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DN-19
A SIMPLE ISOLATION AMPLIFIER USING THE UC1901
The UC1901 Isolated Feedback Generator has other ap-
plications besides providing isolated feedback in switch-
ing power supplies. This IC’s amplitude modulation
system and error amplifier can be used to implement a
very low cost, high bandwidth, isolation amplifier. Isola-
tion amplifiers of this type find use in switching power
supplies, motor controls, instrumentation, industrial con-
trols and medical systems.
The UC1901 generates a programmable high frequency
carrier signal (up to 5MHz) with an amplitude that is con-
trolled by a high gain error amplifier. In a typical feedback
application, this amplifier and modulator are used, in
conjunction with the UC1901’s 1.5V reference and a
small signal coupling transformer, to provide precision
regulation for an isolated switching power supply. Ca-
pacitively coupled feedback around the UC1901 error
amplifier determines the device’s small signal AC re-
sponse, but the DC operating point is determined by the
requirements of the overall power supply loop. By add-
ing an additional winding on the coupling transformer
and a demodulator circuit for this winding, local DC feed-
back can be provided to the UC1901’s error amplifier. In
this mode very accurate DC, as well as small signal, AC,
transfer functions can be established across the isola-
tion boundary.
The configuration of an isolation amplifier using the
UC1901 is shown in the figure below. The drivers on the
UC1901 couple an amplitude modulated carrier to two
matched windings (W2 and W3 ) on a small signal trans-
former. The demodulated signal from winding W2 is used
to provide feedback to the UC1901’s error amplifier while
the demodulated signal from W3 is the isolated output
signal. The use of the feedback winding linearizes the
transfer function of the overall amplifier and allows DC
signals to be accurately transferred. Matching of the two
demodulator windings and demodulator circuits is impor-
tant to maximize linearity and minimize DC offsets. An
optional output buffer and filter will reduce residual car-
rier ripple and isolate the output demodulator from its
load. The internal gain compensation on the UC1901 is
sufficient for stable operation with overall gains down to
12dB. This circuit requires a supply voltage to the
UC1901 that, if not available in the system already, can
be generated using a second similar circuit operating in
the reverse direction.
The primary features of this circuit are:
1. Good Signal Linearity
2. Wide Bandwidth (3dB Bandwidths > 500kHz)
3. High Isolation Capability
4. Low Cost
A Low Cost, High Bandwidth, Isolation Amplifier: An additional feedback winding linearizes the transfer func-
tion of the amplifier by matching the coupling characteristics to the isolated output.
Design Note
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 • FAX (603) 424-3460
