RC OSCILLATORS 



C's at all times. R^ and Cg form a low-pass filter to smooth the control bias. 

 The system works well and the output waveform is quite pure provided the 

 level of oscillation is held down to a volt or two. The level is controlled by 

 the setting of the potentiometer. 



HT + 



-\N\f 



Figure 14.32 



Figure 14.32 



Lamp method — A very much simpler scheme makes use of the temperature 

 coefficient of resistance of metals. The method is equally applicable to 

 phase shift and Wien bridge oscillators. If the cathode resistance of one of the 

 valves is composed of one or more metal filament electric bulbs {Figure 14.33), 

 then, as the valve is in class A, when the amplitude of the input signal increases 

 the average cathode current is unaffected but the R.M.S. cathode current 

 rises. Hence the temperature of the bulb filaments also rise, and so does their 

 resistance. This increases the negative feedback applied to the valve and 

 reduces the loop gain, counteracting the original increase in input. 



For the scheme to be effective the bulb filament temperature must be a 

 fast-changing function of the R.M.S. current, which implies that the rated 

 bulb filament current must not be too greatly in excess of the average valve 

 anode current. The latter is typically 5 mA. It is possible to get rear-fight 

 bulbs for bicycles which consume only 40 mA ; these are very suitable. 



Thermistor method — A popular modern method of stabilizing the level of 

 oscillation is to use a thermistor. It consists of a small piece (about the size of 

 a pinhead) of a material having a very large negative temperature coefficient 

 of resistance. Their symbol is Figure 14.34 and they may be used in various 

 ways. In Figure 14.35, C is large enough to have neghgible reactance even at 

 the lowest frequency of osciUation. The feedback is positively to the first 

 valve grid, in the normal manner for Wien bridge oscillators, and also 

 negatively to the first valve cathode via the thermistor. When the amplitude 

 of the oscillations rises, the thermistor heats up and its resistance falls, 

 increasing the negative feedback. 



A very neat circuit (Figure 14.36) was shown to the author by K. E. 

 Machin. A long-tailed pair is used, so that the feedback to the frequency 

 determining network is negative from the left-hand anode and positive from 

 the right-hand. The thermistor is across the output and heats up if the latter 



223 



