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ILA1062A

Low Voltage Transmission Circuit with Dialler Interface

Integral Corp. 

ILA1062A

Vref is an internally generated temperature

compensated reference voltage of 3.7 V and R9 is

an external resistor connected between SLPE and

VEE.

In normal use the value of R9 would be 20Ω.

Changing the value of R9 will also affect

microphone gain, DTMF gain, gain control

characteristics, sidetone level, maximum output

swing on LN and the DC characteristics (especially

at the lower voltages).

Under

normal

conditions,

when

ISLPE>>ICC + 0.5 mA + Ip, the static behaviour of the

circuit is that of a 3.7 V regulator diode with an

internal resistance equal to that of R9. In the audio

frequency range the dynamic impedance is largely

determined by R1. Fig.2 shows the equivalent

impedance of the circuit.

Leq = C3 x R9 x RP

RP = 16.2 KΩ

Figure 2. Equivalent impedance circuit

At line currents below 9 mA the internal reference

voltage is automatically adjusted to a lower value

(typically 1.6 V at 1 mA) This means that more sets

can be operated in parallel with DC line voltages

(excluding the polarity guard) down to an absolute

minimum voltage of 1.6 V.

At line currents below 9 mA the circuit has limited

sending and receiving levels. The internal reference

voltage can be adjusted by means of an external

resistor (RVA). This resistor when connected between

LN and REG will decrease the internal reference

voltage and when connected between REG and

SLPE will increase the internal reference voltage.

Microphone inputs MIC+ and MIC- and gain

pins GAS1 and GAS2

The circuit has symmetrical microphone inputs. Its

input impedance is 64 KΩ (2 x 32 KΩ) and its

voltage gain is typically 52 dB (when R7 = 68 KΩ,

see Figure 3). Dynamic, magnetic, piezo-electric or

electret (with built-in FET source followers) can be

used.

The gain of the microphone amplifier can be

adjusted between 44 dB and 52 dB to suit the

sensitivity of the transducer in use. The gain is

proportional to the value of R7 which is connected

between GAS1 and GAS2.

Stability is ensured by two external capacitors, C6

connected between GAS1 and SLPE and C8

connected between GAS1 and VEE. The value of C6

is 100 pF but this may be increased to obtain a first-

order low-pass filter. The value of C8 is 10 limes the

value of C6. The cut-off frequency corresponds to

the time constant R7 x C6.

Input MUTE

When MUTE is LOW or open circuit, the DTMF

input is enabled and the microphone and receiving

amplifier inputs are inhibited. The reverse is true

when MUTE is HIGH, MUTE switching causes only

negligible clicking on the line and earpiece output. If

the number of parallel sets in use causes a drop in

line current to below 6 mA the DTMF amplifier

becomes active independent to the DC level applied

to the MUTE input.

Dial-tone multi-frequency input DTMF

When the DTMF input is enabled dialling tones may

be sent on to the line. The voltage gain from DTMF

to LN is typically 25.5 dB (when R7 = 68 KΩ) and

varies with R7 in the same way as the microphone

gain. The signalling tones can be heard in the

earpiece at a low level (confidence tone).

Receiving amplifier IR, QR and GAR

The receiving amplifier has one input (IR) and a

non-inverting output (QR). The IR to QR gain is

typically 31 dB (when R4 = 100 KΩ). It can be

adjusted between 20 and 31 dB to match the

sensitivity of the transducer in use. The gain is set

with the value of R4 which is connected between

GAR and QR. The overall receive gain, between LN

and QR, is calculated by subtracting the anti-

sidetone network attenuation (32 dB) from the

amplifier gain. Two external capacitors, C4 and C7,

ensure stability. C4 is normally 100 pF and C7 is 10

times the value of C4. The value of C4 may be

increased to obtain a first-order low-pass filter. The

cut-off frequency will depend on the time constant

R4 x C4.

The output voltage of the receiving amplifier is

specified for continuous-wave drive. The maximum

output voltage will be higher under speech

conditions where the peak to RMS ratio is higher.

3

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