A BIAS: Right in the middle

How boring if you’re biased right in the middle eh?  You don’t lean to the left or the right, you’re just middle of the road for everything – as if you don’t actually have an opinion.  Well, in audio amplifier designs that’s exactly what you want – a totally neutral middle-of-the-road stance when it comes to music.

Several of you have asked me to expand this little mini series on tubes and transistors by one day to help understand bias, what it means and how it works.  Understand that this is actually a bigger subject than I can comfortably cover in a small daily post but I’ll do my best – and to do this let’s design a single ended amplifier together – right here in this post.

Remember in our understanding of the different types of devices we have two: voltage devices (tubes and FET’s) that are always on and we have to turn them off – and current devices (bipolar transistors) that are always off and we have to do the opposite.  With this understanding in mind we can now easily appreciate that each of these devices can be used in one of two ways: as either a switch (on/off) or an amplifier (moving between the two extremes of on/off).

To make a tube or FET a switch we need to simply turn it all the way off or let it go back to all the way on – and then the opposite for the bipolar transistor.  Just as a side note – in the late 1940′s early computers used vacuum tubes as electronic switches requiring huge rooms and building full of them to perform even simple calculations.  The term “bugs”, associated with computer program glitches, is rumored to have come from this era as the glowing tubes attracted moths to the light that wreaked havoc in the systems.

To make an amplifier we need to stay away from using the device as a switch – we never want to reach the two extremes of always on or always off if music is going to come out of our device without distortion (the always on state is called clipping).  This means we need to use the device in its in between state, and we want to begin our journey right in the middle.

So, let’s design our little amplifier using a single device – in this case we’ll choose a JFET and design a single ended amplifier.  To do this we need a few elements: a battery and 4 resistors.  We want a gain of 10 for our amplifier – meaning if we put 1 volt on the input we will get 10 volts on the output.  This is easy.  Remembering our device has 3 nodes: the signal input (gate) and the 2 battery inputs (drain and source) we place a resistor on the drain and connect it to the + of our battery and another resistor on the source and the – of the battery.  We make sure the + resistor is 10 times bigger than the – resistor (this sets our gain).  We’re almost ready to amplify our signal.

On the input side (gate in a FET, grid in a tube and base in a transistor) we want to make sure the device sits right in the middle of the battery voltage – this is important because we want our musical signal to have room to move – and move somewhere away from the two extremes (always on or always off).  This process is called biasing the amplifier.

Let’s imagine we have a 30 volt battery – we want our device to therefore rest at half of that or 15 volts.  In a FET, tube or transistor this is really easy, we simply use two more resistors on the input that have a ratio such that we get 1.5 volts at their meeting point.  One goes to the – of the battery and the other to the + and when our little amplifier sees this, it produces 15 (1.5 x 10) volts on the output.  Bingo, we have an amplifier.  The output of this amplifier is taken at the junction of the + resistor and the drain.

Now, when you put our phono cartridge on the input, the tiny voltage is amplified and we get a 10 times larger signal on the output which makes music!

Sorry this was so technical and it’s the last one that will be – promise.  Tomorrow we wrap up the series with some comments about how all this ties together.

Let me leave you with one interesting notion, that of field effect.  Remember that in a tube or FET there is no physical connection between the input and the output – everything is controlled through a mysterious “field” that goes through the air and magically turns the device off or somewhere in between on and off?  How the heck is that possible and is there an example of this in real everyday life?

Sure – and this is one I delight in leaving people with because it gets your brain spinning.  Picture yourself driving in your car down a busy city street.  Suddenly, the light ahead turns yellow and you slow down.  The light turns red and you stop and wait.  The light turns green and you go.  Sound familiar?  This is the same exact way a field works – only this time the field is working on you.  In the traffic example a change in small particles called photons has this effect and in our transistor the particles are called electrons.  In both cases there’s no apparent physical connection or touching yet control happens perfectly.

Now you know you’re no different than an amplifier.  Just make sure you sit in the middle and don’t go running any red lights.

Paul McGowan

Leave a Reply