Transistors / Amplifiers. What am I missing? - Page 1
Forum section: electronics / beginners

author: omax1784
author_rank: Newbie
timestamp: December 05, 2023, 12:58:20 am
content: I've been reading about transistors and amplification as I'm getting into tinkering around with old tape machines, mixers and what not. In my book and many other sources online, they all mention things like "input high impedance - reverse bias" and "output low impedance - forward bias" and I'm looking at the schematic and I don't see it. I see the opposite. I'll attach a photo from my book. How is the input reverse biased and output forward biased? I see 2 DC sources, the collector source which looks reverse biased and the emitter-base DC source which looks forward biased. The input is coming in on the base while the DC voltage in that leg shared with the input signal is forward biased not reverse biased. Neg to N type. Why am I wrong?

author: CaptDon
author_rank: Super Contributor
timestamp: December 05, 2023, 01:45:32 am
content: The statement makes no sense? In both of those circuits, the NPN example and the PNP example the base is forward biased with the correct polarity on the collector. In the examples the collector supply voltage would have to be 10 volts and probably more like 12 to prevent clipping. The base to emitter is certainly forward biased. When you view the transistor as two diodes the collector to base voltage would appear to be reverse biased and if it were 'forward' biased the collector supply current / voltage would conduct right out through the base connection which we know it doesn't. In that textbook example the base bias battery would have to be high enough and the base resistor of proper value to set the idle point at the emitter resistor to about half the collector voltage. Some textbook examples are themselves over simplified or the verbage is over simplified to make a point and the point becomes less clear the more you examine the example.

author: rstofer
author_rank: Super Contributor
timestamp: December 05, 2023, 03:55:32 am
content: W2aew video 366

author: AlfBaz
author_rank: Super Contributor
timestamp: December 05, 2023, 08:50:34 am
content: Quote from: rstofer on December 05, 2023, 03:55:32 amThere's also 356

author: MrAl
author_rank: Super Contributor
timestamp: December 05, 2023, 12:39:50 pm
content: Quote from: omax1784 on December 05, 2023, 12:58:20 amThis might help.Here is a drawing with the internal 'diodes' shown.The original 'diodes' are drawn in red, the collector base 'diodes' are drawn in blue.You should be able to see the forward and reverse bias schemes now.Keep in mind a diode is forward biased when its anode is more positive than the cathode, and reverse biased when the cathode is more positive than the anode.The cathode is depicted as the tip of the arrowhead.

author: omax1784
author_rank: Newbie
timestamp: December 05, 2023, 11:06:35 pm
content: Thanks for the input, but I think everyone agrees that a) the base-emitter is forward biased, and b) the base-collector is reverse bias. NPN or PNP, doesn't matter. It's specifically the signal input (reverse) that I don't understand. The input is connected to the base, the emitter-base DC source, and ground. Somehow this means that the input is across base-collector which is reverse bias. Why can't I say the input is across base-emitter? It's right there, look it at, even sharing a nice little DC source with the base.

author: TimFox
author_rank: Super Contributor
timestamp: December 05, 2023, 11:17:01 pm
content: Yes, when you vary the base-emitter voltage, the collector-emitter current changes.Increasing BE voltage magnitude increases the magnitude of the CE current.From this, drawing in the current direction arrows correctly, with a resistor in series with the collector from its supply voltage:If you increase the base-emitter voltage, the base current of the forward-biased BE junction increases and the increased voltage drop in the collector's load resistor results in a decrease in the collector-emitter voltage.Hence, the inverting action of the "common-emitter" amplifier, whether NPN or PNP.

author: magic
author_rank: Super Contributor
timestamp: December 05, 2023, 11:32:29 pm
content: Quote from: omax1784 on December 05, 2023, 11:06:35 pmNot sure what sources mention those supposed "rules", but they are nonsense as you concluded yourself - anything connected to the base faces two different junctions.If we want to be pedantic, the reality here is "input forward bias - low impedance". That's because the lower impedance junction "wins" with the high impedance one when they are effectively paralleled. Indeed, input impedance of such amplifier is not very high - it's β·RE, because base current varies with base voltage only β times less than collector/emitter current.For really high input impedance amplifiers JFETs are used, where the gate is reverse biased with respect to both source and drain. Or MOSFETs, with completely insulated gate.

author: MrAl
author_rank: Super Contributor
timestamp: December 06, 2023, 01:41:35 pm
content: Quote from: omax1784 on December 05, 2023, 11:06:35 pmHi,Not really.See 6-13 and 6-14 in your drawing.Note that the signal source is connected to the base AND to ground. The emitter is connected to ground through a resistor. Since the base emitter is forward biased it only has a small voltage drop, like 0.7v. With no signal input the emitter could have a very low voltage (unless biased to 1/2 Vcc). The collector has a constant voltage applied to it, so the voltage between the base and collector could be 12 volts.As the signal changes, the current through the base emitter changes, and that changes the current through the collector to emitter, and that means the emitter voltage changes. For a positive input change the current through the base emitter rises, and the current through the collector to emitter rises, and thus the emitter voltage rises.This is also called a Voltage Follower because the emitter voltage approximately follows the input signal voltage.If the input signal was connected to the collector, there would be a huge current into the base emitter, which would probably blow out the transistor or at least cause it to saturate, and that would even be with no signal input at all. The impedance of the signal source would cause a large input current through the base emitter which would not work very well at all.The main reason for the transistor action is the current through the base emitter (or also viewed as the voltage across the base emitter). The voltage across the collector base does not matter as much by a long shot, so changing the collector to base voltage would make hardly a difference on the output voltage. There is a small change that would accompany a forced change in collector to base voltage (see the Early Effect), but it would not be anything like amplification if you did that as it is a minor effect. Thus it would not make too much sense to try to purposely vary the collector to base voltage if you wanted some decent amplification.

author: magic
author_rank: Super Contributor
timestamp: December 06, 2023, 04:08:21 pm
content: Sorry, I missed that we are talking about common collector here.These three configurations are sometimes also known as "grounded emitter", "grounded base" and "grounded collector". It doesn't necessarily mean that the common terminal is connected to literal ground, but it means that it is at a fixed potential and doesn't move with the input or output signal.In CE and CB amplifiers voltage between the grounded terminal and the input terminal is vital for determining collector current, so no one doubts that the signal is applied between the base and emitter or vice-versa. In CC, the exact DC (or even AC, within reason) voltage at the collector doesn't matter as long as the BC junction stays reverse biased, but since the collector is typically "grounded" one can say that the signal is applied between the base and collector, by convention.If you want, draw a version of this circuit with the collector connected to ground and the load connected between the emitter and a suitable positive (PNP) or negative (NPN) supply rail. Then you will see the input signal source appearing exactly across the BC junction.This doesn't necessarily invalidate what I wrote yesterday about input impedance - both CC and CE input impedance is β·RE. CE input impedance is usually fairly low because REis low in these amplifiers. CC input impedance may be higher if REis suitably high, for example when driving a second emitter follower (a Darlington pair).

author: omax1784
author_rank: Newbie
timestamp: December 06, 2023, 10:56:17 pm
content: I appreciate all the helpful answers here. The sources I was quoting in regards to stating CC has high input impedance is the book I had in the photo which is Basic Electronics - Prepared by the bureau of navy personnel, and also electronics-tutorials.ws.Thanks.

author: MrAl
author_rank: Super Contributor
timestamp: December 07, 2023, 09:18:19 am
content: Quote from: omax1784 on December 06, 2023, 10:56:17 pmHello,A lot of the Navy stuff is pretty good.However, I am not sure I like the nomenclature "high impedance" or "low impedance". That's because the impedance is quite application dependent, as someone else alluded to earlier. That means the high or low impedance is a relative term which makes it comparative rather than absolute. That in turn means that "high impedance" is going to be relative to some other impedance, as is "low impedance".For example, a voltage follower has been said to have "high input impedance", while an emitter follower is said to have "low input impedance". However, if the common emitter has (say) a 10 Ohm emitter resistor and the voltage follower (common collector) is driving a 10 Ohm load, the input impedance will be about the same for both.Moreover, if the common emitter has a 100 Ohm emitter resistor while the common collector has a 10 Ohm load, the input impedance for the common emitter will be much higher than the common collector.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 07, 2023, 10:54:58 am
content: For all practical purposes you can use the attached to get a very good estimate of how small signal AF circuits behave.The transistor is in its operating region when the base emitter junction is forward biased. This will happen at about 0.7V for silicon.In the case of the follower, the base bias voltage would vbe set so that 0.7Vcc appears at the emitter. So for a 10V supply, Vb would be typically 5.7VFor the common collector circuit, the collector voltage would be set to 0.7Vcc and the emitter voltage set to a volt or so.Thats easy to calculate: Ve=1V, so Vb=1.7V. Ie=Ib+Ic but as Ib for a smal signal transistor is small (Hfe times smaller) compared to Ib you can say that Ie~Ic.Then Ie=Ve/R3. The Voltage across R4 needs to be 0.7*Vcc, ie R4=(0.7*Vcc)/Ic . For a power transistor Hfe is often quite low and you have to factor in Ib.Why set the emitter a volt or so? Thermal stability. Increasing temeperature casues Vbe to drop in value, that increases Ic amd that heats up the transistor even more in a process known as thermal runaway. With R3 fitted, increasing Ic (Ie) increases Ve and that pushes down Vbe. The same is true for the follower too. Thats negative feedback at work.Transistors arenota pair of diodes- the book is misleading here. Although a transistor will meaure that way with an ohm meter, its not how transistors work. You can get into the physics of it but it wont help you get started with interpreting and designing basic circuits. Unless your doing an exam, dont worry!Get a decent text book. ie Horowitz and Hill.

author: jasonRF
author_rank: Regular Contributor
timestamp: December 08, 2023, 12:15:02 pm
content: The emitter-follower output impedance on that figure is terribly wrong. Definitely not "good enough". I have corrected it in a new version of that image.Just how wrong? Consider the case where the emitter voltage is 5V and R3=1 kOhm. Then Ie = 5mA, re=5.2 Ohms, and the output impedance is about 5 Ohms, not the 1 kOhms that the wrong formula indicates.jason

author: magic
author_rank: Super Contributor
timestamp: December 08, 2023, 12:30:31 pm
content: The "good enough" formula is still good enough when voltage drop across R3 is order of magnitude lower than 26mV

author: Terry Bites
author_rank: Super Contributor
timestamp: December 08, 2023, 03:27:26 pm
content: Quite right, thanks. My brain fart there I think!

author: iMo
author_rank: Super Contributor
timestamp: December 08, 2023, 04:07:50 pm
content: FYI - this guy is producing an unbelievable amount of videos with practical step-by-step design and analysis (with practical EE math) of tons of various circuits with transistors and opamps. Pretty cool, indeed..https://www.youtube.com/@STEMprof/videos

author: MrAl
author_rank: Super Contributor
timestamp: December 08, 2023, 05:20:03 pm
content: Quote from: iMo on December 08, 2023, 04:07:50 pmNow if only his penmanship was to improveI've never seen anyone write the units for resistors as an exponent.It looks like he is writing something like 3.3^k for a 3.3k resistor instead of just writing "3k".See that 500k resistor, written as "500" with an 'exponent' of "k".

author: iMo
author_rank: Super Contributor
timestamp: December 08, 2023, 05:35:03 pm
content: 52 videos in the last month..

author: MrAl
author_rank: Super Contributor
timestamp: December 10, 2023, 02:20:22 pm
content: Quote from: iMo on December 08, 2023, 05:35:03 pmHi,Yeah that's an achievement in itself I guess, lots of work.Kind of makes ya want to make a video about something

author: MathWizard
author_rank: Super Contributor
timestamp: December 11, 2023, 02:08:15 pm
content: Quote from: iMo on December 08, 2023, 04:07:50 pmI had a look, I'm sure there's lots of good video's there.I need a 2nd life so that I will have time to read all the pdf's and watch all the prof's/etc on utube, on EE/math/physics/chem/etc.I like to have something to listen to in the background whether I'm learning equations, or building /fixing circuits, to break the silence. But for ages I'll just put on fiction related stuff, because it can get pretty boring sometimes. And I won't hear a lot of it anyways, I'm so into whatever I'm doing.I should make more time for some video's, and treat them more like classes.