T 0032/84 (Redefining an invention) 06-08-1985

Reasons for the Decision

1. The appeal complies with Articles 106 to 108 and Rule 64 EPC.

2. The Examining Division considered that the circuit constituting the subject-matter of the application did not enable a reference voltage to be obtained and consequently decided to refuse the application solely on the basis of Article 83 EPC (page 4, last paragraph). It therefore concerned itself only with the alleged failure to function of the circuit shown in Figure 5 of the application and did not express any opinion as to the claims. The appellants request that the patent be granted on the basis of the claims in their present form (page 1, lines 2 and 3 of the appeal statement).

A reading of Claim 1 makes clear that the decisive objection is that based on Article 83 EPC as set out in the decision to refuse the application. Before reaching a decision on the present appeal the Board considered it necessary to reconsider the question of whether the application as initially filed provided sufficient information to a person skilled in the art to enable him to carry out the invention, as required by Article 83.

3. This makes it necessary to consider the functioning of the circuit shown in Figure 5 and the description as filed. The circuit and its mode of operation are covered by the present Claim 1. Pages 12 and 13 of the description explain how the reference voltage is generated in the circuit shown in Figure 5 when the switching signals 0'1, 02 and 03 are applied in three successive stages. The following is a brief description of the three stages designated (a), (b) and (c) using the terminology of the application. (a) Stage 1: The switches I', I", I3, I'3 are closed. The switches I2 and I'2 remain open as long as the switches I'1, I"1, I3, I'3 are closed. Consequently the plates of the capacitor C' are charged to the supply potential V'1; (b) Stage 2: The switches I3 and I'3 remain closed as long as the switches I'1, I"1, I2 and I'2 are open. This causes the capacitor C' to discharge through the transistors T'2 and T'1 until the voltage VD'1-VS between the gate and source of the transistor T'1 is equal to the threshold voltage VT'1' of this transistor and the voltage VD'2-VS between the gate and the source of the transistor T'2 is equal to the threshold voltage VT'2 of this transistor. Consequently the difference in potential VD'1-VD'2 between the gates or drains of the transistors T'1 and T'2 is equal to the difference in threshold voltage VT'1-VT'2 of the two transistors T'1 and T'2. (c) Stage 3: The switches I'1, I"1, I3, I'3 remain open as long as the switches I2, I'2 are closed. The difference in threshold voltage VT'1-VT'2 occurs between the terminals S' and S".

The Board doubts whether at stage (a) the two electrodes of the capacitor (C') are at the same potential. The effect of closing the switches I' and I" is to produce a parallel arrangement between the terminal at potential V'1 and the terminal S, the first branch being composed of the two transistors I'1 and T'2 in the conducting state in series and the second the transistors I"1 and T'1 also in the conducting state. As these transistors have different characteristics and different threshold voltages the current passing through the two branches should also differ. Since, therefore, the branches act as voltage dividers through which different currents flow, the potentials of points D' and D" should be different. The Board considers the essential aspect of the circuit's operation to be the fact that at stage (b) the switches I'1 and I"1 are open while I3 and I'3 are closed. In these circumstances and allowing that the capacitor C' has been charged by the non-zero difference in potential between points D' and D", it could discharge into the series circuit comprising I3, T'2, T'1 and I'3. Since the transistors T'1 and T'2 have different threshold voltages one of the them - namely that having the higher threshold voltage, for example T'1 - would be blocked before the other one. There would then no longer be any continuity in the series circuit, the capacitor C' would no longer discharge and the source/drain voltage and current would disappear. At the end of stage (b), therefore, there would be a difference in potential between the two electrodes of the capacitor C' equal to the higher threshold voltage of the two transistors T'1 and T'2. It is thus impossible for a reference voltage VT'1 and VT'2 to be obtained in this manner at the terminals of the capacitor C'. This conclusion bears out the Examining Division's decision that the circuit concerned did not function as described unless accompanied by other means such as additional capacitances enabling the gate/source voltages of the transistor T'1 and T'2 to be maintained at their threshold level. It would seem important that during stage (a) closure of the switches I1' and I" makes the transistors T'1 and T'2 become conductive with gate/source voltages higher than their threshold value while during stage (b) the opening of switches I'1 and I" leads to a decrease in both the gate/source voltages of the transistors T'1 and T'2 and in their gate/source currents until they are both blocked, two events which are probably not simultaneous. To enable this circuit to function as described the gate/source voltage of the two transistors, which is equal to the threshold voltage, must remain constant long enough for the potential of each electrode of the capacitor C' to become equal to that of the transistor gate to which that electrode is connected via one of the switches I3 or I'3. The invention therefore makes it possible to obtain a reference voltage equal to the difference between the true threshold voltages of the transistors T'1 and T'2. This principle is not to be found in any of the citations contained in the search report.

4. The idea of using the true threshold voltage of a transistor is disclosed in that portion of the description relating to Figure 4 which gives a detailed explanation also applicable to Figure 5. It describes how the switch I1 is only closed for a fairly short periodt and then opens, enabling the threshold voltage T2 to be used after a certain time as a reference voltage. It is also clear that the capacitor (c) is an essential component of this circuit since once the switch I1 has opened it is this capacitor that enables a voltage to exist between the gate or drain (point A) on the one hand and the source (point E) of the transistor T2 on the other. Since at the moment the switch I1 opens this voltage is higher than the threshold voltage of the transistor T2, the capacitor (c) will discharge until its voltage is equal to the threshold voltage of the transistor T1. The difference in potential at the terminals of the capacitor (c) that is available as a reference voltage is thus similarly equal to the threshold voltage of the transistor T2.

5. In the description as filed the aim of the invention is stated on page 2, lines 13 to 21 and its subject-matter defined on page 2, line 22, to page 3, line 5. It is clear from the latter passage that, in the words of Claim 1 as initially filed, the invention relates to "means for producing a difference in the threshold voltages of the transistors T'1 and T'2, which difference constitutes the said reference voltage". (i) This shows that in its original form the application was concerned with the generation of a reference voltage equal to the non-zero difference in the threshold voltages of two transistors specifically chosen for this purpose. Since the applicant used the references T'1 and T'2 in specifying the subject-matter of the application on pages 2 and 3 it is clear that Figure 5 of the application, as the only one showing these references, illustrates the subject-matter.

(ii) In order to explain the functioning of this circuit in which the threshold voltages of two transistors (T'1 and T'2) are generated the applicants begin by describing the mode of generation of a threshold voltage for a single transistor (T1 in Figure 4). As is clear from the description and the explanations given above the capacitor (c) is an essential component of this circuit. Moreover a continued reading of the application reveals the following statement on page 11, lines 33 to 37:

"The operation of the reference voltage generator shown in Figure 5 will be better understood from the following explanation. The generator utilises two threshold voltage measuring circuits of the type previously described when referring to Figures 4a and 4b." The Board of Appeal considers that just as the circuit of the transistor T'1 shown in Figure 5 has an additional capacitor equivalent to the capacitor (c) in Figure 4a, the circuit of the transistor T'2 should have its own capacitor equivalent to (c).

(iii) The Board considers that a person skilled in the art having read the application, and in particular lines 33 to 37 of page 11, would be suprised not to find illustrated in Figure 5 - in addition to the capacitor (C') - capacitors similar to those shown in Figure 4a. He will then find that the circuit shown in Figure 5 cannot function unless he takes account of the statement made on page 11, lines 33 to 37 quoted above which involves the existence of additional capacitors equivalent to the capacitor (c) shown in Figure 4a. (iv) The Board of Appeal is of the opinion that in view of what has been said above the application provides sufficient information to enable a person skilled in the art to form a complete concept of the invention and to carry it out. (v )Since the teaching given in the application is quite unambiguous to a person skilled in the art the fact that the applicants have omitted to refer to the additional capacitors and to show them in Figure 5 should not be interpreted against them.

6. In paragraphs 3 to 5 above the Board has considered whether the application as originally filed complied with the requirements of Article 83 EPC. It has not taken into account the information, arguments and explanations advanced by either the examiner or the applicants. Since as pointed out above the application provides sufficient information to enable the invention to be carried out in practice this question must be answered in the affirmative. The Board does not, therefore, need to consider the question of compliance with Article 123(2) EPC.

7. The additional capacitances involved here are, for example, the parasitic capacitances that occur in an integrated circuit and an inherent feature thereof.

A person skilled in the art is perfectly aware of this fact and also knows that it is possible to ensure a very specific value for these capacitances during circuit manufacture utilising this knowledge when designing them. Further information on this subject will be found on pages 30 and 31 of "Principes et applications des circuits intégrés" (1972) by H. Lilen, to which the appellants refer and in which such capacitances are described as "structural capacitances". This thinking is also reflected in the application when it states on page 9, lines 31 to 33, that "the small-geometry transistor T2, the MOS transistor I1, and the capacitor C may take the form of an integrated circuit on a semi-conductor substrate".

8. These comments should not be allowed to mask the deficiencies in the text of the application which requires close study for a clear understanding. Thus on page 12, lines 20 to 24 it contains the statement that:

During the first stage the plates of the capacitor C' are charged to the source potential V'1. However three lines later (27 to 29) it is stated that During the second stage the capacitor C' discharges via the transistors T'2 and T'1. The first sentence must be interpreted as meaning that the two terminals of the capacitor C' are at the same potential, thus that the capacitor C' is not charged. C' cannot therefore discharge via the transistors T'1 and T'2 as the second sentence states. This textual error in the application explains why the Examining Division concluded that the circuit shown in Figure 5 could not function as described. It is worth pointing out that such additional capacitances exist either between the gate and source of the transistor (see Figure 4a) or between the gate and the substrate, or - since the gate is connected to the drain - between drain and source or drain and substrate. The capacitance between the drain and substrate may in the case of the circuit shown in Figure 5 be replaced by the capacitance between one electrode of the capacitor C' and the substrate without impairing the circuit's operation. This is because the switches I3 and I'3 are closed while the capacitor is discharging to the level of the threshold voltage of the transistor concerned.

9. The Examining Division's decision to refuse the application is based on alleged non-compliance with Article 83 EPC. Since this is sufficient grounds for a refusal the Examining Division had no reason to continue the procedure and consider the other conditions for patentability, namely novelty and inventive step. The fact that the second communication of 12 January 1983 refers to Articles 52 and 57 EPC does not mean that the application was examined in the light of any criteria other than that of Article 83 EPC.

Since, therefore

- the grounds for refusal under Article 83 EPC have been shown to be incorrect,

- the application has not yet been examined in the light of patentability criteria other than that of Article 83,

- the applicants are entitled to demand equitable treatment, the Board of Appeal is referring the application back to the Examining Division under Article 111(1) EPC.