Wien-bridge oscillator with low harmonic distortion - 2

52 WIRELESS WORLD MAY 1981 and R, implies that if, instead of the net­ be significantly lower, because of the very operating frequency of the oscillator. Fast work of Fig. 3(a) being connected between small input-signal amplitude and the ab­ response-speed, high h.f. gain op.-amps. a signal source Ein and the OV line, it was sence of any internal transfer errors be­ can therefore be used without problems. connected between two signal sources + E x tween the inverting and non-inverting in­ and -Ey, where these are sinusoidal and puts, than is the case for an identical For these reasons, it can be expected identical in frequency and the negative amplifying element in a series-feedback that the residual harmonic distortion of sign implies phase opposition, as shown in configuration. 3,4 this oscillator design will be exceedingly Fig. 3(b), then a small, in-phase signal • The time-delay errors in the second am­ small, and measurements on two proto­ would exist at the point 'X', at the fre­ plifying stage (A2) no longer contribute to types have indeed shown this to be the quency of maximum transmission, (fo), if loss of stablility in the system, but only to a case. So far as can be determined, the +Ex was slightly greater than -2Ey• very small compensatory shift in the residual distortion - almost exclusively This could then be used as a positive­ feedback signal in a circuit such as that shown in Fig. 4, to sustain oscillation at R the frequency fo. Indeed, such a circuit will work quite well, and will sustain a Thermistor Rs constant output magnitude of oscillation if a thermistor is employed, as shown, to make the gain of the second, inverting, Output amplifier stage dependent on the ampli­ tude of the input signal. However, there is, in practice, a small snag with such an R arrangement, and that is that the inverted negative-feedback signal applied to the in­ put of Al will suffer an additional phase error due to the internal time lag within Figures in brackets Rp. Rs and Rin chosen to A2, and this will cause unwanted h.f. insta­ refer to diagnom 3b SUIt individual thermistor employed bility if '3rd generation' high speed op.­ amps. such as the CA 3140, or the 174 1 S, Fig. 5. Final form of new configuration in low-distortion oscillator are used in the realisation of this circuit. It is, fortunately, an easy matter to re­ solve this difficulty if the circuit is recast in 0·01 the form shown in Fig. 5, in which the ·009 , (Includes negative-feedback signal, equivalent to ,50 Hz "hum" I -Ey ·005 in Fig. 3(b), is derived from the am­ plifier AI> and the positive-feedback signal "- is obtained from the output of the second � , "- inverting amplifier A2• :z ... o This configuration offers several signifi­ � 0001 cant advantages. o .. l­ V) • The input signal to Al is extremely a .0005 small, since it is only required to be EOUl/2M, where M is the open-loop gain of A I - typically 100dB for a good modern op.amp. i.c. - and, as pointed out by the author in an earlier article2, with semi­ 0 0001 conductor amplifiers the non-linearity of 1 such devices is essentially an input charac­ FREQUENCY (Hz) teristic, dependent on the magnitude of Fig. 6. Measured total harmonic distortion of improved oscillator of Fig. 5 the input signal. • The second-stage amplifier is operated as a shunt-feedback element, and the non­ Fig. 7. New oscillator with external optoelectronic amplitude-control circuit. Silonex linearities of such a stage can be shown to (formerly National Semiconductors) cell, Type NSL395, is obtainable from Cheston Electronics Ltd., Vanguard House, 56 Oughton Street, Ormskirk, Lancs. Tel: 069572456 1/ Photoconductive cell 10k NSL395 10}! 47k Si 3k3 56k Si Si I1 3k3 3k3 3k3 \�------------------�vr------------------�/ Oscillator Full wave rectifier Feedback control circuit Output Note: PC1 and LED1 are in optical contact Document Outline WW 1981 May p51 WW 1981 May p52 WW 1981 May p53