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A circuit is described which converts a staircase input waveform into a linear current ramp through a coil. The coil inductance forms part of a second-order Bessel filter.
English (United States)
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Bessel Filter Coil Driver Circuit
A circuit is described which converts a staircase input waveform into a linear
current ramp through a coil. The coil inductance forms part of a second-order
In a CRT display system a digital-to-analog converter produces a staircase
input waveform which is to be converted into a linear current ramp through an
inductive deflection coil. A staircase waveform would cause an unacceptable
ripple if used to drive the coil without smoothing. We have found that a Bessel
filter driver circuit produces a good approximation to linear smoothing.
In the circuit diagram, a staircase input current is applied to one terminal of
an operational amplifier, the output of which is connected via an emitter follower
transistor T to one end of the deflection coil with inductance L in series with a
current-measuring resistor R3. The input terminal is connected to the emitter of
transistor T by R1 and C in parallel and also via resistor R2 to the other end of
The gain of the circuit is determined by the resistors R1, R2
and R3. Coil inductance L forms part of a second-order Bessel
filter whose response is governed by the time constants L/R1 and R3C.
These are selected to give the best approximation to a linear rise
of current I through inductance L. A small current is maintained
through resistor r to reduce transients through the operational
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