# Op-Amp AC amplifier with highpass and lowpass filters
* For use with [MCP6001](https://ww1.microchip.com/downloads/en/DeviceDoc/MCP6001-1R-1U-2-4-1-MHz-Low-Power-Op-Amp-DS20001733L.pdf) or similar amplifier.
* Works with mV inputs centered around GND
* Few components, 0603 passives for easy soldering
* Schematic drawn on the PCB, so you don't need any stupid documentation
* Designed at ELAB, the PCB can be found [here](https://elab.kth.se/parts?q=opamp&p=2048)
* MCP6001 and "equivalent" amps can be found [here](https://elab.kth.se/parts?q=mcp&p=1630)

![Image](images/pcb.png)

# Basic configuration

![Basic configuration](images/basic.png)
## Behaviour of the example configuration:
* 11x gain
* Bandpass 20Hz - 15kHz (approx, not textbook perfect!)
* Supply voltage 3-5V
* Makes 10mV signal centered around GND into a 110mV signal centered around 2.5V
* Output centered arround VCC/2
* To directly drive headphones you must add a DC blocking capacitor! (between OUT and your headphones, 100uF should be fine)

## Easy adjustments for people in a hurry:
* More gain? -> Increase R2 with respect to R1
* Pass higher frequencies? -> Reduce Clp or Rout (or both)
* Pass lower frequencies? -> Increase both Rb+ and Rb- (both should be same)
* Lower center voltage of output? -> Increase Rb+ relative to Rb-
* Achive the opposite of any of the above? -> Do the opposite of any of the above

Note! The MCP6001 has a GBW of 1MHz. This means, if the gain is set to 100x, the amplifier will drop 3dB at 1MHz/100x = 10kHz! For very low input frequencies (<5Hz), you may need to increase C1 and C2. 

# Design details

## C1, Rb+ and Rb- together form a combo bias and highpass filter
* Since the op-amp does not have a negative voltage rail, the signal can not be less than GND
* Therefore, the input must be biased - pulled towards some positive voltage
* If Rb+ = Rb-, then the bias will be VCC/2 - the signal will be centered around VCC/2
* Adjusting them relative to each other allows you to pick a different center voltage
* The parallel resistance of Rb+ and Rb-, together with C1 forms a HIGHPASS FILTER
* Increasing the resistance lowers the pass frequency

## R1 and R2 set the gain
* Gain = R2/R1 + 1
* Gain is limited by the GBW (gain-bandwidth product) of the op-amp used
* MCP6001 has a GBW of 1MHz, so max frequency = 1MHz / gain (3db loss at that frequency)
* If you need higher frequencies / more gain, you can use a different op-amp, like an LMV721

## C2 makes the output centered arround the biasing frequency
* Whatever voltage Rb+ and Rb- form, will be the center frequency of the output
* In essence, the gain for DC is 0 (since the gain resistors R1 and R2 have not DC path to GND)
* C2 just needs to be big enough, when it is big enough it does not affect filtering

## Rout and Clp can be used for an output lowpass
* Classic low pass filter, nothing to see here

## Rout and Clp can instead be used for an output highpass
* Just swap the position of the capacitor and resistor. It's ok, they fit.

## Cbypass must be between 100nF and 1uF
* This is to ensure stable operation of the op-amp

## An additional bypass near the VCC / GND pins of the header
* An additional bypass capacitor may be placed there, if necessary

# Soldering tips

![Soldered board](images/soldered.jpg)
* Solder under the stereo optical microscope, look into the eyepiece, not the LCD display
* Start with the Op-amp
* Then the resistors and capacitors
* Last the pin header
* If you make the pins face down, the PCB looks cool on a breadboard
* If you make the pins face up, its easy to re-work the soldering

# Simulation

![Simulation](images/simulation.png)
[Falstad simulation available here CLICK CLICK](https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEDYCYDMqDsrUE4AOLSAgsJANiSQBZ1kBTAWjDACgBDEXG8CyEOnQU+AvMijh48btAR48WAnnQIMqGpAR1Z8wnHR5IRinSxTpbAO7cN4GrzwiwDqGwBOIJ-cciCAgXJ4Dy9nV29UEkkguDYAY1CfPiTeGGlICFRoGgV-BCxVBTAcIihYDLcAc0TIghq7ALYAJUT-RJo6gJBNCS6YBGtbXmKRHiG3TzGxoVGGi1ibGfGlsH4J8bHVgTHA4IStwUjRbgQRVPL4BSvrm4UQJiyclQINCnQS9EhzuAhINmqDphAmslo1FsJjsI6gc-p4occ6Khjrs-glEQjMONhswwEj0NB+OgCAgCMIEFoaBpFGUflVupixujUKdJH8AB73KjIXgUJFKbrgOoASQAdgAHACuABcADoAZwAFgBLSoKsUcOVymUykUcEUAE3lACMlRr5XEAPYAWyNFrYHNwSE+Akw5k+9GGdQAglaxQAbJUAMyVcQ4UqVFpF9qk5jAkHoYAhcbOgpATSNTGjrGBsZ0xQEnrTRplYujefIIGI4DAdULAGElVGHRRxFTuHgkcykYWmuwOTXO2puFoTrRU01UIMDps1syROzwFg3cVhy6jvXJxyicMVzQl+B16mAPLSyWyuXBv1ShjuQbog4dYGNDk5WPyZDoYa8gU1kBHgCqAAqbD5skLi8EwphJEi5IgP4cBlL8aBsBa3DmEiBbGOIWDQGAeAdDg2D5AQvLYDStKBGwQA)