High frequency differential to single-ended converter

[0002] The present invention relates to a high frequency converter circuit for converting a differential output signal from a differential amplifier to a single-ended output signal and, more particularly, to a high frequency, i.e., frequencies above eight hundred (800) MHz, circuit and method for combining differential output signals into a single-ended signal and simultaneously achieving an impedance match from the differential amplifier and the input of a single-ended output stage.

[0003] In many high frequency communications systems it may be necessary to convert differential output signals from a differential amplifier into a single ended signal. For example, in some wireless telephone communication systems operating near or at 900 MHz there is a requirement that a single ended transmission signal be utilized to drive the antenna for transmission. However, typically, differential signals are utilized which requires a converter circuit to derive the single ended signal before it is applied to an input of a single-ended output amplifier stage prior to driving the antenna. Matching of the output impedance of the converter circuit to the input impedance of the output amplifier stage is also required.

[0004] In a known prior art differential to single-ended converter one side of the differential outputs of the differential amplifier is simply thrown away, being coupled to ground, while the other output is coupled to the input of the single-ended output amplifier stage. This technique suffers from several disadvantages such as low gain, poor isolation, to name a few. Moreover, the use of an inductor is relatively expensive wherein production cost are desired to be maintained as low as possible.

[0005] Hence, a need exists for a low cost improved differential to single-ended converter for converting high frequency differential signals to a single-ended signal and simultaneously achieve impedance matching to a following single-ended stage while eliminating the disadvantages mentioned above with known prior art circuits.

[0006] FIG. 1 is a partial block and schematic diagram of one embodiment of the present invention; and

[0007] FIG. 2 is a partial block and schematic diagram of another embodiment of the present invention.

[0008] Turning now to FIG. 1 there is shown differential amplifier 10 which amplifies the differential output signals supplied from oscillator 12 to the differential inputs thereof. The differential output signals from amplifier 10 are supplied to external outputs 9 and 13. A differential to single-ended converter circuit of one embodiment of the present invention is illustrated comprising capacitors 14 and 16, of generally the same capacitance, which are coupled respectively to the outputs 9 and 13 as well as transmission line 18 coupled between the capacitors 14 and 16 at terminals 15 and 17. The other ends of capacitors 14 and 16 are coupled to a voltage reference potential such as ground. A phase shifting element or transmission line 18, although shown as being a quarter wavelength (λ/4) transmission line of given characteristic impedance at the given frequency of the differential signals applied at outputs 9 and 13, may be of any odd quarter wavelength multiples of the frequency of the differential signals. An output of the converter is taken at terminal 19 and is applied to the input of externally connected single-ended amplifier stage 20 which amplifies and buffers the single-ended signal from the converter. The amplified single-ended signal is then illustrated as being applied from amplifier stage 20 to the input 22 of a matching stage 24 the output 26 of which may be applied via to an antenna (not shown).

[0009] In operation, the differential output signals from amplifier 10 through the load capacitors 14 and 16. Transmission line 18 rotates the phase of the current through capacitor 14 by 180 degrees such that it is now in phase with the current through capacitor 16 and will be combined therewith. Hence, twice the current is now available to drive the input of the single-ended amplifier 20. Thus, the converter circuit of FIG. 1 will have twice the gain of the prior art converter afore described. The converter circuit of FIG. 1 also provides good common mode rejection in that any common mode signals appearing at terminals 9 and 13 are canceled by λ/4 transmission line 18 causing these signals to be out of phase at output terminal 19.

[0010] Additionally, the converter circuit described above provides conjugate matching to the input impedance of amplifier 20. This matching results from the input impedance of single-ended amplifier 20 looking capacitive while the output impedance looking back into the converter is inductive. The equivalent circuit appears as a parallel LC network resonant at the operating frequency. Therefore the gain of the overall circuitry is maximized.

[0011] Turning to FIG. 2, a corollary converter circuit to that shown in FIG. 1 is illustrated. Once again, a pair of differential output signals are provided by oscillator 28 which are applied to the differential inputs of amplifier 30. Differential amplifier amplifies these signals to provide differential output signals at external output terminals 31 and 33. The converter circuit of the subject embodiment of the invention is shown as including a capacitor 36 the terminals 32 and 34 are coupled to external terminals 31 and 33 and a pair of λ/8 transmission lines 38 and 40 coupled across capacitor 36 and to a voltage supply, V_CC, at terminal 42.

[0012] Once again, the differential signal applied at output terminal 31 will be shifted in phase through transmission lines 38 and 40 to be in phase with the differential signal applied at terminal 33. The two signals are thus combined into a single-ended signal and applied to the input of single-ended amplifier 44 at terminal 37. Again, the output of amplifier 44 may be supplied at terminal 46 to the input of a matching stage 48 to drive an load at terminal 50.

[0013] Although the capacitive impedance is illustrated in the Figures as being realized by physical capacitors 14, and 16 it is well understood to those skilled in the art that open ended transmission line stubs of odd multiples of λ/4 wavelengths could be used.

[0014] Hence, what has been described are a pair of high frequency differential to single-ended converter circuits that provide high gain characteristics, common mode rejection, high linearity and good isolation to other circuits.