High Frequency Mixer Stage

The invention relates to a high frequency mixer stage comprising a field-effect transistor tetrode having two gate electrodes, with the high frequency input signal being fed to one of the gate electrodes. A mixer stage of this type is known, for example, from DE-PS German Pat. No. 21 59 592. In the known circuit, a field-effect transistor tetrode is combined with an ordinary field-effect transistor. The high frequency input signal is fed to one gate electrode of the tetrode, while the oscillator frequency from a parallel resonant circuit is fed to the second gate electrode of the tetrode. Connected to the drain electrode of the tetrode is a further parallel resonant circuit with which the intermediate frequency component is filtered out and coupled out inductively.

The known circuit is relatively sophisticated, since in addition to the tetrode a further field-effect transistor is required and a special local oscillator must be provided.

The object underlying the invention is to provide a high frequency mixer stage which is of particularly simple and easy-to-integrate design.

According to the invention there is provided a high frequency mixer stage wherein the operating point of the tetrode at the second gate electrode is set such that the mixer stage is converted into a frequency changer by the self-excitation which occurs.

This self-excitation is preferably produced by means of an inductive load at the second gate electrode. A GaAs barrier layer field-effect transistor is, for example, suitable as tetrode.

The high frequency mixer stage according to the invention is of extremely simple construction because the single active component determined by the tetrode replaces the preliminary and high frequency amplifiers, respectively, the local oscillator and the mixer. The tetrode is very simple to produce in a manner known per se in integrated semiconductor technology.

The invention will now be described in greater detail, by way of an example, with reference to the drawing where the FIGURE is a view of a preferred circuit for the high frequency mixer stage.

A tetrode T with the two gate electrodes G.sub.1 and G.sub.2 is shown in the FIGURE. This tetrode is, for example, a GaAs MESFET which can be employed in the 10 GHz range. The high frequency input signal is fed to the gate electrode G.sub.1 via the transformer U, while the oscillator frequency f.sub.osz decoupled therefrom is fed to the gate electrode G.sub.2. This local oscillator frequency is generated by means of an inductive load Z.sub.L. Between the drain and source electrodes of the tetrode there is a parallel resonant circuit which is comprised of the capacitor C.sub.1 and the inductance coil L.sub.1 and is aligned to the differential frequency between the oscillator frequency and the input signal frequency. The intermediate frequency ZF is inductively coupled out of this parallel resonant circuit.

In one embodiment, a tetrode with the following transistor characteristics was used:

GaAs MESFET tetrode with a channel width of 200 .mu.m and gate electrodes with a length of 1 .mu.m (gate 1) and 2 .mu.m (gate 2), respectively;

saturation current I.sub.DSS =25 mA,

maximum oscillation frequency f.sub.max =20 GHz,

transconductance at gate 1 g.sub.m1 =15 mS (u.sub.g2 =0).

transconductance at gate 2 g.sub.m2 =10 mS (u.sub.g1 =0).

The gate electrode G.sub.2 was loaded with an inductive load .vertline.Z.sub.L .vertline. in the order of 56.OMEGA. to generate the oscillator frequency f.sub.osz. The oscillator frequency then amounted to 9.0 GHz. With a signal frequency f.sub.sign of 10.08 GHz there is an intermediate frequency ZF in the order of 1.08 GHz which is filtered out at the resonant circuit C.sub.1, L.sub.1.