The two basic amplifiers
In a non-inverting amplifier, the signal goes to the + input and the gain is 1 + Rf ÷ Rg, where Rg goes from the − input to ground. The input impedance is very high, so it doesn't load the source. In an inverting amplifier, the signal goes through Rin to the − input, and the gain is −Rf ÷ Rin, so the output is upside down. Its input impedance equals Rin.
Bandwidth
An op-amp's gain falls with frequency. The gain-bandwidth product, from the datasheet, divided by the noise gain (1 + Rf ÷ Rg for both circuits) gives the bandwidth. A 1 MHz op-amp with a gain of 100 has only 10 kHz of bandwidth, which is fine for audio speech filters but not much more. Use a faster op-amp, or split the gain over two stages.
Tips
- Keep resistors between about 1 kΩ and 100 kΩ for low noise and low offset.
- For single-supply circuits, bias the + input at half the supply and AC-couple the signal.
- Decouple the supply pins with 100 nF close to the chip.
Radio amateurs use op-amps for audio filters, microphone preamps and active antennas. The RC calculator sets the filter frequencies.
Questions
What is the gain of a non-inverting amplifier?
1 + Rf ÷ Rg. With Rf = 47 kΩ and Rg = 4.7 kΩ, it's 11.
What is the gain of an inverting amplifier?
−Rf ÷ Rin. With 47 kΩ and 4.7 kΩ, it's −10.
How much bandwidth will I get?
The gain-bandwidth product divided by the noise gain. A 1 MHz op-amp at a gain of 11 gives about 90 kHz.
Can I get a gain below 1?
With an inverting amplifier, yes. A non-inverting amplifier always has a gain of at least 1.