Derivative Controlled Magnetic Amplifiers A. D . S G H N I T Z L E R

A S S O C I A T E M E M B E R

T ^ H E C I R C U I T s h o w n in F ig . 1 is of a low-level fast-A response m a g n e t i c ampl i f ie r , as d e v e l o p e d for such

low-level s ignals as a r e p r o v i d e d b y t h e r m o c o u p l e s , s t r a in gauges , e tc . Rect i f ie rs a r e n o t i n c l u d e d in t h e i n p u t c i rcui t , t hus e l i m i n a t i n g t h e res t r i c t ion w h i c h t h e y i m p o s e on t h e m i n i m u m signal vo l t age . L o w drift , fast r e sponse , a n d h i g h ga in a r e o b t a i n e d b y t h e use of a c o m b i n a t i o n of d e l a y e d posi t ive v o l t a g e - c u r r e n t a n d n e g a t i v e c u r r e n t -vo l tage feedback . T h e d e l a y e d n e g a t i v e f eedback c u r r e n t -vol tage f rom a resistive l oad is a d d e d t o t h e s ignal vo l t age in t h e con t ro l c i rcu i t of t h e first s t age so t h a t it ampl i f ies t h e a v e r a g e i n c r e m e n t s of s ignal vo l t age p e r ha l f cycle of p o w e r s u p p l y vo l t age . T h e posi t ive v o l t a g e - c u r r e n t feed­back is ad jus ted to j u s t c o m p e n s a t e for rectif ier l e a k a g e a n d e d d y c u r r e n t losses in t h e second s tage . T h e funct ion of t h e second s tage is to ampl i fy a n d s u m t h e o u t p u t vo l t age of t h e first s tage .

I n a 2-stage ampl i f ier , drift is a l m o s t en t i r e ly d u e t o var ia t ions in p a r a m e t e r s of t h e first s tage . T h e first s t age funct ions as a de r iva t ive ampl i f ie r a n d t h e second s t age as a n in t eg ra to r . S ince t h e i n p u t s tage o p e r a t e s u p o n t h e a v e r a g e i n c r e m e n t of s ignal vo l t age p e r ha l f cycle r a t h e r t h a n t h e a v e r a g e s ignal vo l t age p e r ha l f cycle , t h e effect of va r i a t ions in t h e b a c k res i s tance of t h e rectifiers a n d of core m a g n e t i z i n g c u r r e n t u p o n t h e o u t p u t is d i m i n i s h e d b y t h e r a t i o of t h e i n c r e m e n t t o t h e e n t i r e s ignal vo l t age .

Re fe r r ing to F ig . 1, cores I a n d I I a r e p a r t s of ha l f -wave

POSITIVE DERIVATIVE AMPLIFIER GIO

CORE I Λ LOW PASS FILTER

13ion X 4

NEGATIVE DERIVATIVE AMPLIFIER

Fig. 1. Complete amplif ier incorporat ing ha l f -wave der ivat ive amplifier

amplif iers c o n n e c t e d in p u s h - p u l l . Ii a n d 12 a r e c u r r e n t sources w h i c h b ias t h e 2 ampl i f iers t o z e r o o u t p u t w h e n t h e i n p u t is ze ro . Ef is t h e s ignal vo l t age a n d Ef is a nega t ive feedback vo l t age p r o p o r t i o n a l to t h e load c u r r e n t . T h e b a r over t h e vo l t age s y m b o l i nd i ca t e s t h e a v e r a g e vo l tage over a hal f cycle . W i t h a resistive load , E^ is d e l a y e d 1 cycle b y t h e ampl i f ie r . W h e n cond i t i ons a r e such t h a t jS^has b e e n c o n s t a n t for 1 cycle or m o r e , £ ^ = £ < a n d t h e i n p u t to t h e ha l f -wave ampl i f iers cores I a n d I I is

Fig. 2. Trans fer characterist ic

1 30 f, (JKiLiiyoiTS)

ze ro . H o w e v e r , if £ ^ is inc reas ing , i .e., Ei>Ef

s ince Ef is d e l a y e d in t h e ampl i f ie r . T h e n a vo l t age Ei—Ef exists b e t w e e n t e r m i n a l s Χχ a n d X 4 , s u c h t h a t Χχ is pos i t ive w i t h r e s p e c t t o X4. V o l t a g e of th is p o l a r i t y is ampl i f i ed b y c o r e I w h i l e c o r e I I is unaf fec ted a n d its

dE o u t p u t r e m a i n s ze ro . I f E^ is dec rea s ing , i .e., dt

' < 0 ,

Ei<Ef a n d t e r m i n a l Xi is n e g a t i v e w i t h r e spec t t o t e r m i n a l X4. I n this case , t h e vo l t age Ei—Ef= — \Ef—15< is ampl i f ied b y c o r e I I w h i l e c o r e I is unaf fec ted a n d its o u t p u t r e m a i n s ze ro . T h u s t h e o u t p u t of co re I is p r o p o r t i o n a l to posi t ive i n c r e m e n t s of s ignal v o l t a g e , w h i l e t h e o u t p u t of co re I I is p r o p o r t i o n a l t o n e g a t i v e i n c r e m e n t s of s ignal vo l t age .

C o r e s I I I a n d I V a r e p a r t s of a ful l -wave m a g n e t i c ampl i f i e r w i t h sufficient pos i t ive f eedback for i t t o funct ion as a n i n t e g r a t o r . T h e pos i t ive f eedback is p r o v i d e d b y t h e A'4 w i n d i n g s on t h e cores , h is a c u r r e n t source w h i c h p r o v i d e s b i a s b y m e a n s of t h e Ni w i n d i n g s . T h e N2 w i n d i n g s a r e t h e c o n t r o l w i n d i n g s a n d t h e w i n d i n g s t h e l oad w i n d i n g s .

S ince th is c i r cu i t b e h a v e s as a n i n t e g r a t o r s u m m i n g t h e i n c r e m e n t s of s ignal v o l t a g e ampl i f i ed b y cores I a n d I I , t h e o u t p u t c u r r e n t is a l i n e a r func t ion of t h e i n p u t vo l t age . T h e t ransfer c h a r a c t e r i s t i c is s h o w n in F ig . 2.

If t h e n e g a t i v e c u r r e n t - v o l t a g e f eedback is d e l a y e d e i t he r b y a n i n d u c t i v e l o a d o r a low-pass filter, t h e first d e r i v a t i v e of t h e s igna l v o l t a g e b e c o m e s a c o m p o n e n t of t h e l o a d vo l t age . T h u s t h e t o t a l t i m e c o n s t a n t of a sys tem i n c o r p o r a t i n g th is ampl i f ie r c a n b e effectively r e d u c e d . T h i s is espec ia l ly v a l u a b l e in s u c h a p p l i c a t i o n s as d r i v i n g a h i g h - s p e e d t e l e t y p e r e l ay , p r o v i d i n g exc i t a t i on to t h e field w i n d i n g of a g e n e r a t o r , p r o v i d i n g exc i t a t i on to t h e c o n t r o l w i n d i n g of a 2 - p h a s e se rvo m o t o r , o r o p e r a t i n g a m a g n e t i c c l u t c h . Digest of paper 54-375, "Derivative Controlled Magnetic Amplifiers," recommended by the AIEE Committee on Magnetic Amplifiers and approved by the AIEE Com­mittee on Technical Operations for presentation at the AIEE, Fall General Meeting, Chicago, 111., October 11-15, 1954. Scheduled for publication in AIEE Communication and Electronics, 1954.

A. D. Schnitzler is with the Naval Research Laboratory, Washington, D. C.

N O V E M B E R 1954 Schnitzler—Magnetic Amplifiers 1021