You’ll need a multimeter for this – we’ve been using Amazons own “Amazon Commercial 6000 Count Compact Digital Multimeter” which you can buy through the link – we can’t say its better than any other but we like it and it’s not let us down yet.
Now you have a multimeter, you’ve built your board…but you need to bias the transistor(s) – don’t panic, it’s actually easy! You can read a lifetimes worth of content in books and online about the calculations needed to accurately work out which resistor you need based on the datasheets of the particular transistor you’re using – but there’s an easier way!
The basics are like this, the transistor needs a certain level of voltage supplied to the COLLECTOR pin (this varies per transistor so please check!) in order for the transistor to work.
Too little voltage – it will be starved and wont work properly.
Too much voltage – it also wont work properly.
So the key to this is getting the right voltage into the transistor. That usually happens in one of 2 ways, either a fixed resistor or a trimmer.
If you have a trimmer (our boards use trimmers!), it’s easy to adjust the resistance just by turning the dial on the trimmer. If you have a fixed resistor (just a standard resistor) – it’s a good idea to socket this first and you can swap and change resistors until you get the desired voltage to your collector pin.
The images below show a trimmer and a socketed resistor – both are feeding the collector pin of the transistor on the relative boards.
You’ll need power and ground connected to your board to get the bias working – without power…you’ll get a zero reading…and yes…we’ve done that!
Set your multimeter to voltage reading (this is the V with a solid line, and a dotted line below it).
Connect the negative lead (black) from the multimeter to a ground point on the board – this can be the main ground connection from the power, or the ground side of a resistor etc.
Next, connect the positive lead (red) from the multimeter to the COLLECTOR pin of the transistor – it can be awkward doing this the first few times but you’ll get the hang of it.
You’ll notice a reading on your multimeter – this is the voltage being delivered to the COLLECTOR pin.
Depending on the board and what the build guide says, you can/will need to adjust the trimmer/resistor to get the voltage correct – a lot of the time its a ballpark number and you can tune it by ear, if it sounds good when you play it…you’ve got it!
If it’s a trimmer – just slowly turn the dial and you’ll notice the voltage change either up/down – once you get to the right voltage, you’re done!
If it’s a resistor, just pop it out and increase/decrease the size of the resistor accordingly until you get the right voltage, then you can solder it in.
The first few times you do this – it will be a pain in the ass and frustrating – but you’ll get the hag of it!
Our next article will be on measuring HFE and Leakage on germanium transistors with the Peak DCA55, so check back for that soon!
