Datasheet

LM13700

SNOSBW2E –NOVEMBER 1999–REVISED MARCH 2013

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Figure 35. Voltage Controlled State Variable Filter

Voltage Controlled Oscillators

The classic Triangular/Square Wave VCO of Figure 36 is one of a variety of Voltage Controlled Oscillators which

may be built utilizing the LM13700. With the component values shown, this oscillator provides signals from 200

kHz to below 2 Hz as I

is varied from 1 mA to 10 nA. The output amplitudes are set by I

× R

. Note that the

peak differential input voltage must be less than 5V to prevent zenering the inputs.

A few modifications to this circuit produce the ramp/pulse VCO of Figure 37. When V

is high, I

is added to I

to increase amplifier A1's bias current and thus to increase the charging rate of capacitor C. When V

is low, I

goes to zero and the capacitor discharge current is set by I

The VC Lo-Pass Filter of Figure 32 may be used to produce a high-quality sinusoidal VCO. The circuit of

Figure 37 employs two LM13700 packages, with three of the amplifiers configured as lo-pass filters and the

fourth as a limiter/inverter. The circuit oscillates at the frequency at which the loop phase-shift is 360° or 180° for

the inverter and 60° per filter stage. This VCO operates from 5 Hz to 50 kHz with less than 1% THD.

Figure 36. Triangular/Square-Wave VCO

Product Folder Links: LM13700