There are many ways of generating analog sawtooth waveforms with oscillating circuits. Here’s a method that employs a single supply voltage rail to produce a buffered signal whose frequency can be varied over a range from 10Hz to 1MHz (Figure 1).
Figure 1 The sawtooth output waveform is the signal “saw” available at the output of op amp U1a. Its frequency is set by the value of resistor R6 which can vary from 120 Ω to 12 MΩ.
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U3, powered through R5, uses Q2 and R6 to create a constant current source. U3 enforces a constant voltage Vref of 1.2 V between its V+ and FB pins. Q2 is a high-beta NPN transistor that passes virtually all of R2’s current Vref/R6 through its collector to charge C3 with a constant current, producing the linear ramp portion of this ground-referenced sawtooth.
Op-amp U1 buffers this signal and applies it to an input of comparator U2a. The comparator’s other input’s voltage causes its output to transition low when the sawtooth rises to 1 volt. U2A, R1, Q1, R8, C1, and U2b produce a 100 ns one-shot signal at the output of U2b, which drives the gate of M1 high to rapidly discharge C3 to ground.
The frequency of the waveform is 1.2 / ( R6 × C3 ) Hz. With the availability of U3’s Vref tolerances as low as 0.2% and a 0.1% tolerance for R6, the circuit’s overall tolerances are generally limited by an at best 1% C3 combined with the parasitic capacitances of M1.
Waveforms at several different frequencies are seen in Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, and Figure 7.
Figure 2 10 Hz sawtooth for an R6 of 12 MΩ.
Figure 3 100 Hz sawtooth for an R6 of 1.2 MΩ.
Figure 4 1 kHz sawtooth for an R6 of 120 kΩ.
Figure 5 10 kHz sawtooth for an R6 of 12 kΩ.
Figure 6 100 kHz sawtooth for an R6 of 1.2 kΩ.
Figure 7 1 MHz sawtooth for an R6 of 120 Ω.
Figures 3 and 4 show near-ideal sawtooth waveforms. But Figure 2, with its 12 MΩ R6, shows that even when “off,” M1 has a non-infinite drain-source resistance which contributes to the non-linearity of the ramp. It’s also worth noting that although U3’s FB pin typically pulls less than 100 nA, that’s the current that the 12 MΩ R6 is intended to source, so waveform frequency accuracy for this value of resistor is problematic.
Figures 5, 6, and 7 show progressive increases in the effects of the 100nS discharge time for C3 and of the finite recovery time of the op amp when its output saturates near the ground rail.
These circuits do not require any matched-value components. Accuracies are improved by the use of precision versions of R4, R6, R7, and U3, but the circuit’s operation does not necessitate these.
Christopher Paul has worked in various engineering positions in the communications industry for over 40 years.
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