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I've spent hours reading on buffers, non-inverting amplifiers and inverting amplifiers. I get the concept (though I don't understand all the formulas) and I can't make a simple voltage-follower out of a RC4558 Dual general-purpose op-amp.

Here's what I'm working with:

1V peak-to-peak sine wave @ 1kHz from a frequency generator output (50 ohms)

RC4558
Pin 1 - Output
Pin 2 - Inverting Input
Pin 3 - Non-inverting Input
Pin 4 - Ground
Pin 5 - NC
Pin 6 - NC
Pin 7 - NC
Pin 8 - +12 VDC

Signal directly connected to the Non-inverting Input. Output feedback is connected to the Inverting Input.

The output wave is 300mV, squarish / pulsating wave.

What am I doing wrong?

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    \$\begingroup\$ Please share a schematic, even a hand-drawn one. Also, what's the DC bias of the input sine wave? Is it AC coupled, or DC coupled? \$\endgroup\$ – Anindo Ghosh Feb 13 '13 at 19:27
  • \$\begingroup\$ As long as you don't have >10 reputation you can just add a link to the image and someone will edit your question to make it an image. \$\endgroup\$ – user17592 Feb 13 '13 at 19:50
  • \$\begingroup\$ Also, is the output wave 1 kHz or some other frequency? \$\endgroup\$ – Brian Drummond Feb 13 '13 at 19:52
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    \$\begingroup\$ I'm thinking biasing is missing, that op-amp needs 3 volts headroom at each rail (ugh!). \$\endgroup\$ – Anindo Ghosh Feb 13 '13 at 19:54
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Yes, of course you can use RC4558 op-amps for line-level audio. They have been used over the past 30 something years in lots of low end gear. They work, but the quality isn't very good: poor noise and distortion figures, poor driving ability of loads in the less than 10K range. Use something better like an LM4562 or the inexpensive and good NE5532.

It's not a great idea to leave unused devices unconnected. Connect the output to the inverting input, and convert the noninverting input to your V/2 voltage reference (or the ground in a proper dual-voltage circuit).

From your description of the circuit, you are omitting important biasing details. Since you are working with a single power supply between 0 and 12V. The center voltage is at 6V. That is to say, when there is no signal, you want the output of the op-amp to be approximately 6V. This means that the non-inverting input will have to be at 6V. The easiest way to do that is to bias that input to 6V with some resistors, and then couple the signal with a capacitor, so that the signal's AC swings are superimposed on the 6V bias.

enter image description here

Legend:

  • R1, R2 and C1 create a "phantom ground" reference voltage. The point between R1 and R2 is at 6V DC. Capacitor C1 ensures that this point has a low impedance to ground for AC signals, and also helps to reduce power supply noise at this phantom ground.
  • R3 conveys the 6V phantom ground to the + input. Because it is a 10K resistor, it establishes a 10K input impedance (common for line level audio). R3 also has another important role: it provides a path for an input bias current to flow into or out of the + input. Without this resistor, the only path is through the capacitor C2, which blocks direct current.
  • C2 couples the input signal to the + input, superimposing it on the 6V DC. The 5uF value is chosen so that it cleanly passes 20 Hz and up when combined with the 10K input impedance. Try to use a film capacitor. If you raise the input impedance, you can lower the capacitance. E.g. for a 100K input impedance, you'd need only a 0.5 uF capacitor for the same frequency response.
  • C3 couples the output signal to the next output stage. This output coupling cap can usually be omitted. If the next device needs one, it supplies it.
  • R4 and R5 are DC return resistors. If the previous or next device is DC coupled and sources a small current, these resistors complete the circuit for that current. Because otherwise your input and output are open circuits with respect to DC, due to the coupling capacitors. These resistors can usually be omitted, since most devices take care of their own currents internally. You certainly do not need R4 if the C3 coupler is omitted, because the output stage of the op-amp is a low impedance voltage source (sink for current).
  • The unused U1B is connected as described earlier. The phantom ground is conveyed to its + input, and it is in feedback.

Note: when constructing such a device, do not route the jack sleeve connections to a ground trace on the circuit board, even though the circuit suggests that. That creates a "pin 1 problem". The jacks should be grounded directly to the power supply (or if there is a conducting chassis, to the metal chassis, which is then grounded). If there is radio-frequency interference picked up by the cable shields, this approach prevents routing it to the circuit board where it will easily find its way where it isn't supposed to.

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  • \$\begingroup\$ Nice detailed answer - people can learn a lot from it. But don't forget the bypass caps! \$\endgroup\$ – Fred Hamilton Feb 13 '13 at 23:14
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From the limited information in the question, one likely issue is that the input signal is not biased to within the common mode input voltage range of the op-amp. This would cause the output to be driven to the ground rail, and only noise or such peaks as reach the operating range, will show up on the output.

Also, this op-amp is not designed to tolerate inputs down to Ground or below, hence operating without such biasing could compromise the op-amp.

A voltage follower such as below should work for you. I have also added a 5 KHz cut-off RC filter at the output, as a safeguard in case high frequency noise shows up in the output.

Voltage follower

(The not-quite-straight lines are due to it being rather late here)

The 0.1 uF decoupling capacitor showin in the schematic needs to be physically as close to the power and ground pins of the op-amp as possible.

Also, the uncommitted op-amp cannot be left floating as I have done: This app-note from Maxim provides some insights into both the why and the how of dealing with unused op-amps in a package.

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  • \$\begingroup\$ You left out the bypass cap! I'll give you a little while to fix this before -1 since I know you know better. \$\endgroup\$ – Olin Lathrop Feb 13 '13 at 21:00
  • \$\begingroup\$ @OlinLathrop It's called "artistic license". Added now. \$\endgroup\$ – Anindo Ghosh Feb 13 '13 at 23:39

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