Transistor Amplifier
A versitile PCB layout for common topologies
Table of Contents
Introduction
I hate working with breadboards, I'm terrible at wire management and I have no where safe to put the breadboard and the project ends up breaking up.
So wherever possible I prefer to use a PCB, ideally something someone else made but I was shocked to find no suitable simple transistor amplifier board. It seems like the kind of thing you'd get for pennies on ebay.
I set about fixing that and hopefully making something someone else finds easy. The board was laid out with home etching in mind as well as making moving from sketched schematics to the board layout simple.
Theory of the Board
Most transistor amplifiers have similar layouts with parts which can be swapped out i.e. a resistor or a diode or omitted. I wanted my project to support a relatively wide range of tologies and be chainable.
Features
- Common Emitter
- Common Collector
- Emitter Resistor
- Emitter Resistor with bypass capacity
- Collector Feedback
- Fixed Bias
- Divider Bias
- Diode Bias
- In and output capacitors
Layout
So here's the idea, you can pretty much omit, jumper or populate each part. For example, R2 could be omitted for a fixed bias (using only R1), it could be a diode with R1 to form a reference bias. R1 and R2 could be omitted with Rf forming a collector feedback bias. It's all pretty self explanatory once you look at some common layouts.
Jumpers allow output to be taken from either the collector or the emitter.
Figure 1: Here is my prototype sketch of the layout.
Schematic
You'd probably never have all these parts populated. The capacitors and output enabled are intended to be jumperable.
Figure 2: Here is the schematic most of these will be unpopulated or have jumpers in practice.
3D view
Figure 3: Topside 3D view of the PCB
PCB
I've ordered a set of boards for testing from JLCPCB.
| Manufactorer | JLCPCB |
| Layers | 2 |
| Width | 72.4mm |
| Height | 36.8mm |
| Boards | 5 |
| Cost | £6.90 |
Figure 4: File submitted for JLCPCB manufactoring.
Figure 5: Board back from JLCPCB
Home etch files
I etch at home but Gerber files are attached below. The resist is mirrored about the horizontal axis which is typically needed, the layout is not.
Figure 6: Layout of the parts and drill holes
Figure 7: For home etching REVERSED
I've made a few of these (well the original version) and it worked easily with a home etching setup checking on OSH Park it estimates 3 boards for $21 (87x52mm 2 layer). That's pretty steep but this boards has loads of dead space most of it down to those axial resistors (R1/2/c/e). One option would be to rotate them or but I couldn't help trying to squeeze it down a bit. Lets call the above the Phat model and we now have…
Slim Model
I've also made a slim version (which is untested and I think tricky for home etching). But should be cheap to have made (OSH Park estimates $7 compared to the phat versions $21 for 3 boards).
Here the resistors are soldered in a radial format (axial resistors can be folded over).
I don't fancy getting in there and soldering the rats nest around the transistor but it's probably less tricky than it looks.
You could always scale it up and make a larger PCB but the tracks would be very thick and it might be hard to keep the iron hot enough especially with the ground fill.
3D view
Figure 8: Topside 3D view of the PCB
Layout
Figure 9: Layout of the parts and drill holes
Usage
My first use of this was for a simple electret microphone as in figure 10. You can see the abuse of the layout here to add most of a bandpass filter on this board it's output goes through a final input resistor to complete it. It's a collector feedback bias NPN amplifier with a BC547C (hfe ~650).
Figure 10: Electret microphone board
Changes
- 05-04-2021
- Boards ready for shipping from JLCPCB
- 31-03-2021
- Mark transistor Emitter Base and Collector in both models. Add CCSABY and OSHW logos and attribution. Update 3D Renders. Ground fill back copper.