LA4160 3. Power Amplifier The power amplifier circuit consists of the differential stage (TR22, 23), predriver stage (TR41), driver stage (TR42, 44), and output stage (TR47,48). Bias resistor R36 of TR20 at the first stage is attached externally and the input impedance depends on this R36. Input pin (4) whose potential is nearly equal to the ground potential can be connected direct to the variable resistor without using an input coupling capacitor. The closed loop voltage gain is obtained by the following equation. R19 VG (pwr) = 20 log ( R18 + Rf ) Since R19 = 20kΩ, R18 = 50Ω are set, approximately 52dB max. is available if Rf = 0 is taken. Proper Cares in Using IC 1. Printed Circuit Board For the printed circuit board layout, refer to the printed circuit pattern example in principle. It is necessary that the grounding points of the power amplifier input (variable resistor), pin (3) feedback resistor (Rf), and bias resistor (R35) should be adjacent to one another. Especially, it should be noted that if a common impedance exists in the variable resistor grounding, the distortion, residual ripple, and cross interference (leak of signal between preamplifier and power amplifier) may worsen. 8 14 LA4160 7 1 R1 To preamplifier Make adjacent to one another. VR R35 2. Oscillation [Measures against oscillation] a) Preamplifier Since oscillation may osccur when used at a low voltage Approximately 8 gain, a capacitor of approximately 10pF must be connected 9 across pins (8) and (6) for phase compensation. If the 6 Preamp. capacitor connected across pins (8) and (6) has a large RL 0.01μF capacitance at a low load impedance, further oscillation 1000pF (Polyester film may occur, and in this case the load pin must be grounded capacitor) through a capacitor of approximately 0.01μF (Polyester film capacitor). The input pin must be grounded through a capacitor of approximately 1000pF for the purpose of prevention of radio disturbance and oscillation. No.870-7/11
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