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Simple Adjustable Analog Current Mirror

IP.com Disclosure Number: IPCOM000104793D
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 4 page(s) / 87K

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Related People

Goetschel, CJ: AUTHOR [+2]


A digitally adjustable current mirror is disclosed which can be used to adjust circuit parameters under digital control. Some examples are:

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Simple Adjustable Analog Current Mirror

      A digitally adjustable current mirror is disclosed which can be
used to adjust circuit parameters under digital control.  Some
examples are:

o   Amplifier gain,
o   Offset voltage,
o   Comparator threshold, and
o   Comparator hysteresis.

      This current mirror is similar to the conventional current
mirrors except that it uses an additional device (under digital
switching control) to 'steal' some of the reference input current
from the mirror, while reference current itself remains constant (see
Fig. 1).

CLASSICAL MIRROR OPERATION - Q1, Q2, R1, Q3 and R3 are the parts of
the conventional current mirror that are commonly used.  Q1 sinks the
input reference current IIN, and, in doing so, develops a base
voltage at node B1 to support that current.  This base voltage is
coupled to the base of Q3, so Q3 conducts the same amount of current
(if emitter resistors R1 and R3 are of equal value).  Q2 is a helper
transistor, to supply the base current required by Q1 and Q3.

ADJUSTABLE MIRROR OPERATION - In this invention, transistors T4 and
Q4 are added to adjust the output current of the mirror.  Transistor
Q4 can be switched On or Off, under digital control of FET T4 at pin
S1.  When Q4 is switched On, its collector steals part of the input
reference current at node C1.  Then Q1 conducts less current, and so
does Q3, since their bases are coupled together.

Notice that resistor R4 must be much larger than R1, so the Q4 only
steals a fraction of the total input reference current, i.e., 1/4 of
the reference current in Fig 1.  MULTI-STEP ADJUSTABLE MIRROR - This
implementation uses multiple binary weighted currents to
subtractively adjust the output current of the mirror, while the
input reference current remains constant (see Fig. 2).

MULTI-STEP MIRROR OPERATION - Transistors Q4, Q8, Q16 an...