An op-amp amplifier with a gain of 10, simulated
Intermediate
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An op-amp amplifier is the circuit that makes a small signal bigger: a microphone, a sensor bridge, a thermocouple. The inverting amplifier is the simplest to reason about. The signal goes in through a resistor, Rin, to the op-amp’s inverting input, and a second resistor, Rf, runs from the output back to that input. The gain is -Rf / Rin.
The circuit above uses Rin = 1 k and Rf = 10 k, for a gain of -10. It runs from a single 5 V supply, so the non-inverting input is held at 2.5 V by two 10 k resistors and the output swings around 2.5 V instead of trying to go below ground. A function generator puts a 100 Hz sine of 0.2 V amplitude on top of 2.5 V, and a two-channel scope shows the input and the output. The output is a 2 V sine, ten times bigger and upside down.
Turn the generator’s amplitude up to 0.3 V. The output now wants 3 V of swing either side of 2.5 V, more than a 5 V supply has, and the peaks flatten. That is clipping, and it happens where the op-amp’s model says its output stops, not at a limit somebody typed into a drawing.
The op-amp is a single-pole macromodel with an open-loop gain, a gain-bandwidth product, a slew rate, headroom short of each rail and an output current limit. The default model has 10 MHz of gain-bandwidth. Open Analysis in the more menu, choose Frequency response, drive the generator and watch the output from 10 Hz to 10 MHz. The Bode plot is flat at 20 dB, ten times, until the bandwidth runs out near 10 MHz / 11, about 909 kHz, because an inverting gain of 10 spends it as a noise gain of 11.
Switch the op-amp’s model to a 741 and run it again. The corner drops tenfold, to about 91 kHz, because a 741 has 1 MHz to spend. And on 5 V a 741 stops 1.5 V short of each rail, so its output clips at 1.5 V and 3.5 V, which is why it is the wrong part for a 5 V circuit. Seeing that once on a scope is cheaper than learning it on a breadboard.
Change Rf to 22 k and the gain becomes 22. Put a capacitor across Rf and the amplifier becomes a low-pass filter.
Questions
Which op-amp models can I simulate?
A rail-to-rail default, the 741, the TL072 and the LM358, each with its own gain-bandwidth product, slew rate and headroom, and every parameter can be set by hand in the properties panel.
Why does my op-amp output clip on 5 V?
Because the output cannot go past the supply, and many op-amps stop well short of it. A 741 stops about 1.5 V from each rail, which leaves only two volts of swing on a 5 V supply.
Build this for real
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