TL/H/7380 – 20
*
For Stability with High Current Loads
FIGURE 17. Voltage Divider Input
TL/H/7380 – 21
*
For Stability with High Current Loads
FIGURE 18. Intercom
The circuits of
Figures 14 and 16 employ the ‘‘common
mode’’ volume control as shown before. However, any of
the various input connection schemes discussed previously
may be used.
Figure 17 shows the bridge configuration with
the voltage divider input. As discussed in the ‘‘Biasing’’
section the undriven input may be AC or DC grounded. If V
S
is an appreciable distance from the power supply (
l
3
×
) fil-
ter capacitor it should be decoupled with a 1
m
F tantaulum
capacitor.
INTERCOM
The circuit of
Figure 18 provides a minimum component in-
tercom. With switch S
1
in the talk position, the speaker of
the master station acts as the microphone with the aid of
step-up transformer T
1
.
A turns ratio of 25 and a device gain of 50 allows a maxi-
mum loop gain of 1250. R
V
provides a ‘‘common mode’’
volume control. Switching S
1
to the listen position reverses
the role of the master and remote speakers.
LOW COST DUAL SUPPLY
The circuit shown in
Figure 19 demonstrates a minimum
parts count method of symmetrically splitting a supply volt-
age. Unlike the normal R, C, and power zener diode tech-
TL/H/7380 – 22
FIGURE 19. Dual Supply
nique the LM380 circuit does not require a high standby
current and power dissipation to maintain regulation.
With a 20 volt input voltage (
g
10 volt output) the circuit
exhibits a change in output voltage of approximately 2% per
100 mA of unbalanced load change. Any balanced load
change will reflect only the regulation of the source voltage
V
IN
.
The theoretical plus and minus output tracking ability is
100% since the device will provide an output voltage at
one-half of the instantaneous supply voltage in the absence
of a capacitor on the bypass terminal. The actual error in
6
