Permissible Input Research Articles

Amplifiers for precise oscillographic measurements

If the amplifier-oscillograph is to attain the fidelity of frequency reproduction of the unaided oscillograph, the amplifier circuit must be designed, constructed, and used with discretion. In no portion of the circuits, in neither the amplifier itself nor the connected measuring circuits, is it permissible for the associated resistances, capacitances, and inductances to produce time constants so large as to suppress any of the frequencies which the oscillograph vibrator would reproduce normally. Steadiness of the amplifier is a prime consideration, usually attainable with moderately high gain units by having sure electrical contacts and the component parts operated conservatively within their respective ratings. Interference is frequently a source of considerable difficulty and great care must be exercised not to permit it to cause erroneous measurements. A method of compensating for the steady component of the amplifier output current has been investigated and when properly used, has been found neither to impair the sensitivity of the instrument appreciably nor to require critical adjustment. Consideration also has been given to the photographic technique involved in oscillography in order that its full possibilities might be realized. An instrument has been described and discussed which had a sensitivity of about 1.7 millivolts per mm. deflection. When the maximum permissible input resistance of 0.1 megohm was used, a current sensitivity of about 1.7 × 10−8 amperes per mm. deflection was attained. A greater value of input resistance could not be used without the possibility of introducing serious distortion.