Datasheet LM389 (National Semiconductor) - 5

Application Hints Gain Control bypass the unused input, preventing degradation of gain and possible instabilities. This is done with a 0.1 mF capaci- To make the LM389 a more versatile amplifier, two pins (4 tor or a short to ground depending on the dc source resist- and 12) are provided for gain control. With pins 4 and 12 ance of the driven input. open, the 1.35 kX resistor sets the gain at 20 (26 dB). If a capacitor is put from pin 4 to 12, bypassing the 1.35 kX Supplies and Grounds resistor, the gain will go up to 200 (46 dB). If a resistor is The LM389 has excellent supply rejection and does not re- placed in series with the capacitor, the gain can be set to quire a well regulated supply. However, to eliminate possi- any value from 20 to 200. A low frequency pole in the gain ble high frequency stability problems, the supply should be response is caused by the capacitor working against the decoupled to ground with a 0.1 mF capacitor. The high cur- external resistor in series with the 150X internal resistor. If rent ground of the output transistor, pin 18, is brought out the capacitor is eliminated and a resistor connects pin 4 to separately from small signal ground, pin 17. If the two 12, then the output dc level may shift due to the additional ground leads are returned separately to supply then the par- dc gain. Gain control can also be done by capacitively cou- asitic resistance in the power ground lead will not cause pling a resistor (or FET) from pin 12 to ground. stability problems. The parasitic resistance in the signal Additional external components can be placed in parallel ground can cause stability problems and it should be mini- with the internal feedback resistors to tailor the gain and mized. Care should also be taken to insure that the power frequency response for individual applications. For example, dissipation does not exceed the maximum dissipation of the we can compensate poor speaker bass response by fre- package for a given temperature. There are two ways to quency shaping the feedback path. This is done with a se- mute the LM389 amplifier. Shorting pin 3 to the supply volt- ries RC from pin 1 to 12 (paralleling the internal 15 kX resis- age, or shorting pin 12 to ground will turn the amplifier off tor). For 6 dB effective bass boost: R j 15 kX, the lowest without affecting the input signal. value for good stable operation is R e 10 kX if pin 4 is Transistors open. If pins 4 and 12 are bypassed then R as low as 2 kX can be used. This restriction is because the amplifier is only The three transistors on the LM389 are general purpose compensated for closed-loop gains greater than 9V/V. devices that can be used the same as other small signal transistors. As long as the currents and voltages are kept Input Biasing within the absolute maximum limitations, and the collectors The schematic shows that both inputs are biased to ground are never at a negative potential with respect to pin 17, with a 50 kX resistor. The base current of the input transis- there is no limit on the way they can be used. tors is about 250 nA, so the inputs are at about 12.5 mV For example, the emitter-base breakdown voltage of 7.1V when left open. If the dc source resistance driving the can be used as a zener diode at currents from 1 mA to LM389 is higher than 250 kX it will contribute very little 5 mA. These transistors make good LED driver devices, additional offset (about 2.5 mV at the input, 50 mV at the VSAT is only 150 mV when sinking 10 mA. output). If the dc source resistance is less than 10 kX, then In the linear region, these transistors have been used in AM shorting the unused input to ground will keep the offset low and FM radios, tape recorders, phonographs and many oth- (about 2.5 mV at the input, 50 mV at the output). For dc er applications. Using the characteristic curves on noise source resistances between these values we can eliminate voltage and noise current, the level of the collector current excess offset by putting a resistor from the unused input to can be set to optimize noise performance for a given source ground, equal in value to the dc source resistance. Of impedance. Some of the circuits that have been built are course all offset problems are eliminated if the input is ca- shown in Figures 1 – 7 . This is by no means a complete list pacitively coupled. of applications, since that is limited only by the designers When using the LM389 with higher gains (bypassing the imagination. 1.35 kX resistor between pins 4 and 12) it is necessary to TL/H/7847 – 6 FIGURE 1. AM Radio 5