KL34018 IK Semicon Co., Ltd, KL34018 Datasheet - Page 7

• Manufacturer Part Number: KL34018
• Description: Linear Integrated Circuit Voice Switched Speaker Phone Circuit
• Manufacturer: IK Semicon Co., Ltd

LOG AMPLIFIERS
(Transmit and Receive Level Detectors)
receive signals, so as to tell the Tx-Rx comparator
which mode should be in effect. The input signals are
applied to the amplifiers (at TLI and RLI) through AC
coupling capacitors and current limiting resistors. The
value of these components determines the sensitivity of
the respective amplifiers, and has an effect on the
switching times between transmit and receive modes.
The feedback elements for the amplifiers are back-to-
back diodes which provide a logarithmic gain curve,
thus allowing operation over a wide range of signal
levels. The outputs of the amplifiers are rectified, having
a quick rise time and a slow decay time. The rise time is
determined primarily by the external capacitor (at TLO
or RLO) and an internal 500 ohm resistor, and is on the
order of a fraction of a millisecond. The decay time is
determined by the external resistor and capacitor, and is
on the order of a fraction of a second. The switching
time is not fixed, but depends on the relative values of
the transmit
external components. Figure 7 indicates the dc transfer
characteristics of the log amps, and Figure 8 indicates
the transfer characteristics with respect to an ac input
signal. The dc level at TLI, RLI, TLO, and RLO is
approximately VB.
TLO and RLO, which in turn are functions of the
currents sourced out of TLI and RLI, respectively. If an
offset at the comparator input is desired, e.g., to prevent
noise from switching the system, or to give preference
to either the transmit or receive channel, this may be
achieved by biasing the appropriate input (TLI or RLI).
A resistor to ground will cause a DC current to flow out
of that input, thus forcing the output of that amplifier to
be biased slightly higher than normal. This amplifier
then becomes the preferred one in the system operation.
Resistor values from 500 k to 10 M ohms are
recommended for this purpose.
SPEAKER AMPLIFIER
VN), and is noninverting. The input impedance is
nominally 22 kR as long as the output signal is below
that required to activate the Peak Limiter. Figure 9
indicates the typical output swing available at SKO (Pin
15). Since the output current capability is 100 mA, the
lower curve is limited to a 5.0 volt swing. The output
impedance depends on the output signal level and is
relatively low as long as the signal level is not near the
maximum limits. At 3 volts p-p the output impedance is
<0.5 ohms, and at 4.5 volts p-p it is <3 ohms. The
output is short circuit protected at approximately 300
rnA.
causes a portion of the input signal to be shunted to
The Tx-Rx comparator responds to the voltages at
The speaker amplifier has a fixed gain of 34 dB (50
When the amplifier is overdriven, the peak limiter
The log amps monitor the levels of the transmit and
and receive signals, as well as these
the transmit level detector.
effect is that of a gain reduction caused by a reduction of
the input impedance (at SKI) to a value not less than 2.0
㏀.
time of the peak limiter circuit. When a large input signal
is applied to SKI, the voltage at AGC (Pin 17) will drop
quickly as a current source is applied to the external
capacitor. When the large input signal is reduced, the
current source is turned off, and an internal 110 ㏀
resistor discharges the capacitor so the voltage at AGC
can return to its normal value (1.9 Vdc). The capacitor
additionally stabilizes the peak limiting feedback loop.
disabling the rest of the circuit, this may be
accomplished by connecting a resistor from the AGC pin
to ground. A 100 kfl resistor will reduce the gain by 34
dB (0 dB from SKI to SKO), and a 10 k resistor will
reduce the gain by almost 50 dB.
TRANSMIT DETECTOR CIRCUIT
The transmit detector circuit, also known as the
background noise monitor, distinguishes speech (which
consists of bursts) from the background noise (a
relatively constant signal). It does this by storing a
voltage level, representative of the average background
noise, in the capacitor at CP1 (Pin 11). The resistor and
c a p a c i t o r a t t h i s p i n h a v e a t i me co n stan t o f
approximately 5 seconds (in Figure 1). The voltage at
Pin 11 is applied to the inverting input of the Transmit
Detector Comparator. In the absence of speech signals,
the noninverting input receives the same voltage level
minus an offset of 36 mV. In this condition, the output of
the comparator will be low, the output transistor turned
off, and the voltage at XDC (Pin 23) will be at ground. If
the Tx-Rx comparator is in the transmit position, the
attenuators will be in the idle mode (AVacf=75 mV).
When speech is presented to the microphone, the signal
burst appearing at XDI reaches the noninverting input of
the transmit detector comparator before the voltage at the
inverting input can change, causing the output to switch
high, driving the voltage at XDC up to approximately 4
volts. This high level causes the attenuator control block
to switch the attenuators from the idle mode to the
transmit mode (assuming the Tx-Rx comparator is in
the transmit mode). As long as the speech continues to
arrive, and is maintained at a level above the
background, the voltage at XDC will be maintained at a
high level, and the circuit will remain in the transmit
mode. The time constant of the components at XDC will
determine how much time the circuit requires to return to
the idle mode after the cessation of microphone speech
signals, such as occurs during the normal pauses in
speech.
determine the sensitivity of the transmit detector circuit.
Figure 10 indicates the change in DC voltage levels at
CP2 and CP1 in response to a steady state sine wave
If there is a need to mute the speaker amplifier without
The capacitor at Pin 17 (AGC) determines the response
The series resistor and capacitor at XDI (Pin 13)
Rev. 00
KL34018