Question

The input waveform of an AC non-inverting amplifier with single supply is given below. Find the output waveform?

I have been asked this question by my college professor while I was bunking the class.

Question is from AC Amplifiers with Single Supply Voltage topic in portion Operational Amplifier Applications of Linear Integrated Circuits

Answer 1

In an AC non-inverting amplifier with a single supply, the output waveform depends on the input waveform and the amplification factor, which is determined by the gain of the amplifier. The key characteristic of this configuration is that the op-amp is powered by a single positive supply voltage (e.g., +Vcc), meaning the op-amp's output cannot go below ground (0V), which can affect how negative parts of the input signal are handled.

Key Points of the AC Non-Inverting Amplifier with Single Supply:

1. Non-inverting configuration: The input signal is applied to the non-inverting terminal (+) of the op-amp. The output signal will follow the input in phase, meaning if the input goes up, the output also goes up, and vice versa.

2. Single Supply Operation: With a single supply, the op-amp operates between ground (0V) and a positive supply voltage VccV_{cc}Vcc​. This means the output will never go below ground, even if the input signal would cause the output to do so in a dual-supply configuration. Therefore, any part of the input waveform that goes below ground will be clipped to 0V (ground level).

3. Output Signal Clipping: For an AC signal, the output waveform will be clipped at the lower end to the ground level (0V), and amplified in a positive direction. The gain of the amplifier is given by:

   Gain=1+RfRin\text{Gain} = 1 + \frac{R_f}{R_{in}}Gain=1+Rin​Rf​​

   where RfR_fRf​ is the feedback resistor, and RinR_{in}Rin​ is the input resistor.

4. Effect of Input Waveform: If the input waveform has both positive and negative components, the negative part of the waveform will be clipped to 0V, and the positive part will be amplified according to the gain.

Steps to Find the Output Waveform:

1. Determine the Gain: The first step is to find the gain of the amplifier, which will determine how much the input signal is amplified.

2. Clip Negative Components: Any negative part of the input waveform will be clipped at 0V because the amplifier cannot output a negative voltage due to the single supply configuration.

3. Amplify Positive Components: The positive portion of the input waveform will be amplified by the gain factor.

4. Sketch the Output Waveform: After applying the above steps, the output waveform will show the amplified positive portions of the input signal and will clip at 0V for the negative portions.

Example:

If the input waveform is a sinusoidal signal with both positive and negative swings, and the gain of the amplifier is 2, the output will:

• Amplify the positive portion of the input by a factor of 2.
• Clip the negative portion of the input to 0V, since the op-amp cannot go below ground in a single supply setup.

This results in an output waveform that follows the positive peaks of the input signal, with any negative swings flattened at 0V.

Output Behavior:

• For a sinusoidal input, the output will have a sinusoidal shape for the positive parts of the input, and flat at 0V during the negative parts.
• The output waveform will be "rectified" in the sense that all negative portions are clipped to 0V.

This is the basic operation of an AC non-inverting amplifier with a single supply. The output waveform depends on the input waveform's polarity, the gain of the amplifier, and the limits of the single supply voltage.