where y = mT/2; ~ is the quantity depending on the control-signal constant component which is present at the adder input whether the input signal is there or not. This quantity, as well as other instrument noises, limit its maximum sensitivity. Henceforth this quantity will not be taken into account. By adding the output voltages of both channels and extracting the square root from the sum, we finally obtain Uout = ~ [~n K sin q~sin v]2 + [UinK cos q~lz .

(8)

which is reduced to (i) for T = T/2 and y = ~/2. Let us determine the relative output-voltage from ~/2 by the small quantity Ay: 2

8U~

changes due to the departure of angle y

2

AUout _ 2UinK sin~q~sin?c~165 Uout 2~/[Uinf(sin~psin?l~+l~nKeos~l z

A___L 1 sin2q~sin2y'AV 9 Uout -- 2

(9)

Analysis of the expression (9) shows that the differential corresponding to the first (linear) term of the Taylor series obtained by expanding the function (8) is equal to zero at the point y = n/2. Therefore, the error is determined by the second (quadratic) Taylorseries term

6Uout=Isin~cpeos2y.(Ay)2 The maximum error is obtained for ~ =

= T1

. , , qD.( A y p . sin-

(io)

~/2 1

6Uout = -~- (AYY-.

(11)

For a half-period asymmetry up to i0 ~ i.e., for Ay = 10/57, the maximum measurement error does not exceed 1.6% and it can be neglected as compared with the error due to discrepancies between the channel amplification factors and the inaccuracy of the Uct I displacement by ~/2 between channels. LITERATURE CITED 1~

2.

S~LL

A. A. Kharkevich, Spectra and Analysis [in Russian], GIFML (1962). I. V. Latenko et al., Vestn. Kiev Politekh. Inst., Ser. Radio@lektron.,

No. 8 (1971).

STABILIZED CURRENT SOURCE I. I. Lovchev, A. G. Fedotenkov, and M. A. Shevel'kov

UDC 621.316.722.1

Existing current-stabilizing circuits [i] incorporate resistors in series with their supply source, which are many times larger than the load. Such circuits provide stabilized small currents, but they require a high supply voltage (hundreds and thousands of volts). Current stabilizers with nonlinear elements, such as radio tubes or semiconductor devices [2] are economical, but present practical difficulties in stabilizing currents below 10 -7 A, because of the considerable unstable currents in the tube grids and the transistors inverse currents. Current stabilizers in the range of 10-s-10 -12 A are required for measuring the electrophysical parameters of semiconductor materials. The authors of this article have developed an instrument (see Fig. i) which utilizes a vacuum photoelement PE type F1 with a dark current Translated from Izmeritel'naya Tekhnika, No. 12, p. 58, December,

1708

0543-1972/78/2112-1708507.50

1978.

9 1979 Plenum Publishing Corporation

2g V

S2g

&

,_It // r ~ ~A

~a ~ , ~

r

I

{

S3

{

"i'l EM

Fig. 1 not exceeding i0 -z~ A and a volt--ampere characteristic horizontal (working) section slope not exceeding 3%. The stabilized luminous flux produced by the lamp L1 (SM28V-10 W) is transmitted through prisms onto the PE cathode, thus producing in the PE circuit and, therefore, in the entire instrument the stabilized current I s . For the purpose of stabilizing the luminous flux, the lamp L1 is fed from an external stabilized 28 V dc source; the resistors R1 (22 k~) and R4-R6 (of 5.1, 47, and 68 ~ respectively) a~e selected with a margin of power in order to prevent overheating; and the current-carrying conductors are soldered directly to the lamp L1 leadouts, thus eliminating the variable resistance of its contacts. Owing to the application of two prisms for transmitting the luminous flux, the infrared radiations do not heat up the photoelectric element. Moreover, the PE is provided with two screens (steel and ebonite), thus screening it electromagnetically, protecting it from external illumination, and establishing passive thermostatic control. The supply voltage E s equal to i00, 200, and 300 V is fed to the PE from three smallsize batteries type 100PMTsG-U-50ch. This voltage can be tested between the terminals BA and TI. The switches SI-$4, the terminals TI-T3, the plugs PI-P5, and the batteries BI-B3 are insulated from the casing with KeI-F. The maximum load R l is equal to

RZ

=

F-.f - - E r l s Is

'

where Ens is the size of the PE volt--ampere characteristic nonlinear section 60 V).

(not exceeding

The output current is first set with a shorted load. The stabilized current is regulated by adjusting the lamp LI supply voltage by means of resistor RI. The current Is is measured in the range of I-i0 ~A by means of the incorporated class 0.5 instrument MI type MI36/A. Currents below i0 -? A are evaluated from the voltage drop across one of the working standards R7-RI2 (i00, 300 k~, i, 3, i0, 100 M~, and i G~) which is measured with the external instrument EM. The above stabilized current source has the following characteristics: stabilization range of i0-5-I0 -z2 A, output voltage of 0-250 V, and an output-current instability not exceeding 0.3% for load variations of • This instrument can be used for calibrating and testing simultaneously microammeters.

tens of piconano-

1709

LITERATURE CITED i. 2. 3.

T . B . Rozhdestvenskaya et al., Equipment for Precise Measurement of Large Resistances, Small dc Currents, and Testing Methods [in Russian], Izd. Standartov, Moscow (1973). S . D . Dodik (editor), Semiconductor Supply Sources. Designing and Calculations [in Russian], Sovetskoe Radio, Moscow (1969). I . P . Stepanenko, Foundations of the Transistor and Transistor-circuit Theory [in Russian], State Power Engineering Press (GEI), Moscow (1967).

PULSE MEASUREMENTS OF THE emf OF HIGH-RESISTANCE SOURCES V. E. Korepanov, M. A. Rakov, and I. A. Yatsun

UDC 621.317.321.083.72

Electrometric amplifiers are widely used in various industries, as well as in a number of physical experiments. In order to obtain trustworthy measurement results the amplifier input resistance should be at least one order higher than the signal-source internal resistance. This leads to the necessity of designing measuring amplifiers with an input resistance of 10~S-lO ~ ~ or higher. The production and utilization of such amplifiers entails considerable difficulties [i, 2]. Below we describe a method for measuring dc and slowly-changing emf of sources with high internal resistances by means of a measuring amplifier whose input resistance can be made considerably higher than that of the source. The method consists of accumulating the charge on a certain source-equivalent capacitance during a given time interval and then discharging it into the amplifier input resistance. Owing to their large time constant, the high-resistance emf sources can be short circuited for tenths or even units of milliseconds. It is easy to calculate the time for which such an emf source can be closed without changing its voltage by more than the given measurement error. The essence of measurements consists of connecting to the high-resistance emf source with a given capacitance by means of a switch with periodic action and a large duty ratio the measuring amplifier for the time tm determined by the speed of its response. Moreover, the amplifier output-voltage instantaneous value is recorded in the storage device retained there until the next measuring cycle. The spacing t s between measuring cycles is selected to be sufficient for the reestablishment of the initial signal-source voltage level, thus serving to repeat cycles at the intervals T = tm + ts. Let us examine the simplified equivalent circuit of the amplifier input (Fig. i) comprising the signal-source emf E; the equivalent capacitance and output resistance C i and Ri; the switch S; the amplifier input capacitance and resistance C and R; and the voltages UI and U2 at the inputs of the switch and amplifier respectively. It is assumed that the switch resistances in an open and closed state Rre v and Rst meet the requirement Rst <> Ri. By deriving equations for this circuit in accordance with Kirchhoff laws and solving them, we obtain the following expressions for the voltages U: and U2 under closed (V~ and U~) and open (U'~ and U~) conditions of the switch as functions of the time arguments t' and t" :

#

R

~ - -

(i)

9

,

wher~r'=~!=T 2=(c~+C)

n~__~

R~+R

;

Translated from Izmeritel'naya Tekhnika, No. 12, pp. 59-60, December, 1978.

1710

0543-1972/78/2112-1710507.50

9 1979 Plenum Publishing Corporation