Application examples of LM381a

The audio integrated circuit produced by the National Semiconductor Company (NS Company) in the United States has a wide frequency response and low noise. The audio
circuit assembled with it has very excellent performance. This circuit all uses NS Company integrated circuits, as shown in Figure 3-12. The input preamplifier and phono
equalizer adopt LM381, the dynamic noise reduction adopts LM1894, and the DC volume control adopts LM10350. LM381 is a
wide-band , high-gain, low-noise amplifier circuit. It is a dedicated dual preamplifier designed for small signal amplification. amplifying circuit. Its
noise is extremely low. When the signal source internal resistance Rs=600fl and the frequency is 10Hz~10kHz, the effective equivalent noise value at the input end is
0.5r.eVo. LM381 is used in various measuring equipment, small signal wide-band amplifiers and various signals. It is widely used in source circuits.
In practical applications, through external bias adjustment, it can achieve the best noise characteristics and become a high-quality amplifier with wide bandwidth, high gain and low noise
. Its internal circuit and pin arrangement are shown in Figure 3-13, and the application circuit of LM381 is shown in Figure 3-14.
    The interior of LM381 is composed of two independent input preamplifiers, with 60dB isolation between them. There is a
power and a power supply rejection ratio of 120dB. LM381 has extremely high voltage gain (>100dB), Large output
voltage swing and large signal bandwidth (small signal bandwidth is 15MH, large signal bandwidth is 75kH). It can work under a wide power supply voltage of 9V ~ 15V
, and is equipped with complete compensation and short-circuit protection internally. The electrical characteristics are shown in Table 3-2.
    Half of the LM381, IC2, forms the phono equalizer amplifier, and the other half, IC1, forms
the input amplifiers for line, CD player, tape playback, etc. They can be connected to the input terminals through the selector switch SAi.
    LM1894 is a dynamic noise reduction circuit. The output signal after noise reduction is output by pins 4 and 11 (left and right channels). The selection
switch SAz is used to select whether the noise reducer is connected to the circuit. The 1kfl potentiometer connected to pin 5 is used to Adjust the noise reduction sensitivity o
    LM 1035 is a two-channel DC volume control potentiometer, and its output end is connected with an equal loudness volume control potentiometer and
    a J switch.