Op-Amp AC amplifier with highpass and lowpass filters

• For use with MCP6001 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
• MCP6001 and "equivalent" amps can be found here

Basic configuration

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

• 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

Falstad simulation available here CLICK CLICK