Help me get a handle on the transistor amplification MOSFET

[effectsbay]

Hello All

I've been looking at a few schematics for clean boosts on the internet. I really want to 'understand' what I'm doing vs. just soldering components and making it work. I'm really interested in tweaking, and if I don't understand how it works in the first place.. how can I tweak.

Basically, I'm really trying to understand how a transistor is used to amplify a signal. Sorry if this super stupid, but I just don't get it. On that topic, I always get lost on the voltage divider as well.

So I've been looking at a MOSFET transistor BS170. I understand that there is a base, collector and emitter. In my schematic, I think I understand that the input signal goes through a capacitor allowing a little voltage to hit the base keeping it open. At this point, I get lost. I'm not sure where the signal is amplified, and what triggers the amplification. I think I'm still hung up on a transistor as a switch.

Most of the schematics I see, I also see this input signal go between two resistors (I believe this is a voltage divider) which is still confusing me why it doesn't need to go through both resistors first, but having the second resistor after, still draws less volts to the base of the transistor???

Sorry if I'm way off here, just really trying to get a grasp on transistors and some basic fundamentals.

Thanks in advance everyone!
hank

[R.G.]

There exist more than  one kind of transistor. All of them share the property that a changing voltage on a control electrode (base or gate) lets a changing amount of current flow between two other electrodes. Bipolar transistors are not equal to MOSFETs and neither is equal to a JFET.

Try reading the following and let me know what is still mysterious.

http://geofex.com/Article_Folders/How_It_Works/hiw.htm
http://geofex.com/Article_Folders/mosboost/mosboost.htm

[PRR]

> I'm still hung up on a transistor as a switch

Yeah, but it can be a half-good switch.

Most switches "snap" from off to on. Some can be "teased" to "partially on". Like you touch a lamp switch, the lamp comes part-bright? Normally you also get the smell of a burning switch (it's absorbing some of the power which should be in the lamp). But ignore that for a moment.

While at this partially-on point, jiggle the switch. The part-on lamp gets more and less bright.

Attach the switch to your guitar string, drum head, microphone diaphragm. Now the lamp gets more/less bright under command of a small/thin vibrating string or skin.

Lamps don't really respond as fast as strings/skins vibrate, and don't make sound. Replace the lamp with a loudspeaker. Now the speaker cone moves according to the skin/string vibration.

The string/skin only has to move the small switch. The lamp or speaker is moved by some external power source. We have turned a small but interesting vibration plus a large but boring power source into a large interesting vibration. Amplification!

In fact a mechanical switch is a terrible way to amplify. It is impossibly fiddly. Yet one of the telephone inventors (Bell? Grey?) insisted that it DID work, all through the 30+ year patent battle. If brilliant men and lawyers can be confused for so long, we can use the idea as a stepping-stone along a path to better understanding.

(What worked better, was THE mike for 90 years: powdered carbon in a cup. Loose, resistance is high. Press, resistance drops. Add battery and diaphragm, you get a current which has your sound AND is more powerful: Amplification! Interestingly there is no fundamental patent on the carbon mike, and invention credit is tangled.)

Remember how you had to tease the switch to find the part-on point between on and off? That's your "two resistors" at the input. If a transistor is off at 0.5V, and on at 0.7V, we bias it to 0.6V. And like a real switch, this is very fiddly. We may need 0.59V on a hot day, 0.62V on a cold day. A different transistor may need 0.6V and 0.63V. Much of the art of bias is about finding that point every day for every transistor you might use.

> I always get lost on the voltage divider as well.

You are NOT going to "get it" until you master Ohms Law (relation of V, I, and R) and can estimate any of the three parameters at a glance. Marine Boot Camp starts with a million push-ups so you are fit to wrassle any enemy you meet; electronics boot-camp should start with a million Ohm's exercises so you are fit to wrassle any circuit. Then master voltage dividers, so you just glance and have a clue where it comes out. I don't mean 8-digit precision; more like a carpenter looks at a shed and "at a glance" knows he needs 7 sheets of plywood to cover it. May turn out to be 6.234 sheets... often a close answer is good enough.

> MOSFET  ..... base, collector and emitter

Read more slowly. Not only should you master basic 2-leg voltage and resistance before you get into 3-leg creatures, you should comprehend that there are different types of transistors and their legs are named stupidly different.

> Sorry if this super stupid, but I just don't get it.

I can be stupid, but I like this stuff. Even so, it took a few years to sorta understand basic triode vacuum tubes (which are "not that complicated"). It took a whole decade to "get" BJT transistors. Even then, I only had a "bus driver" working concept plus some skill at applying some simple math approximation. It's taken another 30 years to "get" how BJTs (and FETs including hollow-state; at some level they all work the same) work inside.

R.G.'s essays would have helped. But he covers a lot of material. Skim it quick, then take it in VERY small bites, with full digestion of each bite.

Read Everything you can handle (and save the gnarly books for later). Pete http://pmillett.com/technical_books_online.htm has some stuff, mostly past your current path-point, but try Inside the Vacuum Tube. Like Amdahl's There Are No Electrons, he pictures current as little guys running around, a flawed but classic analogy (just don't believe it too far). Yes, it is about Vacuum Tubes.... but transistors work the same with a few specific differences. Yes, you do need to look at atoms first. You don't need a deep grasp of Emission: in solid-state we use crystals where electrons are already loose. Once you have loose electrons, you can read "Movement Of Charges", "Diodes", etc. When you get to "Triodes" (and really "get" how current works), just remember that a tube's wire-grid is replaced with a "crystal junction" in a BJT transistor. A F.E.T. still has a grid, but we mount it on the side and call it a gate. Focus on the basic idea of signal, power, bias, and how you get them together and apart again.

[phector2004]

thanks for the tutorial, R.G!

I've been looking for something basic like this to cover what they didn't teach us in school

[effectsbay]

There exist more than  one kind of transistor. All of them share the property that a changing voltage on a control electrode (base or gate) lets a changing amount of current flow between two other electrodes. Bipolar transistors are not equal to MOSFETs and neither is equal to a JFET.

Try reading the following and let me know what is still mysterious.

http://geofex.com/Article_Folders/How_It_Works/hiw.htm
http://geofex.com/Article_Folders/mosboost/mosboost.htm

The MOSFET article is awesome. I read it 10 times.. took a few times, but I'm definitely understanding the gate, source and drain action. His circuit is exactly the circuit I was looking at, so for a clean boost, I'm understanding the following

Input signal hits the first cap (100), which scrubs the juice, but allows the signal to go through, but after the cap, it's getting hit with 9V. 2 10M resistors (voltage divider) brings the voltage to be the minimum Vt for the gate, which opens it when the circuit is on. The drain is getting partial juice (limited by 5.1K resistor) and that is amplifying the signal  at the source. Source is adjusted is adjusted by 5K resistor pot and sent to the output. I believe the last cap is to only allow signal back to the output. In the schematic I'm using there is a 100K resistor instead of a cap.. confused by that.

What I wasn't understanding before.. but I think I get it now.. is that current is hitting the hitting the drain and gate at the same time, but the amount of volts is differed by the resistors in line. I'm also understanding that the when the pedal is 'on' the gate is open the full time, but the resistor pot on the source is controlling the source.

Let me know if I'm way off.

Thanks so much for providing those links!!!!

hank

[Transmogrifox]

Here's another analogy to help put things together:
Think of a transistor as a valve...actually, this analogy is where the term Valve has come to be a name for vacuum tubes.

V, Voltage --> Water pressure
I, Current --> Volume of water flowing from high pressure to low pressure
R, Resistance --> Constriction of the pipe...or in the case of a transistor (trans-resistor), the constriction of the valve.

Now say this valve can be turned on and off by connecting a pipe to the valve actuator and feeding it with a small pressure.  The valve is constricting a larger pipe that could potentially have a lot of pressure, and perhaps can carry a large volume of water (high current).

Now, if on the input to the valve, you have a pressure that can go positive and negative (pushing and sucking), then say when it goes positive, it is pushing on the valve to open it.  Then a bunch of water begins to flow through the valve in response to the  amount it is pushed open at the "gate".

Then when the input "sucks" the valve, it begins to shut.  If the valve gate started at zero pressure (shut), then nothing happens in response to sucking the valve out.  Now you need a bias in order to see some change on the output due to sucking.  You put another pipe in place to supply a steady pressure so the valve is always about 1/2 way open or so.  Now for your changing pushing/sucking "signal" at the gate, you need to isolate it from the constant pressure....in other words, you only want the gate to respond to pressure CHANGES, and not to a constant pressure.  This could be done by perhaps putting some kind of a diaphragm in the input pipe to keep water from leaking out where the input comes in.  If the pressure difference across the diaphragm changes, then that gets transferred to the other side...and then the valve is modulated, and thus the current through the big pipe gets modulated in response to the little tiny pressure variation at the valve.

Now the resistor found at the "Collector" or "drain" (depending on the transistor) acts to constrict the amount of current that can flow through this circuit.  Looking at the "drain" side of the valve, the pressure drops as the valve opens.  When the valve is significantly less constrictive than the pipe feeding it, the pressure is really low. If the valve turns off, the pressure rises to the pressure of the mains.  This is distortion:  At one side the input signal is no longer followed by the output because the pressure at the "drain" is so low, it can't get any lower.  On the other extreme, the valve is completely shut, and the output limits at the pressure of the mains.  For everything in between, there is a relationship between the pressure at the "drain" and the pressure applied to the valve actuator.  This relationship is P_out/P_applied ==GAIN. 

I hope you can follow that ok.  Anyway, water pressure, flow and pipe resistance is a good touch/feel/see concept that can help understand electricity.

To take the concept further, it is conceivable to build an entirely mechanical amplifier based on the exact same concepts.  It would probably be some kind of Pneumatic amplifier (compressed air or other gas).  If one could do some careful engineering and machining for high speed pneumatic valves, then you could directly drive a speaker cone with some sort of an air piston.  The microphone would be a barometer .

[effectsbay]

Thanks Transmogrifox. That does help. I've made some good headway over the last two days. Still a little foggy, but overall, I'm starting to where things are going.. generally. And obviously, this is one of the easiest circuits (so I hear) so I'll keep it slow and steady.

Thanks!
hank

[petemoore]

  The strings goes < way and the mag-field of the PU produces a voltage which goes < way, this voltage follows the string very closely and is an 'analog, or the electronic signal could be said to be analagous to the string movement.
 The string also swings the other way> so does the electronic analog.
 Not sure how good an analogy the idle bias point of an amp is to an idle, still string, but in the middle fits both, still, and between the extremes of the peaks + = < and - = >. The string and electronic analog switch at a frequency.
 The cap you mentioned at the beginning lets the frequency into the transistor input, but blocks the DC [the ''still, constant pressure of voltage, like a hose off a big tank of water, draw some out [current] and the pressure hopefully stays constant. Every time the string goes <, it feeds a + [half wave] analog into the Q [transistor] input, and the Q follows this analog signal [starts at idle bias swings up and over passes *idle bias to the - halfwave...crossing the *midpoint.
 The + and - half wave are usually identical and shown as a sine wave, a midpoint line going through the middle. Idendical only if symmetrical, if off bias then the symmetry may 'scrunch' the wave on one side [distorting the waveshape].
 Nuff for today.
  Read RG's pages.

[soggybag]

Wow this could be the most informative thread of all time. Maybe we should start a circuit analysis thread for beginners.

I have been trying to do this for myself lately. I take a circuit and try and figure out as much as I can. One of the things that still stumps me still is calculating low pass high pass filters. These appears in every circuit.

I don't want to hijack things but it seemed related.