The invention applies to internal combustion engines, and, in particular, to fuel metering apparatuses having an oxygen measuring sensor in the exhaust flow of the internal combustion engine. 
In such apparatuses, the fuel metering is effected in accordance with the particular concentration of oxygen in the exhaust in such a manner that, to the greatest possible extent, a stoichiometric ratio of air and fuel (in internal combustion engines with externally-supplied ignition) is available in the combustion chambers. 
The control circuits having these oxygen sensors must now contain an integrator which enables the generation, from the signals of the oxygen sensor which are generated as pulses, of a variable direct voltage affecting fuel metering.  The output signal of an oxygen sensor in the exhaust is pulse-like because the composition of the exhaust gas varies in accordance with the extent to which the various output valves are open at a particular time.  Furthermore, an integrator in the control circuit has the advantage that longer-lasting deviations lead all the more to a strong counter-control process, and thus the desired mixture composition can be attained all the more rapidly. 
In a known apparatus for detoxifying the exhaust gas of internal combustion engines German Offenlegungsschrift (laid-open application) No.  2 251 167, the integrator is embodied as an operational amplifier which is counter-coupled capacitively.  It has now been determined that the output signal of this integrator does not possess the necessary and desired constancy over a relatively long period of time such as is particularly desirable when, during certain operational states, such as full load, overrunning or acceleration, the mixture control intended for producing cleaner exhaust gas is suspended, more or less briefly, for the sake of achieving the desired driving behavior. 
Finally, in the known apparatus, a systematic charging and discharging of the integrator presents problems; that is, it is attainable only at very great expense in terms of circuit technology.