C O M P E N S A T I O N OF T H E T H R E S H O L D - V O L T A G E

T E M P E R A T U R E D R I F T IN A D I O D E L I M I T E R

(UDC 621.382.2.088.6)

L. D. K a u r o v

Translated from Izmer i t e l ' naya Tekhnika, No. 9,

pp. 90-91, September, 1966

t ~ D2

Dv ~, 0'in e ~ try

type

DI02A

D 18

Fig. 1

Ds

~;I

~ g l U~

d

Fig. 2

In designing circuits with semiconductors i t is often necessary to use diode l imiters intended for working with signals whose ampl i tude is of the order of units and tenths of a volt . Under such conditions the threshold vol tage temperature drift contributes an appreciable error to the operation of the circuit .

Changes in the threshold vol tage Uth produced by changes in the vo l t - ampe re character is t ic of the l imit ing diode D z and due to ambient temperature variations are compensated in the circuit shown in Fig. 1 by

means of diodes D I and D 3. If diodes D 2 and D 3 are ident ical , the dis- p lacement in the forward branches of their vo l t - ampe re characterist ics compensate each other. Variations in the reverse current Irv(t ~ of diode D z leads to changes in the vol tage drop across divider resistance Rz. The displacement of the forward branch of the vo l t - ampe re character is t ic of diode D 1 reduces these changes. It is impossible to e l imina te comple te - ly the l imit ing threshold drift owing to the fact that variations in the re-

verse diode current have a pronounced nonlinear characterist ic , whereas those of the forward branch of the vo l t - ampe re characteris t ic are l inear.

By using a p iecewise-broken approximation of the diode vol t - ampere character is t ic (Fig. 2) i t is possible to write

U B = E - - U 0 (1)

U . = K E + U o , (2)

where U 0 is the abscissa of the inf lect ion point in the straight branch of the vo l t - ampe re characteristics; K = Rz/(R 1 + R 2) is the transfer factor of

the vol tage divider R I + ~ .

t ~ c

Without compensa- tion 17r 40 . 54 62 76 36

With com- 46 48 I 50 53 56 I0 pensation ]

Without 42 53 I 66 84 106 64 compensa- I t_~n

With corn- 33 [ 33 34 35 36 3

I

pensation I

From (1) and (2) the l imit ing threshold is

Uth = E ( l - - K ) - - U0

on t h e other hand,

(3)

Oth Uin--U~176 (4)

where Uin and Uout are, respect ively, the ampli tudes of the input and the l imi ted signal (the transfer factor of the l imi ter is taken as 1).

From (3) and (4) we obtain E(1 - K) = Uin -- Uou t.

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It is possible to de te rmine from the above expression for a given E the va lue of the transfer factor K:

Uin - - U out K = I

E (5)

From the va lue of the transfer factor K thus obtained, we find the va lue of Rz:

AUfw(t~ (t') R2, (6)

where AUfw(t ~ is the d isp lacement of the forward branch of the diode D 1 v o l t - a m p e r e character is t ic ; Alrv(t ~ is the

var ia t ion in the reverse current of diode Dz; AUfw(t~ =jAt~ AI (t*) = I tz -- I tl ; j is the forwaxd-voltage t empera - rV r v r v

ture coeff ic ient which for germanium diodes is equal to 2.2-2.5 m V / d e g , and for si l icon diodes to 1.85-1.95 m V / d e g .

At ~ = t 2 - - t l .

The values of I ~ and Irtl v axe taken from reference data or ca lcula ted .

From (6) we obtain

Rz -- A Ufw (t ~ K , R1 __ R~ - - K____~R~ A i r v (t ~ g

The tab le shows the measured pulse ampli tudes at the output of a compensated and a noncompensated l imi ter . In assembling the c i rcui t the diodes are not selected.

1231