Currently, mobile phones are operated predominantly with a single operating frequency.  FIG.  7 shows a realization of such a transmitter/receiver operation in a mobile phone being operated with a single operating frequency, e.g., with approximately 900 MHz for GSM, approximately 1800 MHz for DCS or approximately 1900 MHz for PCS.  An antenna 100 being used to transmit signals and to receive signals is connected to a transmitter/receiver change over unit 102.  The transmitter/receiver change over unit 102 comprises a transmitter switch TX and a receiver switch RX.  In the receiving mode, the transmitter switch TX is opened and the receiver switch RX is closed.  To the contrary, in a transmitting mode the transmitter switch TX is closed and the receiver switch RX is opened. 
In the transmitter mode, a power amplifier 104 outputs a transmitting signal in the pre-specified frequency band.  Here, an impedance matching is carried out through an impedance matching circuit 106 such that the output of the power amplifier sees an impedance which in most cases is lower than the impedance of the following transmission branch, e.g., 50 .OMEGA..
However, the circuit design shown in FIG.  7 more and more limits the increasing use of digital mobile telephony since the number of subscribers is continuously increasing while the number of transmitting frequencies and related transmission channels is limited.  Although an increased transmitting frequency of, e.g., approximately 1800 MHz for DCS or approximately 1900 MHz for PCS in comparison to approximately 900 MHz for GSM enables an increased number of transmission channels, this is only possible at the expense of reduced working ranges for the transmitter stations. 
Nevertheless, a combination of different technical advantages for the different approaches through provision of cellular dual band networks and dual band mobile phones adapted thereto seems to be promising, e.g., a combination of the GSM-frequency band with the DCS- or PCS-frequency band. 
Heretofore, in U.S. Pat.  No.  5,774,017 there is proposed a dual-band amplifier for wireless communication, in particular for operation at either the 800 MHz or the 1900 MHz band.  The described dual band amplifier provides desired gain and input/output impedance.  Switching impedance networks are used at the input and output of a power amplifier to provide matching input impedance and a desired output impedance for operation in two bands. 
FIG.  8 shows another option of a corresponding power amplifier output circuit designed for the necessary transmission/receiving operation in a dual band mobile phone.  This approach directly relies on the circuit design shown in FIG.  7. 
Here, the antenna 200 is connected to two transmitter/receiver change over units 202 and 204.  The sending/receiving change over unit 202 comprises a transmitter switch TXa and a receiver switch RXa for a first transmitter frequency.  Further, the transmission/receiving change over unit 204 comprises a transmitter switch TXb and a receiver switch RXb for a second transmitter frequency.  The different switches TXa, RXa, TXb, and RXb are operated in accordance with the different operation frequencies, respectively, as outlined above with respect to FIG.  7.  Further, a diplexer 206 is necessary to join the two transmission paths to the antenna 200 without losses.  For the amplification of the transmitting signals in the frequency bands, there are provided related power amplifiers 214 and 216.  For these power amplifiers 214 and 216 an impedance matching is realized through impedance matching circuits 218 and 220 provided in each of the two transmission branches.  Alternatively, the two power amplifiers 214 and 216 for the two transmitting frequencies can be substituted through a single power amplifier with two output terminals and a downstream impedance matching circuit. 
This direct generalization of the single band transmitting/receiving circuit shown in FIG.  7 leads to the advantage that the different transmission branches for both transmitting frequency bands are completely decoupled.  However, while suitable impedance matching of the different transmitting frequency bands is achieved through the separated and fully decoupled provision of the impedance matching circuits this is only achieved with a high circuit complexity.  On the one hand this leads to an increase in the production costs and on the other hand also the space requirements necessary for such a dual band transmitting/receiving change over unit constitute a barrier for the implementation thereof. 
In view of the above, the object of the present invention is to achieve an improved matching of a power amplifier outputting transmitting signals in different transmitting frequency bands over a single output terminal to impedances of the different transmission branches in a dual band mobile phone. 
According to the invention, this object is achieved through a power amplifier output circuit for a dual band mobile radio unit according to claim 1.  The power amplifier output circuit comprises a first transmitter/receiver change over means for transmitting/receiving a first transmitting/receiving signal, the transmitter/receiver change over means being provided with an input terminal to which a first impedance matching means is connected, a second transmitter/receiver change over means for transmitting/receiving a second transmitting/receiving signal, a transmission branch change over means to selectively connect the first transmitter/receiver change over means or the second transmitter/receiver change over means to a power amplifier outputting transmitting signals in two frequency bands such that a second impedance matching means is provided between an output terminal of the power amplifier and the transmission branch change over means and the transmission branch change over means comprises at least two switching elements being connected in parallel in a branch connecting the power amplifier with the first transmitter/receiver change over means. 
Therefore, for the present invention the stepwise approach to impedance matching in at least one transmission branch of the power amplifier output circuit is of importance as well as the simultaneous use of a plurality of switching elements connected in parallel.  Both measures in functional relationship lead to a significant minimization of parasitic disturbances in the power amplifier output circuit.  At the same time, there is also achieved a suitable impedance matching for the respective frequency bands and transmitting powers in both transmission branches. 
Further, while the use of only a single impedance matching at the output of the power amplifier will not lead to an optimum impedance matching for both transmission branches according to the present invention this is achieved, firstly, through the first common impedance matching at the output of the power amplifier and, secondly, through a further impedance matching optimized for each transmitting frequency band, respectively.  Further, since the common impedance matching is used for both.  frequency bands the circuit complexity may be reduced significantly. 
Still further, the present invention takes into account that the power absorption in parasitic elements of the transmission branch change over means increases when the disturbing real part of the impedance of the transmission branch change over means lies close to the output impedance of the power amplifier.  E.g., the real part of output impedances of practically used power amplifiers lies in the range from approximately 5 to 6 .OMEGA.  while typical connecting resistances of different switching elements lie in the range of approximately 1 .OMEGA.. In case switching elements are inserted in the transmission branch change over unit only after a first impedance transformation, e.g., to approximately 20 .OMEGA.  at 900 MHz for GSM or 50 .OMEGA.  for 1800 MHz for DCS, the power absorption in the parasitic elements is significantly reduced due to a smaller ratio between switching element connecting resistance and impedance level at the input terminal to the switching element, e.g., the ratio being smaller by an order of magnitude. 
According to the present invention the power absorption through the parasitic elements may be further significantly reduced by providing at least two switching elements in at least one transmission branch of the transmission branch change over means.  Through the parallel connection the parasitic resistance and the parasitic inductance due to the necessary switching between the first and the second impedance matching are reduced by a factor corresponding essentially to the number of switching elements connected in parallel. 
Besides the minimization of the absorbed power the switching elements connected in parallel also contribute to an improved impedance matching.  Due to the decreased overall connecting resistance and the decreased overall parasitic inductance between the first and second impedance matching stage, respectively, the overall impedance matching gets less sensitive towards the disturbing influence of the switching elements. 
According to a preferred embodiment of the invention there is provided a third impedance matching means at an input terminal of the second transmitter/receiver change over means. 
Thus, there is provided an optimized matching in the single transmission branches specifically adapted to the respective transmitting frequency and transmitting power, e.g., 3 W for approximately 900 MHz and 1.5 W for approximately 1800 MHz.  However, since part of the impedance matching for the different transmission branches is achieved through the common impedance matching circuit connected to the output terminal of the power amplifier the circuit complexity specifically necessary for the different transmission branches is minimized. 
According to yet another preferred embodiment of the present invention the transmission branch change over means between the second impedance matching means and the third impedance matching means comprises at least one switching element. 
Usually, the transmission branches are provided to output transmitting signals with a lower transmitting frequency, e.g., approximately 900 MHz for GSM, and a higher transmitting frequency, e.g., approximately 1800 MHz for DCS and approximately 1900 MHz for PCS.  Here, it should be noted that the impedance matching at the output of the power amplifier leads to different results for the different frequency bands.  In particular, in the transmission branch for the higher frequency band there is achieved an almost complete matching to the necessary impedance level through the impedance matching means at the output terminal of the power amplifier such that parasitic elements in the related branch of the transmission branch change over means only have a minor influence.  According to this preferred embodiment of the invention, the object is to provide measures against parasitic effects via frequency selective way only for the lower frequency band while minimizing the additional costs for switching elements.  In other words, switching elements are only inserted to an extent necessary for the selected transmitting frequency. 
According to yet another preferred embodiment of the present invention, there is provided a switchable band stop filter between the second impedance matching means and the third impedance matching means to filter the harmonics of the first transmitting signal during the transmission of the first transmitting signal in the second transmission path. 
The power amplifier is usually operated near saturation.  This leads to the generation of harmonics, e.g., at approximately 1800 MHz, approximately 2700 MHz, . . .  in the GSM-transmitting mode and also to the generation of harmonics at approximately 3600 MHz, etc.  in the DCS-transmitting mode.  Usually, harmonics of first order are dominating. 
Although in the GSM-transmitting mode the harmonics at approximately 1800 MHz, approximately 2700 MHz, . . .  are low pass filtered in the first transmission branch, the first harmonic at approximately 1800 MHz of the GSM transmitting mode is not suppressed through a low pass filter in the second transmission branch being only adapted to harmonics of the second transmitting signal at approximately 3600 MHz, etc.  The same holds true for a combination of the transmitting frequencies for GSM and PCS with a transmitting frequency of approximately 1900 MHz.  Generally speaking, this problem arises for power amplifier outputting transmitting signals in a plurality of transmitting frequency bands in case harmonics of the first, lower transmitting frequency lie below the second, higher transmitting frequency or are identical thereto. 
To solve this problem the second transmission branch is advantageously provided with a switchable band stop filter being adapted to suppress specifically during the transmission of the first lower transmitting frequency the first harmonic thereof in the second transmission branch.  This allows for an optimum decoupling of the different operation modes. 
According to yet another preferred embodiment of the invention the transmission change over means between the first impedance matching means and the second impedance matching circuit consists of a first diode of a PIN-type and a second diode of the PIN-type such that the first diode of the PIN-type and the second diode of the PIN-type are connected in parallel.  Preferably, the first diode of the PIN-type and the second diode of the PIN-type are comprised in a single package or housing. 
Thus, during fabrication and operation of the power amplifier output circuit according to the present invention only a single component must be handled and supplied with power.  During fabrication placement of components is essentially unchanged so that approved circuit layouts and fabrication processes may be maintained essentially without any modification. 
According to yet another preferred embodiment of the present invention the first impedance matching means has a first capacitor connected in shunt configuration at its input.  Further, the first impedance matching circuit comprises a second capacitor in series between the input and the output thereof. 
Usually, the impedance matching is achieved in the single transmission branches through a sequence of capacitors and inductivities.  Also different line elements for the connection of the components and parasitic inductivities of the switching elements are considered.  According to the present invention, it is taken into account that capacitors usually are only available with capacitances lying in a prespecified basic grid, e.g., according to 3.3 pF, 3.9 pF, 4.7 pF, 5.6 pF, etc.  The increased number of capacitances in the first impedance matching circuit results in a finer gradation for the impedance transformation and thus in an improved impedance matching.  This is a particular advantage for transmission branches carrying the transmitting signal in the lower transmitting frequency range.