10

LT1684

The component selections for the active tracking supply

lowpass MFB filter configuration follow the relations:

m

2

Example:

Conditions: Output ring tone peak voltage = 100V

Ring frequency = 20Hz

Input duty cycle range = 10% to 90%

Filter Q = 0.707

Set:

Choose:

Then:

m

≈ .005

C2 = 4700pF

(sets m = 0.0047)

(4

π100)(1e–6)(.0047)(0.707)

This filter configuration yields a DC Gain of 100, a corner

frequency of just under 100Hz with gain reduction of only

0.1% at 20Hz, and a 10kHz carrier rejection of greater than

40dB at the output.

Active Tracking Supply Components

Given the previous discussion, implementation of an

active tracking supply system may seem almost trivial.

Active Tracking Supply Lowpass Multiple

However, bootstrapping an amplifier system about its

own output creates a complex myriad of inherent stability

and response issues. Attempting such a configuration

with generic “jelly-bean” components is not recommended

for the faint of heart or type-A personalities. The LT1684,

however, makes for a simplistic approach to active track-

ing component selection.

The high voltage MOSFET transistors used in the circuit

corresponding high voltage supply rail plus the opposite

maximum excursion of the output signal. For example, if

a system is designed with a 240V supply (+ 120V,

–120V) and outputs a ring signal that has a 100V peak

240/2 + 100 = 220V.

Active Filter Tuned Oscillator—

No PWM Input Required

A simple yet effective method of producing a high quality

sine wave is to place a high-Q bandpass filter and a hard

limited gain element in a positive feedback loop. This

circuit will oscillate at the bandpass frequency, producing

a sine wave at the filter output. The product of the funda-

mental component of the limiter and the filter gain at the

bandpass frequency determines the output amplitude.

This type of circuit is commonly referred to as an active

filter tuned oscillator.

HERTZ (Hz)

1

10

100

1K

10K

100K

1684 F02

–50

0

50

APPLICATIO S I FOR ATIO