DIRECT-COUPLED PRE-AMPLIFIERS 



main amplifier which possesses a differential input. For those which are not, 

 i.e. most commercial oscilloscopes, a single-sided output can be taken from 

 either of them. There will be some loss in rejection ratio, and the gain will 

 of course be halved. 



Although this particular pre-amplifier design employs obsolescent valve 

 types, the setting-up procedure will be described because it is quite typical 

 and serves to indicate the line of approach for differential direct-coupled 

 amplifiers in general. The procedure is as follows: 



(1) Earth both cathode follower inputs and turn the d.c. gain control to 

 maximum. Check on the output meter that the amplifier can be balanced 

 by manipulation of the coarse and fine balance controls. Keep it balanced 

 in this manner during operations 2-5. 



(2) Disconnect one cathode follower input from earth and connect it 

 instead to the calibrator output terminal. Connect a measuring oscilloscope 

 to the amplifier output. Inject a 1 mV pulse and measure the d.c. gain. 

 Open S4, set 'a.c. gain' to maximum, replace the calibrator by a sine wave 

 oscillator set to 1,000 c/s, 1 mV R.M.S., and from the oscilloscope check 

 the a.c. gain. 



(3) Switch the oscillator output to square waves of about 1 mV amplitude, 

 200 c/s, close S4 and adjust the 100 k pre-set potentiometer until the repro- 

 duced waveform is as near as possible square. Open S4 again. 



(4) Connect both cathode follower inputs together and inject about 100 

 mV R.M.S. at 50 c/s between them and earth. Turn the gain balance control 

 until the minimum output appears on the oscilloscope. This gets the 

 rejection ratio as high as possible. 



(5) Increase the frequency to 5,000 c/s and turn the trimming capacitor 

 until the minimum output appears on the oscilloscope. This compensates 

 for the capacitance to earth of the static balancing circuit. 



(6) Short the inputs to earth again and carefully balance the amplifier. 

 Turn the d.c. gain control to minimum and re-balance using the 'low gain 

 balance'. Return the d.c. gain to maximum and re-balance using the fine 

 and coarse balance controls. The ampHfier should now remain in balance 

 irrespective of the d.c. gain control setting. 



(7) With the aid of a high resistance voltmeter adjust the output potential 

 controls so that both outputs are at the same potential as earth when the 

 amplifier is balanced; i.e. make sure that the outputs are not superposed 

 upon a steady bias. 



(8) Grid current adjustment. Unshort the input to the cathode follower 

 to be set up and connect it to the calibrator output. Switch calibrator to 

 'direct' and balance the amplifier. Now switch calibrator to 'kMQ'. The 

 grid current will flow along the kMQ resistor, developing across it a poten- 

 tial difference which will unbalance the amplifier. Return the calibrator 

 switch to 'direct' and alter the appropriate cathode follower HT supply in 

 such a direction as to off-balance the amplifier in the opposite sense to that 

 in which it was moved by the grid current. Re-balance using the fine and 

 coarse balance controls. Repeat the procedure until inserting the kMO into 

 the cathode follower grid circuit produces little or no unbalance. The grid 

 current is given by the voltage at the input corresponding to the unbalance 

 produced, divided by 10^ Q, and should be less than 10"^^ amps. 



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