6 Principles of Operation; Figure 6.1 Circuit Block Diagram; 6.2.1 Stereo Operation- Page 16

Micro-Tech Series 

Power Amplifiers

page 16

Operation Manual

6.2.1 Stereo Operation

For simplicity, the discussion of Stereo opera-
tion will refer to one channel only. Mono opera-
tion will be discussed later. For specific circuit 
references, see the block diagram in Figure 6.1.

The signal at the ¼-inch phone jack input 
passes directly to the balanced gain stage 
(U104-A and U104-B). The balanced gain stage 
causes balanced to single-ended conversion 
using a difference amplifier. From there, gain 
can be controlled with a potentiometer. The 
error amp (U104-C) amplifies the difference 
between the output signal and the input signal 
from the gain pot, and drives the voltage trans-
lator stage.

From the error amp, the voltage translator stage 
routes the signal to the Last Voltage Amplifiers 
(LVAs) based on signal polarity. The +LVA 

(Q105) and the –LVA (Q110), with their push-
pull effect through the bias servo Q318, drive 
the fully complementary output stage.

The bias servo Q318 is thermally coupled to 
the heat sink and sets the quiescent bias cur-
rent in the output stage to lower the distortion 
in the crossover region of the output signal. 
D301, D302, D303, and D304 remove the 
charge on the unused portion of the output 
stage based on the polarity of the output signal.

With the voltage swing provided by the LVAs, 
the signal then gains current amplification 
through the Darlington emitter-follower output 
stage.

The bridge-balanced circuit (U104-D) receives 
a signal from the output of the amplifier and 
compares it to the signal at the Vcc supply. The 
bridge-balanced circuit then develops a voltage 

to drive the bridge-balanced output stage. This 
results in the Vcc supply having exactly one 
half of the output voltage added to their quies-
cent voltage. D309, D310, D311 and a trimmer 
resistor set the quiescent current point for the 
bridge-balanced output stage.

The protection mechanisms that affect the sig-
nal path are implemented to protect the ampli-
fier under real-world conditions. These 
conditions are high instantaneous current, 
excessive temperature, and output device oper-
ation outside safe conditions. 

Q107 and Q108 sense output current and act as 
a common current limiter. When instantaneous 
current exceeds the design criteria, the limiters 
remove the drive from the LVAs to limit output 
current to safe levels.

6 Principles of Operation

ONLY ONE CHANNEL SHOWN

Figure 6.1  Circuit  Block Diagram