Abstract:
An integrated amplifier circuit comprising an amplifier transistor connected in a common base configuration with its base zone being connected via an integrated capacitance to the semiconductor substrate of the integrated circuit so as to connect (short circuit ) the base zone to the semiconductor substrate at the operating frequency of the amplifier.

Description:
BACKGROUND OF THE INVENTION 
     As is known, there is often the danger of self-excitation of undesired oscillations in amplifier circuits comprising transistors with a high transit frequency. Undesired oscillations caused by the influence of unavoidable parasitic inductances and capacitances in cooperation with the amplifier transistor occur in a frequency range generally located substantially outside the operating frequency. The parasitic inductances can be traced back to the electrode leads and the parasitic capacitances tto capacitances located in the semiconductor body (e.g. barrier-layer capacitances) and also to external wiring capacitances. Both parasitic component types are also referred to as parasitic reactances. While the operating frequency in a VHF amplifier is, for example, 100 MHz, the undesired parasitic oscillations occur in the upper VHF or UHF range. The undesired oscillations result in reception interference and spurious radiation. 
     The undesired oscillations occur, particularly when the amplifier transistor is used in a grounded or common base circuit or configuration wherein, as is known, the base electrode, at the operating frequency, is short circuited to reference potential. In a common emitter circuit, however, the undesired oscillations are less frequent or easier to avoid. The problem of self-excitation of oscillations is particularly great in integrated circuits since the parasitic inductances are generally larger than in discrete designs. It is known to prevent the undesired parasitic oscillations by providing the leads with coverings of ferrite beads or by placing ferrite beads or resistors in the lead paths to the amplifier transistor. The known solutions do, however, have the disadvantage that the characteristics of the amplifier circuit are negatively, influenced in the operating frequency range, in particular, by an increase in the inherent noise of the amplifier circuit. The relatively high cost of known solutions is a further disadvantage. Since no satisfactory solution has yet been found to the aforementioned problems, pre-amplifiers are generally still made in accordance with the conventional technique. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide an amplifier circuit which enables the manufacture of pre-amplifiers in the VHF range in accordance with the integrated technique using bipolar transistors with limit frequencies of several GHz and which prevents undesired oscillations in those integrated pre-amplifiers. 
     According to the invention there is provided an amplifier circuit comprising an amplifier transistor connected in a common base configuration. The circuit is integrated and the base zone of the amplifier transistor, at the operating frequency of the amplifier circuit is connected, i.e., short circuited, by an integrated capacitance to the semiconductor substrate of the integrated circuit. In this way, the detrimental effect of the parasitic base lead inductance is avoided. 
     The capacitance is of such dimensions as to be at least 20 times larger than the base-to-collector capacitance of the amplifier transistor. It is advantageous to use the invention in amplifier circuits operating in the VHF range (50-300 MHz). 
     The integrated capacitance is preferably in the form of a pn junction. To make the pn junction, a semiconductor zone preferably of a conductivity type opposite to that of the semiconductor area surrounding it is inserted into the semiconductor body. The semiconductor zone is inserted into the semiconductor body for formation of the pn junction by, for example, diffusion or by ion implantation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail, by way of examples, with reference to the drawings, in which 
     FIG. 1 shows the principal design of an amplifier circuit; 
     FIG. 2 shows an amplifier circuit according to the invention; 
     FIG. 3 shows an embodiment of the invention in accordance with the integrated technique; 
     FIG. 4 shows an amplifier circuit wherein the capacitance comprises several strip-shaped semiconductor zones. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the principal design of an amplifier circuit with a bipolar transistor 1 in a grounded or common base circuit, as used, for example, for VHF receivers. In the circuit shown in FIG. 1, the antenna signal is fed to a tuned input circuit 2 where it is preselected. The selected input signal reaches the emitter of the amplifier transistor 1 via the coupling inductance 3. The amplified output signal is fed to a further selection circuit 4 which further selects the signal. In order to avoid undesired oscillations, ferrite beads 5 and 6 are provided in the base and emitter leads in the known circuit of FIG. 1. These known measures do, however, have the aforementioned disadvantage of causing additional noise. In integrated circuits, the oscillation danger is even greater than in conventional circuits. 
     To enable the amplifier to be designed in accordance with the integrated technique, without the occurrence of parasitic oscillations and additional noise caused by external damping measures such as ferrite beads, there is provided in accordance with the invention, in FIG. 2, an integrated amplifier circuit, wherein the base zone of the amplifier transistor 1 for the operating frequency is connected by an integrated capacitance C to the semiconductor substrate (7) of the integrated circuit. The capacitance C which should, therefore, represent a short circuit at the operating frequency, if possible, should be at least 20 times larger than the base-to-collector capacitance of the amplifier transistor 1. 
     The operating potential is preferably fed internally to the base zone, for example, via the integrated resistor 8, in the embodiment shown. The base potential may, however, also be fed and altered externally via the connection 9. The connection 10 is a general substrate connection for other circuit components of the integrated circuit, while the substrate connection 11 is connected to a substrate area located in the immediate vicinity of the capacitance C. Since the connection 11 is directly connected to the capacitance area of the semiconductor substrate, it decreases the interfering influence of interference voltages located on the remaining substrate. 
     FIG. 3 shows an embodiment of the invention in accordance with the integrated technique. Both the transistor and the capacitance are integrated in a common semiconductor substrate 7. The transistor consists of the emitter zone 12, the base zone 13 and the collector zone 14. The semiconductor zone 15 represents a low-resistance collector connection zone. Inserted in the semiconductor substrate beside the transistor is a semiconductor zone 16 forming with the substrate area 7&#39; a pn junction 18 which acts as barrier-layer capacitance and produces the integrated capacitance C. The semiconductor zone 16, the emitter zone 12, the collector zone 14 and the semiconductor zone 15 are of a conductivity type opposite to that of the semiconductor substrate 7. Other circuit components may, of course, also be integrated in the integrated circuit of FIG. 3. 
     The arrangement shown in FIG. 4 differs from the arrangement of FIG. 3 in that the capacitance C is provided by means of several strip-shaped semiconductor zones 16&#39;, 16&#34;, 16&#34;&#39; which form respective pn junctions with the surrounding semiconductor substrate areas. In the embodiment of FIG. 4, the semiconductor zones 16&#39;, 16&#34;, 16&#34;&#39; are contacted by an electrode 17 of comb-type configuration. In addition to the comb-type electrode 17 there is also provided a further comb-type electrode 19 which contacts the semiconductor areas surrounding the semiconductor zones (16&#39;, 16&#34;, 16&#34;&#39;). The strip-shaped semiconductor zones may also be connected to one another by a semiconductor strip.