LM391 Audio Power Driver

External Components (Figure 1)
Component Typical Value Comments
C IN 1 mF Input coupling capacitor sets a low frequency pole with R IN
1
f L e
2qR IN C IN
R IN 100k Sets input impedance and DC bias to input
R f2 100k Feedback resistor for minimum offset voltage at the output this should be equal to R IN
R f1 51k Feedback resistor that works with R f2 to set the voltage gain
A V e 1 a R f 2
R f1
C f 10 mF Feedback capacitor This reduces the gain to unity at DC for minimum offset voltage at the
output Also sets a low frequency pole with R f1
1
f L e
2qR f1 C f
C C 5 pF Compensation capacitor Sets gain bandwidth product and a high frequency pole
1
GBW e
f h e GBW
2q5000C C A V
Max f h for stable design 500 kHz
R A 39k AB bias resistor
R B 10k AB bias potentiometer Adjust to set bias current in the output stage
C AB 01 mF Bypass capacitor for bias This improves high frequency distortion and transient response
C R 5 pF Ripple capacitor This improves negative supply rejection at midband and high frequencies
C R if used must equal C C
R eb 100X Bleed resistor This removes stored charge in output transistors
R O 27X Output compensation resistor This resistor and C O compensate the output stage This value
will vary slightly for different output devices
C O 01 mF Output compensation capacitor This works with R O to form a zero that cancels f b of the
output power transistors
R E 03X Emitter degeneration resistor This resistor gives thermal stability to the output stage
quiescent current IRC PW5 type
R TH 39k Shutdown resistor Sets the amount of current pulled out of pin 14 during shutdown
C 2 C 2 1000 pF Compensation capacitors for protection circuitry
X L 10Xll5 mH Used to isolate capacitive loads usually 20 turns of wire wrapped around a 10X 2W resistor
4