LM13600 datasheet(7/24 Pages) NSC

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Linearizing Diodes (Continued)

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.

Controlled Impedance Buffers

The upper limit of transconductance is defined by the maxi-

mum value of I

ABC (2 mA). The lowest value of IABC for which

the amplifier will function therefore determines the overall

dynamic range. At very low values of I

ABC, a buffer which has

very low input bias current is desirable. An FET follower sat-

isfies the low input current requirement, but is somewhat

non-linear for large voltage swing. The controlled impedance

buffer is a Darlington which modifies its input bias current to

suit the need. For low values of I

ABC, the buffer’s input cur-

rent is minimal. At higher levels of I

ABC, transistor Q3biases

up Q

12 with a current proportional to IABC for fast slew rate.

When I

ABC is changed, the DC level of the Darlington output

buffer will shift. In audio applications where I

ABC is changed

suddenly, this shift may produce an audible “pop”. For these

applications the LM13700 may produce superior results.

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

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

maximizes their linearizing action when balanced against

IN. A value of 1 mA is recommended for ID unless the spe-

cific application demands otherwise.

DS007980-9

FIGURE 2. Voltage Controlled Amplifier

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