Dictionary of Electrical and Electronic Terms
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Sawtooth Waveform

Saw Tooth. A waveform that appears like teeth on a saw. A Saw-Tooth waveform has a slow linear rise time and fast fall time [flyback]. Also refer to Periodic Waveforms.

Sawtooth Waveform

The term sawtooth refers to the shape of the signal, and so could have any rise or fall time as long as the waveform retains the basic shape of a saw tooth.

A Sawtooth Generator. is a circuit that generates a saw-tooth waveform either from an external input or by self-oscillation, as in a relaxation oscillator. A circuit designed to produce a sawtooth function will have a very slow linear ramp which rises from a steady-state level to a peak. Once the peak voltage of the ramp is reached the voltage will fall very quickly back to the starting level. The fall time is much shorter than the rise time, but not instantaneous, although it appears that way in comparison to the rise time. The fall time is also referred to as the flyback when the signal is used as a sweep generator.

Sawtooth Oscillator

A Unijunction Transistor [UJT] is used in this example to function as a sawtooth oscillator. The UJT is selected for its voltage range and frequency range so it matches that of the desired output signal. The resistor and capacitor is selected for the frequency of oscillation, and example component values are provided. No input signal is required for this circuit, other than the supply voltage. The output is taken off the node between the resistor and capacitor.

Unijunction transistor saw tooth signal generator
UJT Saw Tooth Generator

Unijunction Transistor Circuit Operation

The Uni-junction Transistor [UJT] Q1 does not require an input like the transistor circuit example below. The circuit functions as an oscillator and operates off the charging and discharging of capacitor C1. The charging rate or Rise Time is set by resistor R1 and capacitor C1, the capacitor discharges through the UJT. Once the capacitor discharges, it begins to charge through R1 again.

The Emitter and Base [B1] junction begin as off. C1 starting out at 0 volts will charge up until it reaches one diode drop above the Emitter 'E' at which time the UJT will conduct. The capacitor discharges through the UJT [E & B1] while it is conducting. Once the UJT stops conducting the capacitor will begin to charge again. Resulting in a slow rise time [R x C] and a rapid discharge time which determines the oscillation frequency. The output voltage is the voltage drop across R1 and C1. If you add a resistor between B1 and ground the discharge rate of the capacitor will increase. This circuit is also called a Relaxation Oscillator.

2N4870 Unijunction transistor

Of course you can also make the frequency variable by adding a trimmer, as the current adjustment. Replace R1 with a 25k ohm trimmer. The center tap of the trimmer makes the C1 and the Emitter of Q1 connection. The top side of the trimmer stays connected with the supply voltage [20 volts]. While the other end of the trimmer is left unconnected, as in a rheostat configuration.

Sawtooth Generator

Transistor saw tooth signal generator
Transistor Saw Tooth Generator

This circuit configuration requires an input pulse to generate a sawtooth output. The proper input signal would be a square wave which would be used to turn the transistor on and off and define the length of the sweep and flyback portions of the signal. The period of the input needs to match the intended charging time of the timing circuit.

Sawtooth Circuit Operation; The transistor Q1 is a PNP transistor picked to operate at the frequency of the desired sawtooth frequency and of course the required output voltage.

While the transistor is off [in cut-off] capacitor C1 charges through resistor R3 up to Vcc. This charging time is the rising slope of the sawtooth and also called the sweep time in particular applications. The timing of the slope is depended on the values of the resistor and capacitor [R x C]. In this example 5k x 1uF, or 5mS.

Once capacitor C1 charges up to with a diode drop of the Base voltage, transistor Q1 turns on. As soon as the transistor starts conducting, capacitor C1 is shorted and discharges through the collector emitter junction. The rise time of the saw tooth is the time it takes the capacitor to charge through resistor R3 [C1 x R3]. The fall time is the time it takes for the capacitor to discharge through the transistor. R1 is a bias resistor to set the Base voltage.

Sweep Generator

The vacuum tube circuit to the right is another example of a circuit that outputs a sawtooth waveform. This circuit was used as a sweep generator in an oscilloscope or other display. The ramp or sweep portion of the output is used to move the electron beam from left to right across the display, while the flyback or retrace portion brought the beam back to its starting point.

Of course a FET or transistor could also be used to perform the same function as a Triode Tube. This circuit is used as an example to show a vacuum tube being used as a sawtooth generator and a second method of changing the sweep time [a switch]. The switch is used to change the timing of the sweep time, just as the variable resistor is used in the circuit above it.

RC Time Constant

The Time Constant [TC] is a measure of time based on the amount of change in voltage. A capacitor is considered charged or discharged by 5TC.

The important difference with this waveform is that 1 TC [Time Constant] in the rising voltage is much greater than 1 TC for the falling voltage. The rising voltage time constant is based on the values of the resistor and capacitor, while the much faster falling voltage is based on the capacitor and resistance of the UJT [which is much smaller than the physical resistor].

Another important consideration is to only use the linear portion of the capacitors rise time. Only during the first time constant is the ramp linear, or some what linear. As the capacitor is allowed to charge further the charging time slows more and more. Of course the ramp of a saw-tooth is linear over the rise time. The same applies to the discharge time of the capacitor. The longer the discharge time the less linear the discharge becomes.

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