Colorless writings, part 16 – The Transistor

A transistor? As in electronics 101?

Yes. A transistor. A rather simple device that we’ll find in almost any stompbox, in form or another. Transistor is usually a triode. Meaning that there are three terminals and all those three terminals have their own purpose. You know, a tri-ode. Three terminals. As opposite to diode – di-ode, which has two. Simple analogy forward to tubes that may be pentodes and so on. Transistors can be used for a few different purposes, most notably as switches and signal amplifiers. The latter one is is the one i’ll be focusing on. And we usually can find two distinct types of transistors – NPN and PNP. I’ll be focusing on NPN type, but i’ll talk more about this later. Just a second ago i said that transistors have three terminals. These are called a collector, a base and an emitter. The base is where you apply your signal. Collector collects and emitter emits. Can’t get much simpler than this. When i was introducing myself to the magical little devices, the obvious thing to assume was that the emitter emits the amplified signal and collector collects the juice for this amplification. This is not true. Emitter emits the juice to inside the device. Collector collects the amplified signal and this is the output we can get out if it. Somehow i don’t think i’m the only person who’s had this misconception when getting to know his/her transistors.

Let’s take a look at the subject in even simpler terms. Semiconductors in general work in a very simple way. The voltage and/or current is passed through the semi-conducting material in one direction, but the other. are you familiar with a diode? For a diode, we have anode and cathode as the terminals. Cathode is the end with a stripe on it.. If we apply 9V DC voltage to the anode, we’ll end up having a voltage that’s close to 9V DC on the diode’s cathode, as this is the direction the diode passes voltage and current. But if we apply the same voltage to the cathode, the anode won’t have any voltage, because the diode doesn’t pass the voltage or current in this direction. This is the fundamental, basic idea for all semiconductors. That’s what they do. Once you get this, everything will start looking a lot simpler.

How does a diode get us closer to a transistor?

I just explained how a diode works. There’s not a lot of options how that third terminal is connected inside this device called a transistor. Since i’m talking about NPN type transistors, think of the transistor as two diodes. Both anodes tied together as a base, one cathode is your collector and the other is your emitter. NPN stands for negative, positive, negative. Now, if we apply that 9VDC voltage to the base, or two anodes tied together, we’ll have near 9V on both, emitter and collector. This doesn’t amplify anything, but it shows us the idea behind the transistor. Let’s play with a thought of it doing some amplification without going into mathematics. To begin with, transistors amplify current, so we’ll need to give it some. Our guitar output is good source of low voltage AC signal and it’ll have hot and cold leads. For this example, cold is the same we have as DC ground, so  we can use just a single signal path. As we’ll need to apply some voltages to get the device doing the work, we’ll need to place a capacitor in series with the base and apply the signal to the free end of that cap. This cap will pass the signal to the base, but block the DC voltage from the base from getting to your guitar. Now it all comes down to bias. I know. It’s a concept that isn’t as clear as pancakes for most people. To get the amplification going, we’ll need to have a voltage on the base, which has to be smaller than the voltage on collector. If we take 1M resistor from the 9V supply to the base and another one with 100k value from base to ground, we’ll end up with something like 0,8V DC voltage on the base. That’ll work. For the emitter, we’ll take a small resistor from it to ground. Let’s say 470 ohm. Finally, we’ll need something like 10-12k resistor from supply to the collector and an output cap to block the near supply voltage from our real output.

What happened?

We applied low bias voltage to the base with our signal. Ground potential at the emitter and the bias, or reference voltage pushed the signal towards the collector. Which collected it and made it to pushed the signal out with way higher current than what we applied to the base. That’s the amplification of  a small signal in the simplest possible way. Four resistors, two capacitors and a single transistor. This is the configuration we’ll find inside EHX LPB-1 Linear Power Booster.Sure. This information doesn’t take you very far. But i’m hoping it’ll offer enough snack for thought, so that digging deeper may become slightly easier. There are tons and tons of good articles and videos around the internet if you feel like going further.

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