Join Barron Stone for an in-depth discussion in this video Protect against large signals, part of Electronics Foundations: Semiconductor Devices.
- Some electronic circuits are sensitive and their components can be easily damaged if they receive too large of an input signal, much like how loud noises can hurt my sensitive ears. If somebody is speaking to me at a reasonable volume - - Hey Ben, how are ya? - I'm doing fine Ray, thanks - that's fine. But if Ray's yelling in my ears, - Who are you talking to?! - that's too loud and can damage my ears, so I need a way to protect myself from those loud noises, like earmuffs.
- I still don't know who you're talking to! - [Ben] When working with electronics, a clipper circuit can be used to protect sensitive equipment by limiting the amplitude of input signals. The clipper will remove any parts of a signal that are above or below a certain threshold voltage. For example, the equipment I'm using right now to record this video, uses an amplifier to boost the audio signal coming from my microphone, and then it sends that amplified signal to an analog-to-digital converter connected to my computer.
If for some reason the output voltage from the amplifier was too powerful, maybe I talked too loud into the microphone, I could potentially damage the analog-to-digital converter chip. So, to protect this sensitive converter chip from the amplifier, I can insert a clipper circuit between them. The clipper will limit the signal from the amplifier to keep it within a safe voltage range. If I was only concerned about protecting my circuit against negative voltages, then I could use this simple circuit with the diode and a resistor to clip out negative voltages below a certain threshold.
When the input voltage from the signal source is positive, the diode will act like an open circuit, so current will flow passed it and the output voltage will basically be unchanged. But if the input voltage turns negative and drops below the forward voltage of the diode which is around point six volts, then current will flow through the diode and back through the resistor to the signal source. At the output of the clipper circuit, the voltage will only drop as low as that forward voltage of the diode.
So the output will never drop below negative 0.6 volts. To demonstrate that, I've built a clipper circuit to remove negative voltage on my breadboard using a 1N4148 diode and a one kilo ohm resistor, and I've configured my function generator to provide a 10 volt peak-to-peak sine wave as the input signal. My oscilloscope is displaying the input signal from the function generator on channel one in yellow, and the output signal from the clipper circuit on channel two in blue.
I can see that this circuit allows the positive parts of the AC signal to pass through more or less undisturbed, but it limits the output voltage from ever dropping below negative 0.6 volts, which is the forward voltage of the diode. If instead of clipping off the negative parts of the input signal, I want to keep those and cut out the positive parts, all I need to do is switch the direction of the diode by turning it around. Now the clipper circuit is limiting the voltage from ever rising above point six volts, but it allows the negative part of the AC signal to pass through it freely.
If I wanted to limit the positive and the negative output voltage from my clipper circuit, from ever going above or below certain thresholds, I can do that by using two diodes - one in each direction. In this arrangement, if the input voltage ever goes above the forward voltage of the diode on the left, that diode will turn on and drain current to ground, thus limiting the output to always be less than 0.6 volts. And a similar thing will happen in the negative direction.
If the input voltage ever drops below negative point six volts, the diode on the right will turn on allowing current to flow through it, and ensuring that the output voltage remains above negative 0.6 volts. When I send a small input signal shown in yellow through the dual-direction clipper, the output voltage, shown in the blue, will be more or less that small. But if I increase the amplitude of the input signal to be much larger, the clipper circuit will restrict the output to stay within plus or minus 0.6 volts.
The range in my clipper circuit doesn't have to be limited by the forward voltage of the diodes I'm using. I can adjust the range of the clipper circuit by incorporating other circuit elements to provide reference voltages to bias those diodes. In the example shown here, the 1.4 volt source on the left branch combined with the diodes forward voltage of 0.6 volts, will prevent the output voltage of this clipper circuit from ever rising above two volts. And on the other branch, a negative 2.4 volt source, along with the upward pointing diode, will prevent the output voltage from ever dropping below negative three volts.
Keep in mind that the circuit shown here is simplified by showing three different voltage sources to illustrate the concept. In practice, those reference voltages would be generated using some combination of other circuit elements that act as a voltage regulator.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals