How to Build a Ramp Generator with Transistors

Ramp waveform

In this project, we will show how to build a ramp generator circuit using transistors and a few other simple components, resistors and a capacitor.

This circuit requires no integrated chips.

A ramp generator is a signal generator which generates a ramp waveform. This waveform increases steadly as the capacitor is being charged until it hits its peak and then decreases even more dramatically as the capacitor is discharged.

Being that this waveform repeats over and over again, this generator can be seen as an oscillator.

If you connect to an output device such as an LED, it will turn the LED first with low brightness and as the amplitude of the ramp increases, the LED will get brighter. As the ramp hits its peak and starts to descend in amplitude, the LED fades out until it turns off. The process then repeats itself over and over again.

The details of how to build this circuit and how exactly it works is described in detail below.

If you have an oscilloscope, you can check the output waveform that the circuit produces. If you do not have an oscilloscope, then you can simply connect to an output device such as an LED to see the fade in, fade out process.

Components Needed

  • 2 2N3906 PNP Transistors
  • 2N3904 NPN Transistor
  • 4 10KΩ Resistors
  • 1KΩ Resistor
  • 10nF ceramic capacitor

So in this circuit we use 2 PNP transistors and 1 NPN transistor.

Although we've designated to use the 2N3906 as the PNP transistor and the 2N3904 as the NPN transistor, you really could use any other type of PNP and NPN transistor that you have. It doesn't have to be these.

The datasheet for the 2N3906 PNP transistor is found here: 2N3906 PNP Transistor Datasheet.

The datasheet for the 2N3904 NPNP transistor is found here: 2N3904 NPN Transistor Datasheet.

Even though the transistors operate different, from a back view of the transistor, right side up (with the terminal legs of the transistor pointing downward), from left to right, the terminals of the transistor are EBC: Emitter, Base, and Collector.

So when connecting the transistors, this information is needed.

Ramp Generator Circuit Built with Transistors

The ramp generator circuit we will build with transistors and a capacitor and resistors is shown below.

Ramp generator circuit

Below is the breadboard version of the above circuit so that you can see the exact wiring of the circuit on a breadboard.

Ramp generator breadboard circuit with transistors

For this circuit, we are using 5VDC.

All the way to the left of the circuit, are a 1KΩ resistor and a 10KΩ resistor in parallel. These resistors in parallel set up a bias voltage for the PNP transistor. This allows for the PNP transistor to turn up and allows current flow from the emitter to the collector of the PNP transistor. The current flowing through this transistor is about 30 microamperes. This is the same current that gets dumped into the 10nF ceramic capacitor, charging it up. This forms the rising edge of the ramp waveform. As the capacitor charges up more and more and the charge gets larger and larger, the ramp on the waveform steadily rises.

Once the voltage rises to a level high enough on the capacitor that turns on the PNP transistor that it is connected to the anode the ceramic capacitor, then the capacitor begins discharging. Think of it this way. When there is no power to the circuit, no current can flow because there is no power. Once we turn on the power, the first PNP transistor (to the leftmost part of the circuit) acts as a current source for the capacitor. The capacitor gets charged up by the PNP current source. As it gets charged, the voltage across the capacitor increases. Know that the voltage across a capacitor is proportional to the amount of current that charges it up. As the current flows into the capacitor, its voltage increases. Once the voltage reaches a certain threshold, the peak of the ramp waveform, it is high enough to turn on the second PNP transistor. Once it turns on the PNP transistor, this PNP transistor turns on the NPN transistor. Both transistors are now operating in saturation mode and are fully conducting. Being that the voltage at the capacitor is now high enough to turn on the transistor, the transistor goes from cutoff (not conducting) to saturation (fully conducting). Now that current can flow through the transistor, the capacitor discharges its current through the transistor. Once all the charge from the capacitor has been discharged from the capacitor, then there is not enough voltage to turn on the PNP transistor. Therefore, the 2 rightmost transistors no longer conduct current. The process starts over with the leftmost transistor, the current source, charging up the capacitor again.

This creates the constant ramp waveform of charging and discharging of the capacitor.

The 2 10KΩ resistors in parallel form a voltage divider. Since the voltage supply is 5V, it divides the voltage in half at the midpoint between the 2 resistors, creating 4.5V of power. This voltage is the bias voltage necessary for the collector of the NPN transistor and the base of the PNP transistor. This voltage is needed so that both transistors can turn on. Know that bipolar junction transistors (BJTs) always need biasing to the base of the collectors in order to operate. With the 4.5 V, biasing power is provided to the base of the first transistor and the collector of the second transistor. However, in order to turn on the first transistor, sufficient positive voltage is needed at the collector of the first transistor. This only occurs when the voltage of the capacitor reaches a certain threshold.

So this is a basic ramp generator built using transistors, resistors, and a capacitor.

There are variations of this circuit which can be done.

If you use a larger-sized capacitor, this would increase the time period of the signal. This is because with a larger capacitor, more charge can be held across the terminals. So since more charge can be held, it takes a longer time to charge up. Therefore, the ramp is longer. In the same way that a greater charge is held across the terminals, it takes a longer time to discharge this capacitor, since it stores more charge. So the incline and decline of the ramp are longer, making for a longer period and decreased frequency.

So the capacitor definitely affects the frequency of the signal. So if you try a larger capacitor such as a 100nF capacitor or a 1μF capacitor, you'll definitely see a decreased frequency. The same thing applies in reverse, decreasing the value of the capacitor increases the frequency. So if you use a 1nF capacitor, you'll see an increased frequency.

Another variation you could use is to adjust the amplitude of the supply voltage. By increasing the supply voltage, we increase the amplitude, and, thus, the peak of the ramp waveform. When we increase the voltage, the capacitor can now charge up to a higher voltage. So if we increase the 5V we are currently using to, say, 12V, you'll see that the amplitude of the signal rises. In the same way, if we decrease the supply voltage, the amplitude and, thus, peak of the signal decreases.

So these are variations we can do is to adjust either the frequency or amplitude of the ramp waveform.

To see how this circuit works in real life, see the video below.

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