2.1 Telephone-Line Transceiver
The telephone-line transceiver of
Figure 7
provides a full-
duplexed connection through a PCMCIA, miniature trans-
former. The differential configuration of receiver portion
(UR), cancels reception from the transmitter portion (UT).
Note that the input signals for the differential configuration of
UR, are the transmit voltage (Vt) and Vt/2. This is because
R
match
is chosen to match the coupled telephone-line imped-
ance; therefore dividing Vt by two (assuming R1
>>
R
match
).
The differential configuration of UR has its resistors chosen
to cancel the Vt and Vt/2 inputs according to the following
equation:
DS100128-54
FIGURE 5. Pulse Response per
Figure 4
1.3 Input Bias Current Consideration
Input bias current (I
B
) can develop a somewhat significant
offset voltage. This offset is primarily due to I
B
flowing
through the negative feedback resistor, R
F
. For example, if I
B
is 90nA (max room) and R
F
is 100 kΩ, then an offset of 9 mV
will be developed (V
OS
=I
B
x R
F
).Using a compensation resis-
tor (R
C
), as shown in
Figure 6,
cancels out this affect. But the
input offset current (I
OS
) will still contribute to an offset volt-
age in the same manner - typically 0.05 mV at room temp.
DS100128-33
FIGURE 7. Telephone-line Transceiver for a PCMCIA
Modem Card
Note that Cr is included for canceling out the inadequacies of
the lossy, miniature transformer. Refer to application note
AN-397 for detailed explanation.
2.2“Simple” Mixer (Amplitude Modulator)
The mixer of
Figure 8
is simple and provides a unique form
of amplitude modulation. Vi is the modulation frequency
(F
M
), while a +3V square-wave at the gate of Q1, induces a
carrier frequency (F
C
). Q1 switches (toggles) U1 between in-
verting and non-inverting unity gain configurations. Offset-
ting a sine wave above ground at Vi results in the oscillo-
scope photo of
Figure 9.
The simple mixer can be applied to applications that utilize
the Doppler Effect to measure the velocity of an object. The
difference frequency is one of its output frequency compo-
nents. This difference frequency magnitude (/F
M
-F
C
/) is the
key factor for determining an object’s velocity per the Dop-
pler Effect. If a signal is transmitted to a moving object, the
reflected frequency will be a different frequency. This differ-
ence in transmit and receive frequency is directly propor-
tional to an object’s velocity.
DS100128-59
FIGURE 6. Canceling the Voltage Offset Effect of Input
Bias Current
2.0 APPLICATION CIRCUITS
This section covers the following application circuits:
1. Telephone-Line Transceiver
2. “Simple” Mixer (Amplitude Modulator)
3. Dual Amplifier Active Filters (DAAFs)
a. Low-Pass Filter (LPF)
•
b. High-Pass Filter (HPF)
•
5. Tri-level Voltage Detector
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