Linearizing Diodes (Continued) Applications-Voltage Controlled Notice that in deriving Equation 6 no approximations have Amplifiers been made and there are no temperature-dependent terms. Figure 2 shows how the linearizing diodes can be used in a The limitations are that the signal current not exceed I /2 D voltage-controlled amplifier. To understand the input biasing, and that the diodes be biased with currents. In practice, re- it is best to consider the 13 kΩ resistor as a current source placing the current sources with resistors will generate insig- and use a Thevenin equivalent circuit as shown in Figure 3. nificant errors. This circuit is similar to Figure 1 and operates the same. The potentiometer in Figure 2 is adjusted to minimize the effects Controlled Impedance Buffers of the control signal at the output. The upper limit of transconductance is defined by the maxi- For optimum signal-to-noise performance, I should be as ABC mum value of I (2 mA). The lowest value of I for which large as possible as shown by the Output Voltage vs. Ampli- ABC ABC the amplifier will function therefore determines the overall fier Bias Current graph. Larger amplitudes of input signal dynamic range. At very low values of I , a buffer which has also improve the S/N ratio. The linearizing diodes help here ABC very low input bias current is desirable. An FET follower sat- by allowing larger input signals for the same output distortion isfies the low input current requirement, but is somewhat as shown by the Distortion vs. Differential Input Voltage non-linear for large voltage swing. The controlled impedance graph. S/N may be optimized by adjusting the magnitude of buffer is a Darlington which modifies its input bias current to the input signal via R (Figure 2) until the output distortion is IN suit the need. For low values of I , the buffer’s input cur- below some desired level. The output voltage swing can ABC rent is minimal. At higher levels of I , transistor Q biases then be set at any level by selecting R . ABC 3 L up Q with a current proportional to I for fast slew rate. 12 ABC Although the noise contribution of the linearizing diodes is When I is changed, the DC level of the Darlington output ABC negligible relative to the contribution of the amplifier’s inter- buffer will shift. In audio applications where I is changed ABC nal transistors, I should be as large as possible. This mini- D suddenly, this shift may produce an audible “pop”. For these mizes the dynamic junction resistance of the diodes (r ) and e applications the LM13700 may produce superior results. maximizes their linearizing action when balanced against R . A value of 1 mA is recommended for I unless the spe- IN D cific application demands otherwise. DS007980-9 FIGURE 2. Voltage Controlled Amplifier 7 www.national.com
