Transistor Amplification question - Page 1
Forum section: electronics / beginners

author: geratheg
author_rank: Regular Contributor
timestamp: August 12, 2014, 05:18:52 am
content: I've been reading and doing experiments from the Make: Electronics book and had a few questions. I'll number my questions in red, the ones with a letter are the ones for which I am looking for an answer:1)If we had 6V supply and a 100 ohm resistor in series with a collector of the transistor that amplifies at a 24:1 ratio such as the 2N2222 and the emitter to ground (see attached diagram) and no other device is connected in series, even though the transistor amplifies current, the maximum current would be limited by the resistor like so: 6/100 = 0.06 A by ohms law and the transistor cannot amplify to any more current than 0.06 A in this case.1a)Is this correct?1b)It's a little confusing to a beginner like myself to think about amplification because first I thought to myself "Does the transistor make current? Is that how it amplifies it and where do the limits come from?"2)One of the experiments uses a 2N6027 PUT and a 2N2222 with a 0.0047uF capacitor and a couple of resistors (see attached diagram). The speaker barely makes any sounds when the 2N2222 transistor is eliminated.So in the diagram, lets remove the 1K resistor and the 2N2222 transistor (Q3). Now wire 100 ohms to the bottom of the 2N6027 where the 1K was removed from, and the speaker in series with that and to ground.To me it feels like this should work because the PUT isn't supposed to have much resistance inside it once it lets current through (what I learned), and the speaker is basically now wired to the capacitor, which acts as if it's the power source and the speaker isn't going to get much current from the main power source due to the large 470K resistor.2a)The capacitor is theoretically supposed to be able to deliver infinite current if it was shorted, correct?2b)Now, it appears that just above 2.7V is around what the capacitor is charged to before being discharged, is this correct? Or is it 3.3V?2c)My other question: Is the capacitor not able to deliver enough current or is it that it does not store enough voltage for the speaker to make enough sound (it's very very quiet)? Basically, why do we need to add a 2N2222 transistor to make the speaker easily audible? Is it possible to make the speaker louder using just the Programmable Unijunction Transistor?Does the PUT transistor have a maximum current that it can handle and thus limits the amount of current delivered? Is this why the speaker is very quiet?Thank you for all the help!

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2014, 05:32:03 am
content: With regards to 1, Yes, you are correct. A transistor lets you vary the amount of current you 'let through', but it will be limited by the supply voltage and resistance. Transistors are amplifiers roughly the same way that a mechanical valve on a water hose is an amplifier: only a small amount of effort (power) needs to be applied to control a much larger flow of water. They do not 'create' current, they merely control it.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2014, 05:47:33 am
content: I don't really follow your question 2. One problem is your diagram. Try to learn to draw standard schematics rather than the physical layout on a breadboard. It is much easier for other people to follow.From what I can gather, Q2 is being used as an oscillator. It generates a small oscillating signal that provides the AC signal. Q2, the 2N2222 transistor is being used as an amplifier, to provide enough drive current to make an appreciable sound. Without the amplifier, I guess the 2N6027 can't provide enough current to drive the speaker, at least with the components shown.Capacitors can ideally provide infinite current when shorted, but only for zero attoseconds. They have a finite amount of energy stored, you can discharge it as fast or as slow as you want (in reality of course there are limits on both ends). Ultimately, your power has to come through from the power supply. In this case, the capacitors job I think is actually to set the oscillation frequency, it isn't really a energy storage device, although of course it naturally stores energy as part of its job. If you change the resistor and capacitor values together you can probably change the maximum power output of the 2N6027, but I would guess that it can't handle enough to drive the speaker by itself, hence the presence of the 2N2222 as an amplifier.

author: Bored@Work
author_rank: Super Contributor
timestamp: August 12, 2014, 06:04:30 am
content: Quote from: geratheg on August 12, 2014, 05:18:52 amThe actual current amplification of a transistor varies widely within the same series of transistor. You "never" get exactly x24, unless you use a more advanced circuit than just a common emitter amplifier.Further, 24 seems a bit low for a 2N2222. Check the datasheet.QuoteYes. The visual representation, which is also a design aid, is called the "load line". The load line shows how the load (the resistor) limits the behavior of the transistor. The wikipedia explanation is not too great, but should workhttp://en.wikipedia.org/wiki/Load_line_%28electronics%29.QuoteNo, a transistor does not "make" current. It can regulate current, which is used to amplify a signal. There is no current (or voltage) generator in a transistor (we skip special effects like photo sensitivity). The name transistor comes from "transfer resistor", and that describes its basic behavior.QuoteA programmable unijunction transistor? Ups. Haven't dealt with one since several decades. I hardly remember how they or a unijunction transistor work. They are typically used as switches, e.g. in oscillators, not as amplifiers. Have a look in the datasheet. It should give a hint how your circuit is dimensioned , how the transistor is "programmed".QuoteTheoretically for an infinite small amount of time. But in reality, its internal resistance plus any external resistance will limit the current, and as the voltage decays the current decays.

author: geratheg
author_rank: Regular Contributor
timestamp: August 12, 2014, 06:15:38 am
content: Schematic has been attached to first post.The speaker made a quiet sound wired to just the Programmable Unijunction Transistor.Here is my main curiousity: Can the Programmable Unijunction Transistor be used by itself to make the speaker loud enough? For example by varying resistance values. If so, do you have suggestions as to which values to try? If not, why is that?Does the PUT transistor have a maximum current that it can handle and thus can only deliver a limited amount of current? Is this why the speaker is very quiet?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2014, 07:32:55 am
content: Thanks for the schematic.I have never used a unijunction transistor, so I am not really sure how to use them properly. You could try reducing the 470k resistor and increasing the capacitance by the same factor. If it operates roughly the way I think, that should increase the current flow while maintaining the time constant. I don't know what the limit of the device might be. I also don't know if changing the values will change the waveform shape.

author: geratheg
author_rank: Regular Contributor
timestamp: August 12, 2014, 08:07:17 am
content: Ahh, even the author of the book said that those transistors aren't used much anymore lol.I'll try that, I appreciate the input.

author: Bukurat
author_rank: Regular Contributor
timestamp: August 12, 2014, 08:07:38 am
content: The P.U.T conducts when the anode voltage exceeds the gate voltage.Looking at the circuit the gate voltage is 2.7V. The .0047uf capacitor charges through the 470K resistor. When the voltage across the capacitor exceeds 2.7V the P.U.T will conduct, allowing current to flow in a circuit comprising the capacitor, P.U.T., 1K resistor and the base- emitter junction of the 2N2222, back to the capacitor.The capacitor will discharge, the P.U.T will switch off and the cycle will repeat.Thats the basics. As you delve deeper you will be able to calculate the base and collector currents as well as the frequency of oscillation.

author: LvW
author_rank: Frequent Contributor
timestamp: August 12, 2014, 08:11:55 am
content: Quote from: Bored@Work on August 12, 2014, 06:04:30 amYes - and this means: An input voltage (at the base) causes a corresponding output current (collector).Hi geratheg, in order to avoid misunderstandings from the beginning: The transistor - when used as an amplifier - works as avoltage-controlled current source.I think it is necessary to point to this fact because - unfortunately - many transistor descriptions/explanation and even some textbooks try to tell us that the BJT would be current-controlled - perhaps because Ic=beta*Ib looks so nice and simple? Of course, this equation can be used during design of transistor stages but it does not tell us the physical truth. There is an exponential relation between the base-emitter voltage and the collector current. This equation was derived from W. Shockleys famous description of the pn junction.

author: tautech
author_rank: Super Contributor
timestamp: August 12, 2014, 09:17:32 am
content: Quote from: geratheg on August 12, 2014, 06:15:38 amIn short the answer is no.Although the datasheet indicates they can pulse sufficient current, it is of very short duration and therefore unlikely to sufficiently energize a speaker coil.Hence the need for a transistor.I have seen them used for phase angle control of SCR's in a welder.2 UJT's each producing multiple pulses for each half of a 50 Hz mains sinewave.Datasheet attached