Neglecting the VCEsat of the bridge transistors and the VBE of the diodes:

=

1

2

where

(1)

DC TRANSFER FUNCTION

In order to be sure that the current loop is stable the following condition is imposed :

1 + sRC = 1 + s

(pole cancellation)

(2)

from which RC =

(Note that in practice R must greater than 5.6 K

Ω)

The transfer function is then,

1

(3)

In DC condition, this is reduced to

•

1

=

0.044

[

A

V

]

(4)

OPEN-LOOP GAIN AND STABILITY CRITERION

A

β =

1

sR subF C

•

1

=

C

1

(5)

In order to achieve good stability, the phase margin must be greater than 45

° when | Aβ | = 1.

1

2

must be | A

β | < 1 (see fig. 7), that is :

| A

β | f =

1

2

=

(6)

Figure 7. Open Loop Frequency Response

7/12

L292