Datasheet

LM13700

SNOSBW2E –NOVEMBER 1999–REVISED MARCH 2013

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APPLICATIONS

Voltage Controlled Amplifiers

Figure 23 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 kΩ resistor as a current source and use a Thevenin equivalent circuit

as shown in Figure 24. This circuit is similar to Figure 22 and operates the same. The potentiometer in Figure 23

is adjusted to minimize the effects of the control signal at the output.

Figure 22. Linearizing Diodes

For optimum signal-to-noise performance, I

ABC

should be as large as possible as shown by the Output Voltage

vs. Amplifier 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

(Figure 23) until the output distortion is below some desired level. The output voltage swing can then be set at

any level by selecting R

Although the noise contribution of the linearizing diodes is negligible relative to the contribution of the amplifier's

internal transistors, I

should be as large as possible. This minimizes the dynamic junction resistance of the

diodes (r

) and maximizes their linearizing action when balanced against R

. A value of 1 mA is recommended

for I

unless the specific application demands otherwise.

Figure 23. Voltage Controlled Amplifier

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