Biasing Class
AB power output stage
Emitter
resistors RE1 & RE2 are the key for biasing output
stage.
Besides bias, they
give us thermal stability, negative feedback and overload
protection, and improve THD. (Total Harmonic Distortion)
Voltage across RE1
& RE2, can be used for current sensing and protection
purposes circuits as well.
RE1 &RE2 are set
at 0.1 Ohm because is a good value for minimum power losses
(Resistor dissipation) and as well for current sensing
(Protection under short circuit or over current conditions).
(e.g.): For
minimum crossover distortion and low sink current lets use
3mA, for example.
VRE1=0.003A x 0.01
Ohm=0.3 mV
Vbe=0.5V, then V
across RD shall be Vbe1+Vbe2+0.6mV
Vs=VRD+Vbe(Q3)+Vbe(Q4)=2.0006V
If RB=4.7K Ohms
then RC=(2-0.6)/(0.6/4700)=1.4V/0.127mA=10.9K
Note: Only RC
controls the output bias current, RC can be adjusted to any
value between 8.2K and 11K with a trade off being THD versus
Sink current.
RD is selected for the maximum driver bias desired
and maximum speed to turn off the output stage, for a lower
Slew factor RD shall be 33 Ohms to 220 Ohms.
