# Understanding Schematics

Hello there,

I have basic knowledge of electronics, but this schematics is getting really hard to understand, Can someone please explain it to me whats is happening here, Its an Amplifier for a very small signal.

I am also willing to pay, if someone can design the pcb and schematics for me in eagle, which I could order at a board shop.

I am also willing to pay, if someone can design the pcb and schematics for me in eagle, which I could order at a board shop.

Please click on 'report to moderator' and ask them to move this to Gigs and collaborations', the folks there take paid commissions.

What don't you understand about the schematic? I don't know every detail but there are 3 amplifying stages with gain control on the last one. The capacitors across the feedback resistors limit the highest frequency amplified, so I guess it's a filter as well. I can't be more specific, but I am sure someone else can.

It is simply a high gain, low frequency audio amplifier. It amplifies the output of the HB100 microwave motion detector front end.

There must be plenty of web sites out there with tutorials on basic op-amp circuits.

Break the circuit into sections. There are 3 amplification stages using opamps. The first is a non-inverting gain stage, Operational amplifier - Wikipedia

The subsequent two stages are inverting amplifiers, Operational amplifier - Wikipedia

Above these there is a virtual ground provided by a voltage divider, and this goes to the inverting stages, Virtual Ground Circuits

The top right has the power supply part of the circuit, drawn backwards to make it more confusing I guess...

Between the stages are capacitors to ac-couple, losing any DC offset. The bandwidth of the circuit appears to be from a few Hz to a few 100Hz. The 2.2nF capacitors set the high frequency roll off together with the 1M resistors.

Note this circuit is problematic in that the gains are 101, -122, -122, giving an overall gain of 1,500,000.
For this to be stable careful attention needs to be paid to layout and supply decoupling, you' probably want the
first stage supply rails to be further decoupled to avoid unwanted feedback affecting it. With this much gain
poor layout risks making an oscillator. Its also likely to be sensitive to pick-up on the input, and would benefit from being in a screened metal box.

The LM358(*) is a poor performer at best, and isn't suited to this application with high gains. A precision
rail-to-rail device with low input offset voltage and better bandwidth would be ideal. High gain stages have
less bandwidth in opamp circuits (which is why the amp is in 3 stages to start with).

(*) The 358 starts to struggle at audio frequencies, most opamps are good for 100's of kHz or more.

Thanks a lot everyone who replied and Thanks Mark for taking the time for all the explanation. I will be going through all links for better understanding.