Rail-to-rail enter and output (RRIO) op amps, each FET and BJT, have been round for a very long time, however the prospects they supply for versatile waveshape technology with easy circuits can nonetheless shock. Determine 1 is an instance, outputting a wide range of precision waveshapes whereas utilizing only one twin RRIO op amp as the one lively system.
Determine 1 Buffered triangles, sawtooth waves, and sq. waves from two op amps.
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Right here’s the way it works.
Amplifier A2 integrates the distinction between A1’s rail-to-rail output and waveshape management R2 to supply triangle output Tout. The time fixed of the mixing is ready by R1C1 which mixes with R2 to supply constructive and damaging ramp charges of:
dV+/dT = VR2/R1C1 and
dV–/dT = (VR2 – Vdd)/R1C1.
As proven in Determine 2, a symmetrical triangular waveshape outcomes if VR2 = Vdd/2, a constructive ramp sawtooth if VR2 < Vdd/2, and a damaging sawtooth if VR2 > Vss/2.
As might be famous later, sq. wave Pout’s obligation issue can also be managed by R2.
Determine 2 Tout waveshape versus R2 the place a symmetrical triangular waveshape, a constructive ramp sawtooth, and a damaging ramp sawtooth might be realized.
The Tout waveshape is symmetrical round Vdd/2 and its peak-to-peak amplitude is ready by R3 over a variety extending from 0 (totally CCW) to Vdd (totally CW) as proven in Determine 3.
Determine 3 Tout amplitude versus R3 the place the Tout waveshape’s peak-to-peak amplitude is ready by R3 over a variety from 0 to Vdd.
A1 acts as a comparator triggered by the distinction between Tout (as scaled by R3’s setting) and the Vdd/2 reference voltage offered by R5/R6. Optimistic suggestions looped via R4 makes A1 periodically flip-flop and reverse the route of the Tout triangle ramp. The ratio of R4 to R3 determines the height excursions of Tout required to make A2 change states and thereby units Tout’s Vpp amplitude.
A1’s rectangular 0 V to Vdd sign is offered because the Pout output, with an obligation issue that goes from 0% to 100% as R2 is diverse from 0 to Vdd (totally CW).
Oscillation frequency is affected by all three pots, however Tout waveshape, amplitude, and Pout obligation issue are all mutually unbiased. Subsequently, the quickest and easiest way of setting the varied parameters (the tactic that leads to the least “tail-chasing”), is to set R2 and R3 first to acquire the specified waveshape and amplitude, then modify R1 to the specified frequency.
Relying considerably on the selection of Vdd, complete present draw is ~2mA unique of output loading. The utmost oscillation frequency utilizing the TLV2372 is ca:10kHz however quicker amplifiers can be found if extra pace is required, as are op amps able to dealing with extra voltage. And naturally, a bipolar provide would accommodate bipolar outputs. Chef’s selection!
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Almost 100 submissions have been accepted since his first contribution again in 1974.
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