M A G N E T I C T R A N S I S T O R I Z E D DC A M P L I F I E R S

R. Y a . B e r k m a n , B. L. B o n d a r u k , a n d L. I . R a k h l i n

UDC 621.317.791

The app l i ca t ionof magne t ic modulators (MM) in modern equipment does not as ye t correspond to their poten- t ia l i t ies . The high met ro logica l parameters obtained in laboratories as the result of precision adjustments are often lacking under conditions of mass production, and the devices thus produced are very compl ica ted in adjustment and unrel iable in operation.

This circumstance is probably due to the insufficient development of that part of the general theory which deals with the errors of MMs and measuring circuits as a whole. As a result of this the MM operating conditions and parameters recommended in l i terature are far from always opt imal for a t ta ining the required metro logica l param-

eters. For example , in an MM operation close to the given exci ta t ion current, the ~rrors due to residual zero drift in overloading are v i r tua l ly unavoidable. Moreover, the exci ta t ion frequency is then one order tower than the one

tolerated for the given thickness of mater ia ls under other operating conditions.

Considerable diff icult ies arise in implement ing in devices based on MMs a given set of requirements. Thus, the error due to the residual zero drift in overloading can be reduced by increasing the exci ta t ion field, but the

errors due to the external magnet iz ing field will then increase as wetl as the content of even harmonics in the ex- c i ta t ion circuit .

The PhysicomechanicaI Institute of the UkrSSR Academy of Sciences has carried out a number of invest iga- tions for determining the MM parameters and operating conditions which would ensure the highest possible me t ro - logical characteris t ics for the equipment based on these devices.

As a result of this the basic sources of errors in magne t ic modulators and measuring circuits were obtained and methods for their reduction were evolved. Sui table relationships between the MM and measuring circuit parameters were found for ensuring stable conversion characteris t ics of modulators [1] with a tolerated technological dispersion of their core mate r i a l ' s parameters. The required MM-exci ta t ion condition was found for raising considerably the exc i ta t ion frequency of technological ly s imple cores made of a re la t ive ly thick rolled iron [2], and vi r tua l ly for e l imina t ing at the same t ime the residual zero drift in overloading.

An MM designwas developed in which the effect of the control windings capac i tance was e l imina ted [3]. This made it possible to produce the control winding with vi r tual ly any number of turns and, therefore, to raise sharply the MM current sensitivity. The par t icular feature of this design consists of the fact that it provides a high input resistance with a considerably higher exc i ta t ion frequency, i .e , , it ensures at the same t ime a high operat ing speed for the device .

The units and e lements of the c i rcui t were selected, with the specific requirements for measuring circuits taken into consideration, in order to provide the highest possible met ro log ica l parameters for the device as a whole.

On the basis of the investigations carried out at the Institute, with the par t ic ipat ion of members of the Elec-

tronic Measuring Instrument-Making Depar tment of the Livov Polytechn~cal Institute, a series of high-speed dc am- plifiers with a high input resistance was developed and manufactured.

Below we provide a brief descript ion of the c i rcui t and technica l characteris t ics of the basic instrument a m - pl i f ier with a high operat ing speed and a large input resistance (dc microvolmanoammeter ) . The instrument whose block schemat ic is shown in Fig. 1 consists of a converting dc ampl i f ie r with a large negat ive feedback. The dc

Translated from I zmer i t e l ' n aya Tekhnika, No. 4, pp. 56-57, April, 1972. Original ar t ic le submitted June 17, 1969.

I @1972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West i7th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever

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Input _ _ ~ i Outp nt

Fig. I

vol tage fed to the instrument (or the voltage drop across the shunt

in measuring current) is converted into an ac voltage, amplif ied, rect i f ied with a synchronous detector , and transmitted through the output dc amplif ier to the feedback circuit . The measurement ranges are switched by changing the dc feedback factor by means of the divider 1 in the ampli f ier overal l feedback circuit , or by switching shunts in measuring current. The total ampl i f ica t ion

factor of the system (with feedback) can then be changed from 3160 to 10.

The correct se lect ion of various circuit units and e lements determines to a considerable extent the ach ieve- ment of high met ro log ica l parameters for MM instruments. For instance, in order to ensure high zero stabil i ty, it is necessary to min imize the ampl i tude of even harmonics (mainly the second harmonic) in the MM exci ta t ion vol t - age. I n this connection, and also from the point of view of power relationships, the modulator 4 is exci ted by the rectangular waveform vol tage generator 8 through the in te rmedia te even harmonics LC fi l ter 3. Moreover, the stray even harmonics which are produced by the fi l ter e lements owing to their smal l nonlineari ty are made negl igibly small . The 20-kHz vol tage of the driving generator is fed simultaneously to the phase-sensi t ive detector 9 and the frequency divider 6. The J n-kHz vol tage drives the output stage, which feeds the magnet ic modulator 4 through the filter.

The measuring channel should meet the requirements of high select ivi ty , a small slope of the phase-frequency

character is t ic , and a wide dynamic range. These requirements are met by providing the measuring circui t with the input divider 2, the magne t ic modulator 4, the input LC fil ter 5, the ac ampl i f ier 7, the phase-sensi t ive detector 9, and the dc ampl i f ie r 10. The ac vol tage amplif ier 7 consists of four ampl i fying stages and a phase-regulator for setting the in i t ia l phase-shif t of the ampli f ied signal with respect to the phase-sensi t ive detector ' s reference voltage. In order to improve the dynamic characterist ics of the circuit , the phase-sensi t ive detector 's output vol tage is l i m - ited. In overloading, the negat ive vol tage is l imi ted with a diode, and the positive vol tage by the saturation of the dc output ampl i f ier ' s input transistor.

Measures have been adopted in the output ampl i f ier for raising zero stabi l i ty. The biasing of the first two stages' transistors is mutual ly compensated and a large negat ive vol tage feedback is provided for the entire amp l i -

fier.

Below we list the technica l characterist ics of the dc microvol tnanoammeter .

It has a vol tage range of 5 tN-1V; a current range of 0.5 hA-100 gA; an instrument ampl i f ier ' s basic error

of 0.5%; an entire instrument 's basic error (determined by the indicat ing instrument 's precision class) of 1.5%; a set peak noise level of ~ 0.3 pV in the band of 0.05-1 Hz; an rms error of 0.1 pV; an input resistance of 1.5 Mr2 for the ranges of 100 pV-3 mV and of 500 Mt; /V for the remaining ranges; a nominal vol tage drop of a mV across the in- strument 's terminals for measuring current on the scales; an operating speed of 5 msec (the set t l ing t ime of the output vol tage for a sudden change of the input signal); for measuring dc voltages on a background of ac voltages, the instantaneous v a h e of the input var iable should not exceed :L 3.6 mV in the range of 100 pV-3 mV, and it should

not exceed 120% of the difference between the upper and lower scale values on the remaining ranges; the instrument is provided with an addit ional 40-dB suppression filter of the industrial frequency of 50 Hz; the instrument can with-

stand a short overloading of up to 100 V on al l ranges without losing its precision class; i t has a zero drift of 1-2 yV during 24 h of normal operat ing conditions after half an hour's heat ing up; i topera tes normally in ambient t empera- tures of +5 to +45~ it operates from 220 V, 50 Hz mains. Mains vol tage variations of - 1 5 to + 10% are tolerated. The instrument has a power consumption not exceeding 10 VA, overal l dimensions of 360 x 300 • 120 mm and a

mass of 9.5 kg.

The results thus obtained show that with an appropriate select ion of parameters and operating conditions it is possible to use magnet ic modulators with a second-harmonic output for producing highly re l iab le high-speed dc vol tage amplif iers which can be successfully used in high-precis ion laboratory and industrial equipment.

1.

L I T E R A T U R E C I T E D

R. Ya. Berkman and B. L. Bondaruk, "Stabi l i ty and sensit ivity of second harmonic magnet ic modulators in re la t ion to the operating conditions of their exci ta t ion and measurement circuits ," in: Select ion and Trans- mission of Information [in Russian], Naukova Dumka, Kiev (1967).

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2. R. Ya. Berkman and B. L. Bondaruk, "Possibility of raising considerably the excitation frequency of magnetic modulators," in: Selection and Transmission of Information [in Russian], Naukova Dumka, Kiev (1968).

3. R. Ya. Berkman and B, L. Bondaruk, Author's Certificate No. 230895, Byull. Izobr., No. 35, (1968).

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