LM13700N datasheet(6/24 Pages) NSC

Nome de Peças	LM13700N

Descrição Electrónicos	Dual Operational Transconductance Amplifiers with Linearizing Diodes and Buffers

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Circuit Description (Continued)

Q

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:

I

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)

Notice that in deriving

Equation (6) no approximations have

been made and there are no temperature-dependent terms.

The limitations are that the signal current not exceed I

D/2

and that the diodes be biased with currents. In practice, re-

placing the current sources with resistors will generate insig-

nificant errors.

Applications:

Voltage Controlled Amplifiers

Figure 2 shows how the linearizing diodes can be used in a

voltage-controlled amplifier. To understand the input biasing,

it is best to consider the 13 k

Ω resistor as a current source

and use a Thevenin equivalent circuit as shown in

Figure 3.

This circuit is similar to

Figure 1 and operates the same. The

potentiometer in

Figure 2 is adjusted to minimize the effects

of the control signal at the output.

For optimum signal-to-noise performance, I

ABC should be as

large as possible as shown by the Output Voltage vs. Ampli-

fier Bias Current graph. Larger amplitudes of input signal

also improve the S/N ratio. The linearizing diodes help here

by allowing larger input signals for the same output distortion

as shown by the Distortion vs. Differential Input Voltage

graph. S/N may be optimized by adjusting the magnitude of

the input signal via R

IN (Figure 2) until the output distortion is

below some desired level. The output voltage swing can

then be set at any level by selecting R

L.

Although the noise contribution of the linearizing diodes is

negligible relative to the contribution of the amplifier’s inter-

nal transistors, I

D should be as large as possible. This mini-

mizes the dynamic junction resistance of the diodes (r

e) and

DS007981-8

FIGURE 1. Linearizing Diodes

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