarily have equal Amplitude. There are two different IM3, i.e., the IM3low referred to the spurious tone 2ω2−ω1 and the IM3high referred to the spurious tone 2ω1−ω2 with IM3 being the maximum between IM3high and IM3low. The intercept of the third order IP3 of a non-linear system is linked to the parameter IM3 by the relation:
where IIP3 is the intercept of the third order in input to the non-linear system and Pi is the power of the signal in input to said non-linear system; since in a real amplifier we have the parameters IM3high and IM3low, we also have the parameters IIP3high and IIP3low: In addition we have that the intercept of the third order OIP3 relative to the signal in output from the linear system is given by:
OIP3[dBm]=IIP3[dBm]+G[dB]
where G is the gain in decibels of said non-linear system. For OIP3 we have the parameters OIP3high and OIP3low.
A low noise amplifier that presents good performance in terms of linearity is shown in the article “Effect of out-of-band termination on intermodulation distortion in common-emitter circuits”, IEEE MTT-S Dig., vol. 3, pages 977-980, June 1999 by V. Aparin and C. Persico. The low noise amplifier described comprises a bipolar transistor in common-emitter configuration and particular circuits for the polarization of the transistor, for the input adaptation and for the output adaptation. The parameter IM3 calculated for a bipolar transistor Q1 in common-emitter configuration, shown in
At the frequency Δω, since the capacitance C3 is a short circuit, the capacitances C1 and C2 are open elements and the values of the impedance offered by the inductance Lb and by the microstrip ML2 are almost nil, we have the input impedance Zs(Δω) that is equal to the resistance R1.
At the frequencies ω and 2ω the capacitances C1 and C2 have negligible impedances while the microstrip ML1, having length l1=λ/4, behaves like an open circuit at the frequency ω and like a short circuit at the frequency 2ω. Therefore at the frequency ω the impedance Zs depends on the inductance Lb and on the length l3 of the microstrip ML3. At the frequency 2ω the impedance Zs depends on the inductance Lb, on the length l2 of the microstrip ML2 and on the length l3 of the microstrip ML3. Setting the length l3 and the inductance Lb at the operative frequency ω the resistance R1 and the length l2 of the microstrip ML2 can be chosen to vary the impedance Zs at the frequencies Δω and 2ω to obtain the maximum linearity. The microstrip ML5, having length λ/4, behaves like an open circuit at the frequency ω and like a short circuit at the frequencies Δω and 2ω. Such a low noise amplifier has the disadvantages of poor insulation between input and output, a low stability and the interdependence between the output adaptation and that in input.
In view of the state of the art, an object of the present invention is to provide a low noise amplifier that overcomes the abovementioned inconveniences.
In accordance with the present invention this object is achieved by means of a low noise amplifier comprising a cascode device which includes at least a first and a second transistor having a terminal in common and the output terminal of the second transistor being the output terminal of the cascode device and being coupled to the output terminal of the amplifier. The first circuit means is suitable for the polarization of the second transistor and is positioned between a supply voltage and another terminal of the second. The second circuit means is connected to the output terminal of the cascode device and is suitable for its adaptation of output of the cascode device, with the amplifier perative at a given frequency such that the first circuit means includes a first series of a resistance and a capacitance and the second means includes a second series of a resistance and a capacitance, with the first series being coupled between said other terminal of the second transistor and ground and the second series being coupled between the output terminal of the cascode device and ground. The values of the resistances of the first and of the second series are much lower than the module values of the respective capacitive impedances of the first and of said second series at the given frequency.
Thanks to the present invention it is possible to provide a low noise amplifier that in addition to the qualities of the cascode amplifier has good linearity characteristics.
The characteristics and advantages of the present invention will appear evident from the following detailed description of an embodiment thereof, illustrated as non-limiting example in the enclosed drawings, in which:
The second means B2 comprise a resistance Rb coupled between the base terminal of the transistor T2 and ground and the third means B3 comprise a resistance Rc coupled between the output terminal of the amplifier and ground. The resistances have such a small value that, calculating the impedances Zb, Zl of the second means B2 and of the third means B3 at the frequencies Δω, ω and 2ω, they are negligible at the frequencies ω and 2ω but are not negligible at the lower frequencies, that is at the frequency Δω. In this manner the resistances Rb and Rc improve the linearity of the amplifier without influencing the other characteristics of the amplifier such as noise figure, the output adaptation and the low impedance on the base terminal of the transistor T2. The impedances Zb, Zl calculated for the second means B2 and for the third means B3 are the impedances seen respectively of a terminal of the transistor T2, i.e, from the base terminal, and from the output terminal of the transistor T2, i.e, the collector terminal of the transistor T2.
More precisely the second means B2 comprise a series of a resistance Rb and of a capacitance Cb; the series is connected between the base terminal of the transistor T2 and ground and is such that the value of the resistance Rb is much less, that is by at least an order of size, than the value in module of the capacitive impedance of the capacitance Cb calculated at the operating frequency of the amplifier, that is
The third means B3 comprise a series of a resistance Rc and of a capacitance Cp; the series is coupled between the collector terminal of the transistor T2 and ground and is such that the value of the resistance Rc is much less, that is by at least an order of size, than the value in module of the capacitive impedance of the capacitance Cp calculated at the operating frequency ω of the amplifier, that is
In this manner the resistances Rb and Rc do not influence the other characteristics of the amplifier.
The first circuit means B1 comprise an inductance Lb1 connected to the base terminal of the transistor T1 and to the current generator Ibias, a series of a microstrip ML33, a resistance R11 and a capacitor C33 positioned between a terminal of the inductance Lb1 and ground; between the terminal in common of the resistance R11 and of the microstrip ML33 and ground is positioned a capacitance C22. The inductance Lb1 is connected to the input terminal IN1 by means of the series of a capacitance C11 and of another microstrip ML22; another microstrip ML11 is positioned between the terminal IN1 and ground. The capacitance C33 has a much higher value than the capacitances C11 and C22.
At the frequency Δω, since the capacitance C33 is a short circuit, the capacitances C11 and C22 are open elements and the values of the impedance offered by the inductance Lb1 and by the microstrip ML22 are almost nil, we have the input impedance Zs1 (Δω), that is the impedance seen from the base terminal of the transistor T1, that is equal to the resistance R11.
At the frequencies ω and 2ω the capacitance C22 has a negligible impedance while the microstrip ML11, having length l1=λ/4, behaves like an open circuit at the frequency ω and like a short circuit at the frequency 2ω. Therefore at the frequency ω the impedance Zs1 depends on the inductance Lb1, on the capacitance C11 and on the length l33 of the microstrip ML33. At the frequency 2ω the impedance Zs1 depends on the inductance Lb1, on the capacitance C11, on the length l22 of the microstrip ML22 and on the length l33 of the microstrip ML33. Setting the length l33, the capacitance C11 and the inductance Lb1 at the operating frequency ω the resistance R11 and the length l22 of the microstrip ML2 can be chosen for varying the impedance Zs at the frequencies Δω and 2ω to obtain the maximum linearity.
Number | Date | Country | Kind |
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MI2004A0871 | Apr 2004 | IT | national |
Number | Name | Date | Kind |
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5742205 | Cowen et al. | Apr 1998 | A |
Number | Date | Country | |
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20050242887 A1 | Nov 2005 | US |