AAA/ 



10,000 V 



0-10 V 



Fig. 7- Series resistance constant current source. 



/?a 



IF ^AS60db 



fiA. 



Pig. 8. Simplified diagram of operational 

 amplifier. 



voltage transferred through resistor Rj_ will be 

 determined by this bias current. The effect is 

 such that if the oscillator output level increases 

 the rectified level of the automatic gain control 

 increases, thereby driving more bias current 

 through the control diode and lowering its dynamic 

 impedance. Less positive feedback voltage is 

 applied to the front end of the oscillator 

 restoring the oscillator output level to very 

 nearly its original value. A thermistor- 

 resistor network is used to compensate for the 

 temperature shifts in the control and rectifier 

 diodes. The oscillator maintains the required 

 amplitude stability of less than ^0.1$ over an 

 ambient range of 50 F . 



° — wv 



100 K 



I V 



-o 



t t 



I00K 



2000-n- 



500 -n. 



Eos 

 0-8 V 



Eoa 

 3-11 V 



Fig. 9- Actual constant current amplifier. 



from zero to 2,000 ohms requires a series of resis- 

 tance, B. s? of 2 megohms or greater. Under these 

 conditions the oscillator output would have to be 

 at least 10,000 volts rms which is not practical. 

 By utilizing a high gain amplifier in the opera- 

 tional configuration, the needed current accuracy 

 can be attained and the oscillator output level 

 of 1 volt rms is multiplied by 10 which is needed 

 to drive the staff. 



A simplified diagram of the operational ampli- 

 fier is shown in Fig. 8. If the raw gain, A 

 is at least 80 db and the closed loop gain is 

 20 db or less, there will be a net negative feed- 

 back of 60 db or more. Under these conditions as 

 R3 is varied over the 2,000 ohm range the output 



voltage of the amplifier, E 



changes from 



2 volts to 10 volts as the current, I„, is held 

 constant. This follows from I-j_ being equal to 

 Ig within better than 0.1$ as a result of the 

 60 db of negative feedback around the amplifier. 

 As Ro is varied downward Ip tends to increase 

 causing a slight decrease in amplifier sensing 

 current, Io . The amplifier output voltage, E Qa , 

 is in turn lowered and Lp_ very closely restored 

 to its initial value. It follows that the vol- 

 tage, E os , developed across Ro is proportional to 

 the value of Ro since Ip is maintained constant 

 by the action of the operational amplifier. Again 

 it may be important to state that the preceding 

 is true only if E is constant to within 0.1$. 

 Fig. 9 shows the scheme used in the actual circuit 

 to arrive at a practical value of amplifier input 

 impedance, the principle of operation being the 

 same as for the simplified case in Fig. 8. 



Constant Current Amplifier 



This amplifier is used to drive a constant 

 current through the staff independent of resis- 

 tance. The excitation level of 10 volts rms 

 results in a good signal to noise ratio. In 

 Fig. 7 is shown a circuit configuration in which 

 a resistance is interposed between the oscillator 

 and the staff to approximate a constant current 

 source. Full scale voltage of 10 volts rms will 

 generate a current of 5 milliamps through the 

 2,000 ohm staff. Holding 5 milliamps constant 

 to within i0.1$ as the staff resistance changes 



SIGNAL CONDITIONING 



AC-DC Converter 



The AC-DC converter consists of a high gain 

 amplifier again connected in the operational con- 

 figuration but this time including silicon 

 switching diodes in the feedback path. This tech- 

 nique develops a proportional DC output voltage 

 with relation to the rms value of the input vol- 

 tage. With a pure sinusoid the average value is 

 equal to 0.1t5 of the rms value. This clearly 



97 



