LM13600M datasheet(6/24 Pages) NSC

6 / 24 page

Circuit Description

The differential transistor pair Q

4 and Q5 form a transcon-

ductance stage in that the ratio of their collector currents is

defined by the differential input voltage according to the

transfer function:

(1)

where V

IN is the differential input voltage, kT/q is approxi-

mately 26 mV at 25˚C and I

5 and I4 are the collector currents

of transistors Q

5 and Q4 respectively. With the exception of

3 and Q13, all transistors and diodes are identical in size.

Transistors Q

1 and Q2 with Diode D1 form a current mirror

which forces the sum of currents I

4 and I5 to equal IABC;

4 +I5 = IABC

(2)

where I

ABC is the amplifier bias current applied to the gain

pin.

For small differential input voltages the ratio of I

4 and I5 ap-

proaches unity and the Taylor series of the In function can be

approximated as:

(3)

(4)

Collector currents I

4 and I5 are not very useful by themselves

and it is necessary to subtract one current from the other.

The remaining transistors and diodes form three current mir-

rors that produce an output current equal to I

5 minus I4 thus:

(5)

The term in brackets is then the transconductance of the am-

plifier and is proportional to I

ABC.

Linearizing Diodes

For differential voltages greater than a few millivolts,

Equa-

tion (3) becomes less valid and the transconductance be-

comes increasingly nonlinear.

Figure 1 demonstrates how

the internal diodes can linearize the transfer function of the

amplifier. For convenience assume the diodes are biased

with current sources and the input signal is in the form of cur-

rent I

S. Since the sum of I4 and I5 is IABC and the difference

is I

OUT, currents I4 and I5 can be written as follows:

Since the diodes and the input transistors have identical ge-

ometries and are subject to similar voltages and tempera-

tures, the following is true:

(6)

DS007980-8

FIGURE 1. Linearizing Diodes

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