Handbook for Sound Engineers

720 Chapter 20

of hard relay contacts energized by suitable circuitry.
Upon turn-on these contacts are open and are subse-
quently closed by means of a delay circuit that allows
the amplifier to stabilize before the load is connected.
These same contacts open immediately upon amplifier
turn-off. An additional signal is supplied to this muting
circuitry by means of a low-pass filter connected to the
amplifier’s final stage. If dc is sensed at this point in
excess of a safe value, the circuit acts to disconnect the
load from the amplifier.

The amplifier can be protected against dc at its input
when such is not intended by either transformer or
capacitor high-pass filters. It may further be protected
against radio-frequency signals at the input or output
lines by means of series-connected low-pass filters.
These filters must be designed with care so as to not
unnecessarily restrict the intended amplifier passband.
Excessive heat sink temperature is sensed by an
attached thermal sensor that controls internal cooling
fans or may ultimately interrupt power to the output
stage. Thermally sensitive bias tracking circuitry can be
provided to insure appropriate bias conditions for the
output over a reasonable range of ambient temperature.
Short-circuit protection usually involves monitoring the
currents in the output devices and restricting the drive
applied to the output stage whenever excessive current
is detected with long-term protection still being
provided by the thermal mechanisms previously
mentioned. Such a circuit suitable for the amplifier of
Fig. 20-22 is given in Fig. 20-24. Resistors R 15 and R 16
form a voltage divider sensing the emitter currents of
the output devices. In the event of excessive emitter
current, Q 7 robs base drive from Q 1 while Q 8 robs base
drive from Q 2. Diodes D 3 and D 4 prevent Q 7 and Q 8
from having their collector to base junctions forward
biased under conditions of normal operation. This same
circuit can readily be converted into a dissipation limiter
rather than just a current limiter by referencing the junc-
tion of the R 16 resistors to ground rather than to the
amplifier output terminal.
The amplifier depicted in Fig. 20-25 has an inter-
esting protection mechanism that provides automatic
turn-on muting along with protection of the output stage.

The output stage of this amplifier consists of compli-
mentary MOSFETs connected in the common source
configuration that yields voltage gain and signal
polarity inversion in the output stage. When the ampli-
fier is first energized any incoming signal is muted by
the JFET T1, which provides a low resistance to ground.
As the capacitor connected to the gate of T 1 gradually
charges toward 15 V this condition is relaxed and the
amplifier becomes operative. During normal operation

the currents in the output transistors T 2 and T 3 are moni-

tored at points A and B. An excessive current in either

device will trigger Q 2 which is a voltage discriminator

or Schmitt trigger circuit. The amplifier will then be

muted for a time determined by the RC combination in

the gate circuit of T 1.

20.3.2 High-Power Amplifiers

The demands of the sound reproduction and reinforce-

ment industry for both higher power and higher perfor-

mance amplifiers have brought about the necessity for a

paradigm shift in power amplifier design. The older

time-honored linear designs employing push-pull output

Figure 20-24. Short-circuit protector for the amplifier of Fig.

20-22.

Figure 20-25. Protected MOSFET power amplifier.

D 3

R 16

R 15

R 15

R 16

Q 7

Q 8

D 4

To base of Q 1

To emitter of Q 1

To emitter of Q 2

To base of Q 2

To junction of R 1 and R 2

Signal in

–15 V

+15 V

+15 V

A

45 V

To B 45 V

To A

Load

T 1

T 2

T 3

Q 1

Q 2

+

+

• 
  
  
  
  • B