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LM13700 Datenblatt(PDF) 6 Page - National Semiconductor (TI) |
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LM13700 Datenblatt(HTML) 6 Page - National Semiconductor (TI) |
6 / 24 page 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 www.national.com 6 |
Ähnliche Teilenummer - LM13700 |
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Ähnliche Beschreibung - LM13700 |
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